Build 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.

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.

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.

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.

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

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.

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, to reangle 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 reangled 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.

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.

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

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.

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.

Be sure to check the valve timing during this stage too- this needs to be accurate, and the formula I use was provided by the Bladon Bros, and is called "balancing the peaks".

peak duration= inlet open BTC+ inlet closing ABC +180 divided by 2 minus (inlet opening) = inlet peak

peak duration= ex open BBC+ ex close ATC + 180 divided by 2 minus (ex closing) = exhaust peak

note: for an N6 cam the peaks should be in the 102 to 106 degree range.

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.


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..

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...

Flashback Fabrications 604-888-8785email
PO Box 401Lynden WA98264

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