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How to Build A Racer

My Inspiration

A lot of my inspiration to build an Aermacchi Racer came from the excellent article by Chris Kerber entitled, “Racing the Aermacchi Harley Davidson Sprint“. Chris has raced Aermacchis for almost 2 decades, and is extremely knowledgeable about them. He’s recently updated the information, and the best thing about it is that it’s FREE! Just contact Leslie at Moto Italia USA, phone 1 707 763 1982.

I’m a member of the Aermacchi E-group, and some time ago, there was a lot of discussion about “how to get the best bang for your buck”, regarding building a 350 race motor. The general consensus was to use a 71/72 bottom end, with a 73/74 top end and 5-speed transmission. This means hunting down 2 motors (not impossible), to make one good stocker. I raced my first season with a stock motor, and it would pull an honest 100mph on the straights.

A stock frame can be used, if gusseted. I suggest not using anything later than a 1970; they got too long in the wheelbase and heavy after that. Thus, to build a racer, it’s best to locate (2) engines and an earlier frame, as a starting point. Once you have these three things, approx. $2000 of parts (rims, tires, tank, seat, etc.), plus a lot of hard work will get you an Aermacchi vintage roadracer that is fun to ride, handles well, and makes you feel like a kid all over again!

If you have an Aermacchi racer and want more HP, there is a lot of information available to that end. Using engine parts from Germany, I built my engine to specs provided by Chris and Paul, the Bladon Bros, in England.  Chris and Paul  were very generous in providing me with ALL the specs needed to build a racing motor, and I attribute a lot of my success to how well the engine pulls, with no flat spots. They have given me the OK to pass those specs along:

  • 39mm inlet valve
  • 34mm exhaust valve
  • Carillo connecting rod, 135mm
  • N6 cam
  • 1972 crankcases>best/strongest webbing
  • optimum inlet tract and carb size: 35mm
  • inlet manifold length: 100mm (measured along      centreline)
  • sparkplug: NGK B10EV
  • compression ratio: 11.2:1
  • piston skirt clearance: .004″ – .005″
  • timing advance: 32-35 degrees BTDC
  • ignition system: battery/total loss w/ Lucas Rita electronic modified for Aermacchi; 12 degrees of advance.
  • carb: 34mm Spanish Amal bored to 35mm
  • main jet 220/230
  • pilot 25
  • needle jet 105
  • needle 2C3 using bottom notch
  • 2A1 using top notch
  • slide #3


Building a 392


A lot of the credit for this motor must go to Rudolf Jungjohann in Germany. He’s built a few, at least, and answered many of my questions. Thank you, Rudolf.

You’ll need 73/74 cases. The larger left side flywheel bearing is necessary. The smaller bearing won’t stand up to the additional loads. I thought about using the earlier flywheels and shrinking a hardened sleeve over the LS, but was told that’s not a good idea, so I modified the 73/74 shafts to resemble the earlier flywheels. The flywheels are case hardened to probably .060- .080″. Use a carbide insert and go slow. The left side was going fine until I encountered the oil hole, part way up the new taper. The oil hole is hardened too, and I couldn’t drill in any further, which was a problem because I needed an 8mm thread to hold on my ignition reluctor. I called my friend Dan, who has considerable experience with very hard metals. He managed to get a 10mm thread happening, but not too deep because the tap was protesting so loudly! 10mm was larger than I had planned, so the taper had to be recut, and later, after the flywheels were all assembled and the taper no longer needed, I plugged the hole with a 10mm thread and loctite, then put the entire flywheel assembly back in the lathe, and drilled and tapped the end to 6mm…… much lighter! A 6mm allen screw holds on my ignition reluctor and degree wheel adapter.

Machining the flywheel shaft in the lathe using a steady rest

Machining the flywheel shaft in the lathe using a steady rest.

Dan also set up the flywheels in a CNC mill, and bored the crankpin holes from 30mm to almost 35mm, the size needed for the Yamaha crankpin. Here are the part #s if you’re going this route.

SRX rod IVJ116510100
bearing 93310635B500
crankpin IVJ116810000
thrust washers 90201356K100 (you need 3 of these)

Next, the flywheels went off to the grinder, who surface ground the inner “cheek” of each flywheel to be parallel with the outside face, then flipped it around, mounted it to a magnetic chuck, dialled it up, and internally ground the bores. To get the bores equidistant from the mainshaft centres, he measured the flywheel OD’s, took into account one was .0007″ larger, then used a ball mike to measure from the crankpin bore to the OD of the flywheel, compensating with a little tap here and there.  It worked.  Make sense?

Stock flywheel on top, modified on bottom with SRX rod, hardened sleeve for the LS o-ring to run on, reluctor and degree wheel mount on far left. With a 35mm straight crankpin, this assembly is MUCH stronger than stock.

Stock flywheel on top, modified on bottom with SRX rod, hardened sleeve for the LS o-ring to run on, reluctor and degree wheel mount on far left. With a 35mm straight crankpin, this assembly is MUCH stronger than stock.

The Yamaha crankpin is too long, so it got shortened to 2.380 “.  I used a right angle grinder with a thin cutoff wheel for the rough cuts. You want to get the middle oil hole as close to the middle as possible. Exact centre isn’t possible, you just have to live with it. Then it was back to the grinding shop to have the ends made square, to the final length, plus a small 3 degree chamfer for each leading edge to aid flywheel assembly. Don’t forget the press lube, it really helps. (thanks Ron L.) Next is to drill a 4mm oil hole to match the flywheel oil hole. A $30 carbide drill mounted in the mill with a steady speed and good cutting oil took care of that. A couple of custom 6061 endplugs for the crankpin finishes the job. Rudolf has been using one of these SRX pin / rod combos in a race motor for a decade and has never even had it apart. I can only trust a standard crankpin for 2 seasons in a race motor.  If you think about it, it makes sense. The Yamaha rod and pin was designed for a 600 motor that may get hopped up, so a little 400 Aermacchi with a few mods doesn’t bother it much at all.

'74 left crankcase starting to get modified. The first insert has been glued in, and electric start hump removed.

’74 left crankcase starting to get modified. The first insert has been glued in, and electric start hump removed.

Back to the cases- you’ll need to remove the (4) cylinder studs, clamp it to an angle plate in the mill, dial it up, and bore out the “throat” to accept the larger liner. I wanted to use the earlier sidecases, so that meant a lot of modifying. I made aluminum inserts, glued them in, machined the face square, and tapped threads for the LS cover. I glued an insert over the oil transfer hole, and plugged the shifter shaft hole. I machined a cover for the LS bearing, and installed an oil seal. The cover is also the mount for my ignition pickup.. I shrunk a steel sleeve over the shaft and keyway (for the oil seal to run on), and filled the keyway with more epoxy. The area around the countershaft sprocket was milled away, and a chain guard fabricated and installed.


The bearing still needs an ignition plate and oil seal to cover it.

More mods… the bearing still needs an ignition plate and oil seal to cover it.

Final crankcase complete

Final version complete

The stock Yamaha rod is too wide to fit between the flywheels, so it was milled narrower, ground smooth to relieve stresses and save weight ( 386.4g > 343.6g, then adding a bushing= 367.8 grams), then polished. Shot peening is a good idea, but the local company wanted $50 for one rod, which is the same price for (4), so I passed on that. The Yamaha assembly comes with (2) 1mm spacers for either side of the big end. You really need (2) more .5mm spacers to get the sideplay correct and the rod in the middle, but they’re not available, so get (1) more 1mm spacer, and choose which side to put it on. Remember the middle oil hole in the crankpin was slightly off centre? I put the rod to the same side as the oil hole. Machine up a phosphor bronze bushing for the small end, press it in, drill (2) oil holes, and hone for .0005″ clearance on the wrist pin. You can now do final assembly on your flywheels .

Machining the SRX rod to a thinner profile.

Machining the SRX rod to a thinner profile.

Lightening the small end with a ball end mill. The alloy plugs in the big end are used to hold the rod in the vice for grinding / sanding / polishing

Lightening the small end with a ball end mill. The alloy plugs in the big end are used to hold the rod in the vice for grinding / sanding / polishing.

I was concerned about the balance factor, as I had just added about 100 grams to the crankpin, rod, and piston.  I talked to Rudolf about this. He said balance factor for an Aermacchi is all just theory. He said put it together and just use it. He talked about someone in Germany who went to great trouble to make a very beautiful, modern-style “pork chop” flywheel for an Aermacchi. It was very light, and promised more power! In reality, it vibrated terribly, and they tried every balance factor from 15% to 40%, and nothing worked. Finally, they went back to the stock flywheel, and it was a great improvement. Rudolf also spent 900 Euros on getting one of his flywheels balanced, and it wasn’t any better than before.

The right side cover won’t fit (as I discovered), because the bronze bushing (in the case) that slips over the end of the RS flywheel shaft needs to be bored out slightly- back to the mill with the boring head; I gave it .004 – .005″ clearance. The 74 RS shaft is SLIGHTLY larger than the 72 shaft. You can hardly see it with your eye…

The cylinder needs the old sleeve taken out, then bored for the new sleeve; it gets shrunk in, then bored for the new piston. Don’t forget the cylinder and sleeve will both be shortened to fit the new rod / piston combos.

Fixtures for holding the cylinder head to the rotary table,

Fixtures for holding the cylinder head to the rotary table, to re-angle the intake valve. The key to this is the 3/4″ brass bar with the 1/8″ steel rod that passes through it. The smaller end of the brass bar passes through the old valve guide hole, and the alloy endcap and 6mm allen screw hold it firmly against the valve seat. The 1/8″ rod is at the exact point of intersection of valve stem and rocker face, so the position of the rod must be in the exact middle of the rotary table. Now get the 3/4″ collet of the mill head to slide nicely over the brass bar, and you know it’s all lined up. Remove the brass bar, rotate the head one degree, secure everything, and bore the new valve guide hole.

The intake valve was re-angled one degree. I made some fixtures to hold the head in the mill, clamped to the side of my rotary table. It was designed to rotate around the end of the intake valve stem, so as not to interfere with the rocker / stem relationship. My calculations figured one degree would move the intake head about .080″. When I milled the one degree, it sure seemed huge, and I wished I had only changed it half a degree. But it turned out OK. After boring the new valve guide hole, you need to reface for the valve spring collar, and space (if needed), for the spring itself. The valve seat comes out now, and I did this by TIG-welding around the inside of the seat a few times with a stainless rod, then welding in a big washer. It taps out easily now… Next, the head is flipped over, and you re-centre to bore for the new valve seat. I machined the seat from 4340, heated the head, froze the seat, and hammered it in with a driver. .006 – .007″ is a good interference for a valve seat.

cylinder head mounted to the rotary table. Note how the 1/8" rod is lined up with the axis of the table

Here you can see the cylinder head mounted to the rotary table. Note how the 1/8″ rod is lined up with the axis of the table.

rotary table setup on the mill, 3/4" collet aligned and slid over the brass bar.

In this photo the rotary table is setup on the mill, and the 3/4″ collet is aligned and slid over the brass bar.

head on the rotary table to increase compression and accept the 4mm larger cylinder liner

A different setup… here the head is being milled on the rotary table to increase compression and accept the 4mm larger cylinder liner.

The head gets remounted a little differently on the rotary table, and is milled to raise the compression ratio and accept the larger cylinder liner. Now, trial mockups of the top end can begin. The Ducati piston needs to be cut down on the top about 1mm to get the dome into the combustion chamber more. Next, the cylinder and liner are shortened to give .050″ clearance when the piston is at TDC. The (4) cylinder studs need to be shortened. Sometimes you may need to machine up extra thick washers that are just the right thickness. This should be an easy job, but in my experience it hasn’t always been that way to get it right… To check for valve / piston clearance, I use carburettor springs in place of valve springs. With a larger and reangled intake valve, quite a bit had to come out of the piston. Figuring out WHERE to remove metal is a slow process. If not, the trip to the recycling bin is VERY FAST! Piston crown thickness was .190″, and I took the intake pocket down to .160″, plus I machined .045″ off the valve “margin”, which is thinning the valve head. This made it all work.

Cutaway of cylinder head to show relationship with un-machined Ducati piston.

Cutaway of cylinder head to show relationship with un-machined Ducati piston.

Material to be removed from the Ducati piston.. the intake valve pocket was also deepened .030".

Material to be removed from the Ducati piston.. the intake valve pocket was also deepened .030″.

Machining out the intake valve pocket with a single point flycutter mounted in a boring head.

Machining out the intake valve pocket with a single point flycutter mounted in a boring head.

Adjustable pushrod- simple but effective for determining length.

This is where I discovered the 73/74 RH shaft only has one keyway! You can still adjust the timing 5 degrees either way, but you need to move the pinion gear 1/3 of a turn, and if anybody else looks at it they’ll swear your timing is off a mile! Be sure to make a good drawing with corresponding notes for future reference. I have an adjustable pushrod that I use for figuring out correct pushrod length. It really does make the job easier. I put the stock pushrod in the lathe, mark how short it needs to be, then use a little parting tool to remove aluminum from around the steel end. When the end comes “free”, I then part the tube to length. I use loctite and an arbor press to reinstall the hardened end. Don’t press directly on the ends- I have (2) 6061 blocks drilled so the ends slip through and rest on the steel “shoulders”.

Here are some specs I use as clearance guidelines for the top end:

valve to valve (overlap) .030″ minimum
valve to piston .060″
valve clearance in piston pocket .030″


When you’re building a race motor, take your time, check EVERYTHING! Not checking one thing means it will probably come back and bite you. When race motors blow up, it can be very expensive, time consuming all over again, and demoralizing. Try not to go there.

Home of Flashback Fabrications.

Flashback Fabrications. Yes, we have snow year round in Canada . . .

I finally got the motor together and into the frame, and discovered just one more thing— the lower engine mounting castings are machined .280″ narrower than 71/72 cases—so I had to machine up (2) more 6061 spacers .140″ wider than the previous ones that I just had anodized.

Drain plug detail... to change the oil, only remove the 1/8" NPT plug-eliminate the stock "gush" of oil and corresponding cleanup.

Drain plug detail… to change the oil, only remove the 1/8″ NPT plug-eliminate the stock “gush” of oil and corresponding cleanup.


Out on the track, after sorting out a few miscellaneous bugs like a leaky rocker feed line and a dead battery, I discovered the bike was a rocket off the line, pulled well out of the corners, but lacked top end. Plus, it vibrated no more than when it was just a little 350.  It was time to put on my engine tuner hat . . .


Hints and tips from a few years spent at the track:
  1. Make a list of what you need to bring to the track. My list is on my computer; easy to up-date, easy to print when needed. It’s too easy to leave something at home. One friend left his race boots behind, and had to buy new ones at the track . . . NOT CHEAP!  Either that, or no race.
  2. Figure out what kind of race gas you need. I buy 110 leaded at $5/gal., then I mix it with regular unleaded, usually a 3:1 ratio. My compression ratio is 11.5:1 and I don’t need 110 octane for that. Higher octane only burns slower, reducing HP at some point.  My 3:1 ratio saves a bit of money too.
  3. Unlike Superbike, where a tire is lucky to last a race, I can get two seasons on mine. Some racers change tires each season regardless of wear, and you can sometimes get these tires for free! Right now I’m using the front tire from a 750 racer as my rear tire. Also use these tires on my street Guzzi.
  4. Racers are frugal in other ways too. I have a friend who uses (expensive) synthetic oil in his race bike. After a couple of races he drains it, but saves it to use in his truck! He figures the oil still has lots of life for his V8 or whatever.
  5. Dean turned me on to Honda “spray cleaner and polish”. Very good for cleaning helmet visors and windshields. The small can (5 oz.) has lasted me at least a couple of seasons.
  6. If you’re going to race in the rain, get some surgical latex gloves to put on under your race gloves. This will stop a lot of the dye from soaking into your hands. If you helmet doesn’t have the click-stops to open the visor slightly, figure out another way to hold your visor open 1/2″, otherwise you’ll probably fog-up.
  7. Don’t forget earplugs. Even at tracks that have a 105 dB limit, it’s not hard for me to get a headache, or to have my ears ring into the next day. I’m even considering custom made earplugs. I can’t imagine riding a MotoGP bike with the new 130 dB limit . . .
  8. Learning to race. If you’re new at this, or thinking about Vintage racing, it’s not about big balls and twisting the throttle more. Riding fast is about thinking and being sensitive to what the bike is doing. You learn to go faster in small increments. Riders who go fast “suddenly” usually have a big get-off, then go back and adopt the incremental approach. It’s good to have a goal, or “focus” for each practice session, or race. For example, a more consistent line in turn 2, or dialing in the braking point for turn 5.
  9. The number one goal is always to “Bring it Home”. I’ve seen too many first time racers crash, and never return. When you focus on your riding, the pressure to perform is lessened. When I started Vintage racing, I had absolutely no idea how I’d do. I didn’t want to be last, and just hoped I could be somewhere in the middle of the pack. Well, I was towards the back of the pack, and sometimes I was last. But I was always learning, and kept on improving my bike. My lap times slowly came down.
  10. If you’re having problems with a particular corner, talk to other riders that are faster than you. Learn about things like late-apexing. I learnt many corners by following the faster Honda 160 riders through them. I think a lot about my riding a lot and analyze many of the little details. On the track I try and translate all that into “flow”. The good racers are all very smooth, and consistent. Racing is all about confidence. Work to build that confidence; it’s a great feeling!


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List of Aermacchi Suppliers


Moto Italia

Leslie runs this shop in Petaluma, CA. Most parts, new and used, for most Aermacchis. Friendly service, reasonable prices.

Tel 707 763 1982


Charleston Custom Cycle

Don has a lot of inventory for Aermacchis, but also sells parts for other things, eg. Harley golf carts. Mostly NOS.

Tel 217 345 2577



Paul Hoover is a true expert on Aermacchis. He owns the Aermacchi name, sells service manuals and parts catalogs, installed a Carillo rod in the flywheel of my first race engine, and much more…


Kesco-Hoover / 603 East Carson Av / Cushing, OK 74023
Tel 918 225 1818


Ron Lancaster

He’s the dirt track guru in Illinois. Races them too. Probably overhauls more Sprint engines than anyone. Very knowledgeable.

Tel 815 438 7985


Custom Discount Parts

Name says it all. Plus service too, from expert mechanic, Steve Bentley, who also does restorations.

Tel 912 475 5896


Richard Chiles

Manufactures good quality armature pullers for Sprints. Also sells copies of the Sprint 250/350 manuals online.

8801 Ocean Springs Road, Ocean Springs, MS 39564
Tel 228 875 2104


R.C. Barker Engineering

Bob Barker might not have a single Aermacchi part, but he is the distributor for Dunlop Vintage racing tires. Give him a call.

2933 N.E. 20th Way, Gainsville, FL 32609
Tel 352 378 3715
Fax 352 371 2490




Bladon Brothers

Paul and Chris have been involved in the Aermacchi racing scene for years, and are very knowledgeable. Racing parts available.

Tel 011 44 1978 710 671
Fax 011 44 1978 710 853


Dick Linton

Flies passenger jets for his “real” job. In the meantime, however, has also racked up an enviable record of success in Isle of Man victories with others piloting his machines. Racing parts available.

Fax 011 441 483 268 081

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