ACE Fireball 535 Royal Enfield High Performance Kit

ACE Fireball 535 Royal Enfield high performance engine kit

ACE Fireball 535 Royal Enfield high performance engine kit

What we have here, fellas, is the ACE Fireball 535 Royal Enfield high performance engine kit. You take your stock, single cylinder Royal Enfield Bullet and with the addition of this kit, turn it into a real, ... well, fireball. No, you're not going to challenge any of the boy racers out there, but compared to any stock Royal Enfield, you'll be the man to beat, seriously. This thing is fast. A video showing an impromptu drag race between the ACE prototype and another modified Bullet makes it clear what a jump in horsepower and performance these modifications deliver, the ACE Fireball simply runs away from the other bike.

ACE Fireball 535 Royal Enfield high performance engine kit

ACE Fireball 535 Royal Enfield high performance engine kit

This package was developed over the last 5 years to overcome the shortcomings of some other Royal Enfield modifications that were very race oriented, which meant lousy street manners and questionable reliability. RE owners wanted more performance but good manners and something that would last.

So, what exactly is the ACE Fireball 535? The primary piece is the modified cylinder head. The development and engineering was done under the supervision of Joe Mondello, one of the most famous names in cylinder head modification. In fact, each head is hand signed by Joe Mondello himself, and individually serial-numbered, they've even created a registry for documentation.

Tom Lyons, one of the partners (along with Sumanth Janardhan aka "Chummaji") behind the ACE, sent me a virtual truckload of information on the kit (I think he's very proud of it!) and it's taken me quite a while to digest it all.

ACE Fireball valve gear

ACE Fireball valve gear

The work begins when the customer's cylinder head arrives. It is stripped and pressure cleaned. The head gets new valve seats, the intake and exhaust ports are treated to a full Mondello high-velocity, high-flow porting job, and the seats get a new multi angle valve job. Custom stainless steel Manley Racing valves with narrower stems are installed in new valve guides with modern keepers holding the PAC Racing stress-relieved competition beehive springs. The springs sit on hardened precision-ground spring shims with custom-made titanium upper valve spring retainers on the other end. There are also precision-ground hardened-steel lash caps over the tips of the valve stems and Viton valve stem seals control oil flow. This entire valve spring system is seated into a machined recess which is cut into the head casting for the purposes of making room for this very different valve spring set-up. It is NOT a "drop-in" package, and there is considerable machine work needed to get this valve gear system into the head.

The combustion chamber is coated with a silver-gray Thermal Barrier coating of the CER-MET(ceramic/metallic) type. This slows the heat transfer from the combustion chamber into the head casting, so the head runs cooler, and more heat is retained in the chamber to make more power. It also provides resistance to detonation because the surfaces cool quickly and do not retain heat as much as the metal substrate. The faces of both valves also receive this same coating to keep them running cooler.

The exhaust port is also coated in a black Thermal Barrier coating that is mostly ceramic material. The purpose of this coating is to insulate the alloy head casting from soaking up unwanted heat from the exhaust gas stream as it exits the head through the port.

The result is a completely modern, high performance cylinder head for the Royal Enfield Bullet, something to awaken the giant within.

But wait, ... there's more!

There's also the ACE Magnum Cams. I could go into all of the detailed specs but basically, the Ace cams are designed to be a "hot street," NOT a "full race" cam set. They are targeted at a reasonably higher rpm range for power production, while not going higher than reliable piston speeds would allow with the improved bottom-end parts. The engine remains manageable and "street-mannered" so it idles well and has torque available in the lower and mid rpm range. You can have fun without having to wind it tight.

Wait, what was that about "improved bottom-end parts?" Those are made up of the ACE piston, 3mm larger than stock diameter and forged from 2618 alloy, using Total Seal piston rings and connected to a steel Carrillo connecting rod. As an option, you can get the Thermal Barrier "CerMet" coating on the crown, which keeps the piston running cooler, and the skirt is coated with a Moly Dri-Lube coating.

Rebuilt crank with polished flywheels, Carillo racing rod, and Alpha roller big-end bearing

Rebuilt crank with polished flywheels, Carillo racing rod, and Alpha roller big-end bearing

They also have custom torque plates for keeping the Bullet barrels distortion free during boring and honing plus there is a Mikuni TM32 flatslide carb, a Barnett clutch and a nice exhaust system to top it all off.

Here's ACE number 1 on the dyno, hitting the rev limiter at 6200 rpm:

ACE Fireball number one has about 16,000 miles on the piston and bottom end and 4000 miles on the Mondello head and everything is running great. It also clocked 103 mph on the GPS speedometer, with the bags on! For those of you who have ever ridden a Bullet, you know that's definitely quite a bit over stock. It's ridden every day on the highways between New York and Philadelphia.

Tom sums it up this way:

In essence, the results are a vintage 500 single that rides with the authority and performance of most 650-750 vintage twins like the Triumph Bonneville T120. Torque is available in great quantity from just off idle, all the way to the 6000rpm redline, with a torque peak of 38 ft-lbs @ about 3700rpm. Horsepower peaks at 37.1 at the rear wheel, at about 5700rpm (We've doubled the rear-wheel hp of the stock bike). A very wide and useful powerband. Very easy kickstarting, great idle stability, great riding around town and in the city, and can really burn up the road on the twisties, and hits 103mph. Cruises on the highway at 80mph without problems.

It's a niche-bike that is aimed at the real-world riders who need to "have it all", and not just a race bike. This bike delivers in spades, in every category, including 70-75mpg fuel economy.

Here's Fireball number 1 with the drag race toward the end of the video. Sounds great!

Before anyone asks, yes, Tom has already had discussions with Aniket Vardhan about installing the ACE Fireball kit in Aniket's 1000cc Musket currently in development. Sweet!

OK, what is the price of entry?
Ace Mondello head mods (on customer's head casting) $1000.
Custom hand-matched intake manifold (pinned and flowed), $100
Ace 535 Forged Piston with pin and rings $295. Coated add $50.
Ace Magnum Cams $350/set.

Barrel and bottom end work, Carrillo rods, Alpha bearings, main bearings Mikuni carbs, exhaust systems, etc, P.O.R.

I hope a lot of Bullet owners take a serious look at this kit. It could surprise the heck out of some vintage bikers out there as they get dusted by an unassuming Royal Enfield.

Very, very nice looking kit with lots of development behind it. I like this a lot.

You can find Tom on the Yahoo group for the ACE


  1. Geirskogul says

    I’ve been following this on the Classic Motorworks forums, and it seems like everything has come together. Find an Iron Barrel motor, throw a couple grand at it, and you have a beast of a motorcycle that will also turn heads at every stoplight. This is the way to do it!

  2. lostinoz says

    simply amazing… this should honestly be a mandatory upgrade to all Enfields out there.
    however are these mods for ALL motor styles? considering theyre in talks with Aniket, i would guess its for the older style motors.

    with the higher performance system, in the musket, i would assume there would have to be a clutch upgrade as well, i remember the video of Aniket riding the musket and the clutch slipping as is, imagine TWO of these hot heads doing their number to it.

    “musket 1000 currently in development” i cant wait to see “currently in PRODUCTION” i honestly think thats the only reason i havent pulled the trigger on buying one (royal enfield) yet.

  3. says

    Yes, this kit is for the older-style vintage “iron barrel” Bullet 500 engines with origins from the India factory.

    We use Barnett friction plates in the clutch, with 3 heavy springs, and that holds the power nicely.
    For much higher power mods, such as the potential of the 1070cc Musket V-Twin with 2 Fireball kits on it, there is the Bob Newby racing clutch with belt drive primary, such as are commonly seen on racing bikes.

    • Michael J Peery says

      Hey Tom. I have been reading as much as I can on the Fireball and I still have yet to find out where to begin, who to talk to and where to find the parts. Where are you all located? I am very interested in this up grade. I ride a 1995 500 Bullet and have been dreaming of some performance upgrades and I REALLY like what I am seeing with this Ace 535 Fireball. Help me by pointing me in the direction for getting started. Cheers!

  4. todd says

    I still have to see if I can get an Enfield into California. Next stop, one of these kits. That’s more power that either my GB500 or XR650L. The kit seems like a bargain too.


  5. says

    I wonder if there has been any experimentation with taking a dozen pounds off the crank? That would wake up the engine acceleration wise quite a bit I would think. Re-balance it and you could live without all the extra rotating weight. It’s not a dirt bike so I wouldn’t think traction would be an isuue.

  6. says

    Yes, reducing flywheel mass has been done on the track bikes for many years.
    There are limits, and most don’t try to remove more than 5 pounds. Using the crank from the 350 Bullet is an easy way to do that. Same stroke.
    However, it has been seen that the higher flywheel mass can be a benefit, because of rotational stability over the 4 strokes of travel in a single cylinder. In fact, some racers of vintage long-stroke singles are moving back to heavier flywheels because of that.
    There are pros and cons to the idea.
    We kept the stock flywheel mass for street use purposes.

  7. Mark L says

    You guys look like you are having WAY too much fun! I really need to look for an Enfield.
    I really like the blue one that was on the cover of a print magazine recently…

    This is actually what motorcycling is becoming to me. Fun again!

    Thanks for the smiles watching the video :)

    Mark L.

  8. says

    That’s why these Royal Enfield Bullets are so popular with people who want to have fun, like in the old days.
    You can actually get into them and modify stuff and be creative mechanically.
    And they are very fun to ride. Even the stock ones are addicting, but very sedate..
    But when you light a little fire under them with some mods, they are an extraordinary machine in the vintage way.
    It will never be a fast bike by modern standards, but it is amazing how much fun can be had below 100mph.
    These are some of the most fun bikes on the market today, and I believe that’s why all these folks like me are picking Bullets to build specials and mods around. There are quite a few of us now..

  9. Geirskogul says

    Even though I can’t apply any of these mods to my Enfield, as it’s an AVL model, the appeal of an old-style bike with modern-style support it very appealing.

  10. akumabito says

    Nice! And I agree – two of these in Aniket’s new machine would be EPIC. I realy hope the double-500 goes into production. Depending on what his conversion kit would cost, and the cost of locating two donor engines it might still very well be cheaper than, say a S&S or Ultima twin. That’s got to grab the attention of some custom bike builders..

  11. kneeslider says

    What’s becoming more apparent all of the time are the number of people using the Royal Enfield as the base for really amazing developments. The ACE Fireball follows the gorgeous McDeeb Specials, the Musket V-Twin from Aniket Vardhan, the Carberry V-Twin and many more. They’re already thinking of combining the ACE with the Musket, but how about combining the ACE with one of the McDeeb specials? Killer vintage looks with performance to match. The possibilities are very appealing.

    Who says you need some high power mega racer to have fun and look good?

  12. DWolvin says

    Seconded on all the comments about how fun that looks, and I really like the idea of a Royal that could hit the San Diego freeways without worry…

    And that exhaust note is AWESOME!

  13. baconpocket says

    i wonder if mac motorcycles might be interested in seeing how much power these air-cooled 500cc thumpers are capable of?

  14. kneeslider says

    baconpocket, “i wonder if mac motorcycles might be interested”

    You know, that’s not a bad idea. The original Mac designs might work with the RE single and the power with the Fireball kit might be enough to make it go.

  15. says

    Thank you for all the comments and well wishes. It has been a lot of hard work in the garage and drawing board but the bike is a joy in every way. The fun’s just begun for Tom & myself. Hopefully Aniket has also been making progress with his latest alchemy.
    good to see familiar names here. Would be a pleasure to build you a fireball someday or at least the crank for you to build upon on.

    It’s pretty simple to get an old style Iron barrel bullet legally into california. The best would be to find a used one with a clear title and at least 7500 miles on the odometer. Contact the Ca DMV and have them run a check of the vin as there is the rare instance of certain vins blacklisted in ca regardless of odometer reading.
    Any iron barrel 500cc bullet with a blown engine is a perfect candidate for our kit as the entire engine will be rebuilt and strengthened from the crank up anyway.
    We currently have a customer in napa valley who is contemplating/in the process of starting an e85 Fireball build that will really move on those Cali highways.

    Hawk: thank you! Before kicking that prototype engine over for the first time… I was anticipating a sound similar to a goldie but after the first ride… i heard manx all the way. It lacks the goldie’s tweet on throttle overrun but more than makes up for it with the manx’s rumbling rudeness.

    Mac motorcycles:
    I’m very keen on their bikes and designs. All of them are stunningly beautiful but I do think the roarer would fit the fireball engine like a glove.


  16. Ben Bullet says

    Awesome! That’s what we needed! But I love Bill’s solution too – Bill can you tell us more?

  17. says

    The way this engine is set-up, we have created our desired behavior with the port and cam match.
    In an antique engine such as this one, where we don’t have electronic engine management systems or variable valve timing, there is only so much you can do with width of the useful power curve.
    If you push the power curve too high, you lose it off the lower rpms. That’s what race bikes do, because they are only interested in high rpm performance on the track.
    For our purposes on the street, we needed a wide band performance envelope, and maximizing hp at a narrow band at the top of the rpm range didn’t fit that need.

    We looked at the basic mechanical integrity of the construction of the engine, and determined that piston speeds could be reliably handled up to about 6000rpm without getting into serious/dangerous stress territory. Race bikes can go higher, but they don’t last as long, and are weak in the lower rpm.
    We could reach 6000rpm at the top, and still keep our lower rpm torque, and that combination, along with the piston speed figure for reliability, gave us our answer for redline target. So again, our goal was to get as much power and revs as we could without losing lower rpm tractability, and without going too high and having potential reliability issues.

    Then, we built the engine with parts that were suitable for racing engines, but we limited the rpm range to something that was well within their abilities, to build in a reliability margin. We used lighter and stronger moving parts inside.
    We limited the rpm range by using a port size and cam profile that would just barely reach our intended max rpm, and make max hp right about at that point. We did pretty well at that, and the dyno showed that we hold max hp to about 5900 rpm, and just begins to drop after that. Actually, we are within a couple hp of peak hp, all the way from 5000rpm to 6000rpm, so we can shift at 6000rpm and still be on the peak hp range after the shift. This makes for fast acceleration with our gearing. Broad hp peak zone. Not “peaky”.

    By having a port that is “just enough” to get where we need at max, it gives the best port velocities at lower rpm, so it helps the low-rpm torque. The port isn’t too big, in other words. We made it big enough to get our rpm goal, but no bigger, so that the speeds of the intake flow remain as good as possible in the lower rpms. We use a high port-velocity approach.
    We also used a cam timing technique which spreads the powerband wider, and is good for wideband torque, and has just enough overlap to get where we need to go in the higher rpms, but keeps good cylinder pressure for torque production in the lower rpms. This gave us a very high level of torque, beginning at the lowest rpms, and staying very flat and high for a very wide range. Excellent for a street motor.
    The combination achieved in this careful port/cam matching is responsible in large part for what we have going here.

    We’ve achieved a much better efficiency level with our design, which is exhibited by the fact that we’ve provided a great increase in power, while still retaining approximately the same fuel economy(70-75mpg) as the stock bike. Huge efficiency increase.

    There are alot of details that I could go into, but suffice it to say that this design has been worked over with a fine-tooth comb, to get exactly how we wanted it to go. It has taken years of work. It might look like just a head mod, and a piston and some cams, but I assure you that those didn’t just pop out of my toaster one morning. Alot went into designing exactly how those things needed to be, to get to the goal.
    The apparent “simplicity” is directly attributable to the fact that we designed each of these items to “the T”, in terms of our goals, and we don’t need to add any extra complexity because we designed the performance into the key parts of the engine. And we did it in a unique way, which hasn’t been used on the Bullet in the past(as far as I know).

    Certainly, I could have pulled more peak hp out of this engine if I made it a racing design.
    But that wasn’t my goal. I wanted very good power, very good reliability, and very good street behavior. Because that’s what people need in the real world.
    I didn’t chase spec-sheet numbers, even though I’m aware that alot of folks buy things based on numbers. I was after real-world performance on the road, and that’s where this bike excels..

  18. Tony C. says

    What a beautiful sound! Its great to read about people who take projects like this by the horns. The old enfield has always been a timelessly beautiful bike to me but all i ever heard about from their owners was whine about how slow and unreliable they were. Sounds like all thats been tackled in very intelligent ways. Bravo to you guys. I think its time for me to sell a norton and build up an enfield!
    – Tony C
    Neptune, NJ

  19. Bill says

    It’s one of two special stunt machines made for a recent Harry Potter film (hence the disguised exhaust etc.). The engines are from CCM .They’re the only Enfields I’ve seen that would power wheely :)


  20. joe says

    Interesting upgrades,it’ll be interesting to see how the bottem ends deals with the extra power long term.They where only just adequate in standard tune. Its a pity someone hasn’t brought back the BSA 500 Gold Star engine,with modern engineering practices ,metalergy and oil seal upgrades like they’ve done to the Enfield, the Goldi engine would run rings around any Enfield in looks,performance and sound .

  21. says

    I think the best bet is to upgrade the 350 to the 500 platform first. The 500 engine drops right into the 350 frame. Then you start out with the bigger engine platform, and get more results out of the money you spend. The 350 takes more work to mod, and you can’t get quite as much out of it in the end.

    ABSAF in the Netherlands is doing very nice modern clones of the Goldie engine. You can buy complete engines, or parts, from them.

  22. Ben Bullet says

    Hey Bill, are you the one who built it? I’m really interested, coz I have a similar project – fitting a japanese single into an Enfield frame. I’ve been running Enfields on big overland journeys in Europe and Asia, and now I’m planning another one in South America where I need a really reliable bike :-) (sorry for the Enfield afficionados, but I don’t believe an Enfield will ever be reliable – even with this awesome kit). Any info on the building would help, is there a way to pm you? (so I stop bothering others here ;-)) Checked your website but no contact info.

  23. Bill says

    Ben – contact info on
    flob-a-dob *


    *google ‘flower pot men’ for explanation 😉

  24. Ben Bullet says

    Ha ha, that’s a nice one! didn’t know about these guys. Sort of reminds me of Pat&Mat, the crazy czech handymen…
    Thanks, sent you a pm.
    see you, dandelion

  25. says

    75 hp 1070cc V twin.

    Lets make it do 150mph and take it to BONNEVILLE like Rollie Free and his Vincent!
    Anyone of you guys willing to ride in your undies :-)

    Seriously, joy to the world.
    This has given me the much needed boost to keep going at it on the new cases, after months of sleepless tossing and turning.

  26. Jim says

    This is what motorcycling should be all about — start with a good, reliable but slowish bike, put a lot of thought and talent into it, and you have a good, reliable and fastish bike that’s fun to ride and won’t embarrass you and still run past the gas station. I’ve always liked the R-E for its ’50s vibe; and with this kit it’s a great runner as well. I agree that the EFI machine also deserves this treatment. Maybe the factory will take notice. A 45-hp thumper, anyone? Control the vibration and it would be a serious runner, right up there in Goldie territory without the drawbacks.

  27. Skizick says

    That brings me to it’s weak point. That creepy lookin wing nut atop the air cleaner. A slightly upgraded knurled knob would better befit such engineering expartise.

  28. Anand says

    hi, Ive got a 500cc 2005dec royal enfield, i stay in Bangalore, is there any body in Bangalore who can fit this kit, i.e., if can b fit in 500cc,,i wud like to c how powerful my bullet can b cranked upto..

  29. Geirskogul says

    Comment 40! This is one of the most popular topics on the Kneeslider.

    Maybe I should pony up and donate an engine to Tom (, then maybe he’ll start up on that engine!

  30. Gerry says

    Would just like to say that I have been picking Tom’s brain on the Royal Enfield forum for quite some time. There is quite a large following there not just because Tom’s articles are so informative but mainly because of accessibility to ask forum members advice. Tom has never steered me wrong and has always been wiling to share his experience with others. I for one have taken blatant advantage as I am mechanically “inclined” and not genius. With added info, tools, enthusiasm, and friends of like obsessive compulsive interests some seriously fun things can occur. Thank you to you also Mr. Crowe for this wonderful article. I’m happy to see Chumma, Tom and Aniket’s work recognized.
    Gerry, aka 500KsGerry

  31. Anant Krishna says

    Hi Tom !

    Is it possible for you to export this kit to India ? Complete with crank, carb head and big end bearings ? As it wont be really convenient to ship individual heads and crank to you.
    I can assure there are a lot of royal enfield owners here itching to get their hands on something like this !
    – Anant

  32. says

    Hi Anant,
    My partner Chumma is coming to Bangalore in January, and is going to start the preliminaries toward getting the Fireball 535 into India, for everybody there who would like it.

    Our longer term plan is to have good distribution of our items inside India.
    And we are also working with our friend Aniket Vardhan to bring the Musket V-Twin to India too.
    We recognize the demand from India for this stuff, and we will do our best to serve it as well as we can.
    I think that this may well be the “rennaissance” for the Bullet, and open up a whole new world of Fireball lovers, and Musket V-Twin lovers.
    When all is said and done, we will have the Fireball 535 single, a 700 V-Twin Musket, and a 1070 V-Twin Musket. And if we can get them all into India as kits to convert existing bikes, then they will all be available there.

    Exciting times!

  33. Anant Krishna says

    Hey Tom,
    Thats great news ! Hope the wait wont be for too long !
    If you are planing to have a distribution chain through authorized RE workshops/ spare outlets. One thing you will have to keep in mind that they are trustful and honor the agreement by not copying your design.
    Also not every mech here has the talent here to get such performance kits running right. Most of them can hardly get an enfield to run like stock! I could help you with a list of talented individuals or workshops who can get this kit running right.
    Exiting times indeed !
    – Anant

  34. Rowan Brecknell says

    I bought a Bullet 500 a month ago and I have 2 cars and I rarely drived them. I love the Bullet so much. Every-one here has Harley Davidsons and a different rumble turns heads. I’m wrapped in the old style with modern parts availability.


  35. says

    i think royal Enfield is one of best bike in the world because its onroad & offroad…too beter then the bike i have 1982 modal royal enfield it was to gud on that time …..

  36. Steve says

    Hi guys.
    I am totally in love with your work, well done!
    I’m currently running a Hitchcocks 612 and am curious as to what you think about using one of your heads and cams on my long stroke bottom end?? Still utilising my Hitchcocks piston also or would your piston be a better bet?
    I’d love a bit more power out of my bike without losing its road manners, hope using your parts with give me this.


  37. Blood Nut says

    Love the Work on this bike, just goes to show some boys still love their toys, and that’s the way it should be! (:{~

  38. Jesuraj. S says

    Hello Tom & Chumma’s Team Members,

    I like and admire your powerful bullet Ace Fireball 535.
    I came to know about you via musket v twin creator Aniket Vardhan.

    I want to know about your performance parts dealers in India. Please give the contact details.

    Best Wishes,

    Jesuraj. S

    Aniket VardhanTom Lyons

  39. says

    Hi Guys,
    Sorry for not checking back in here for a while. Been busy!

    To answer the questions:

    Steve, the ACE parts work very well with the long stroke 612 kit, and it is already in use by some people. They report excellent results, and one said, “I’m now a happy camper!”.

    Cris, we currently have two locations. One in NJ, and one in TN. There is a facility which we can use part time in California by special arrangement. But we can also work by mail/UPS shipping, and many people who build their own engines at home with our parts use this option. Please use our contact info, and/or see our website forum.

    Jesuraj. S, We have plans to come to India, but have not been able to get away from here to do that yet. We would like to do that soon. We expect that our first meetings will be in Bangalore.

    Thanks to everyone for your kind comments.

  40. edmund miller says

    Dear Tom,

    I have searched searched and searched for your email address and would love it!



  41. vikram says

    m frm mumbai.
    need to knw how much will the kit cost me in mumbai..and where do i get it installed..
    i have a 4s electra.
    need to knw all the kinds of modifications possible.