920 Race Engine Teardown

Those cast piston, i think it is surprising and interesting that they could whitstand that thin topland until you used nitros, Ken.

Even if they had been forged it is not too thick a topland i think, maybe a good squishband helps?

I have a book by Smokey Yunick with some very interesting stuff about ring packages and ringland, things have progressed since then but obviously he was ahead of his time so the information is still relevant.

I

SteveA said:

Alan, trigger it from the crank....

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Because I'm using the old Norton racing clutch with the TTI box the mainshaft is intended for the commando clutch, so it is 6mm too long. I use a Jawa speedway sprocket floating on a splined carrier (fitted onto the taper) and a single row primary chain - all of which makes it difficult to get anything in behind it. I think even with a belt drive it would not be easy. I've got hold of a couple of hall effect senders, one of which I can adapt to work off the cam. I think I still have a problem. My friend fitted a Nippon Denso unit to a Ducati 750SS which worked off the crank. He made about 3 rotors before he got one to work properly - the correct shape is very important. The Harley Dyna 2000 unit works off the cam, however is too big and a bit expensive. The Nippon Denso unit is very cheap and has quite a good advance curve. I think I will end up waiting until I can afford a Power Arc unit or.... ?

acotrel said:

SteveA said:

Alan, trigger it from the crank....

Click to expand...

Because I'm using the old Norton racing clutch with the TTI box the mainshaft is intended for the commando clutch, so it is 6mm too long. I use a Jawa speedway sprocket floating on a splined carrier (fitted onto the taper) and a single row primary chain - all of which makes it difficult to get anything in behind it. I think even with a belt drive it would not be easy. I've got hold of a couple of hall effect senders, one of which I can adapt to work off the cam. I think I still have a problem. My friend fitted a Nippon Denso unit to a Ducati 750SS which worked off the crank. He made about 3 rotors before he got one to work properly - the correct shape is very important. The Harley Dyna 2000 unit works off the cam, however is too big and a bit expensive. The Nippon Denso unit is very cheap and has quite a good advance curve. I think I will end up waiting until I can afford a Power Arc unit or.... ?

Click to expand...

We will also get shot for thread hijacking, but Ken may also chime in with how it is done, but you are working on the wrong side of the pulley/sprocket....Steve Maney sells a crank triggered Boyer that sits on a plate mounted outboard of the pulley.....Norman White has a better mechanical installation, but its the same location (ish) outboard. Of course if you fall off you will wipe it off....but you won't be doing that will you

billet said:

Those cast piston, i think it is surprising and interesting that they could whitstand that thin topland until you used nitros, Ken.

Even if they had been forged it is not too thick a topland i think, maybe a good squishband helps?

I have a book by Smokey Yunick with some very interesting stuff about ring packages and ringland, things have progressed since then but obviously he was ahead of his time so the information is still relevant.

I

Click to expand...

+1 on Smokey's books. I've got them all, and they are a great resource. They are mostly based on experience with water cooled V8 pushrod engines, but some of the info is still applicable to our old air cooled twins. I looked at his recommendations when I originally modified these pistons. He said that in their experience, a .125" top land allowed enough heat to soften it, and it would squeeze down and capture the ring, but at .150" it would run on the track all day with no problems. That's why I thought I'd be OK with .145" to land, and it did work well for a lot of racing miles. In otherwise equivalent engines, the piston will run a little hotter in an air cooled engine than with water cooling, so I might have been closer to the limit than Smokey's data would indicate. I'll be running a wider top land with the forged pistons for future nitrous engines, at least .200", and probably closer to .275". I think I might also use ceramic heat shield coatings for combustion chamber and piston top.

Ken

Before i die i want to press Peel to max and for over mile long runs will spray water on fins. First have to get ahead of major let down$ and get Peel engine home.

I have read posts here about cylinderhead drilled to clear the through bolt heads and adding two more through bolt in the back of the cylinder.
Then plug the original counter bores with a slight interference i think and drill it out for the 3/8 bolt itself.
I`ts an solution i like but i have an idea to take it a bit further.
The two trough original bolts at the back are sitting closer to the cylinder wall by 1,5mm, not good, why not make a pinch bolt and get it all the way thru cylinder head, then countersink from above the cylinder head for a bolt?
It would be nice to replace the four other through bolts with pinch bolts thru the cylinder heads if it was possible, maybe at the back with theads on the underside of the cylinder head with some kind of connecting bolt?(left/right threaded)
I think it would give a more even and robust clamping and not distort the cylinder bore so much.
What i have in mind is of course the 920 cylinder.

Good or bad idea?

Sten