My '72 750 torn down again

It is shameful to admit but I advance my timing "just a little bit" after setting it with a strobe at 28 degrees. It felt just a bit "down on power" and one guy had said he got 5 extra HP on a dyno by advancing his a few degrees. I did mine by hand just a smidgeon (because I didn't have a helper around.) and felt just a bit more power and heard no knock. Then I forgot to check to see how far I had actually advanced it and took off for a long trip up to Kidron OH from Charlotte NC. It lasted the first couple hundred miles and then suddenly sounded and felt different. An off idle knock that sounded more like a clang. Well, I didn't know what was causing it so I just drove it home via Columbus OH and Lexington KY and Asheville NC. Quite a long ride and in that huge rain storm the entire way. Then I drove it to Barber and back. Comstock heard it down in Alabama and advised me to tear it down. The Lucas Rita gave up the ghost down at Barber so I have no idea how bad I screwed up the timing. Walridge sold me a Boyer for $130 at the Vintage Festival and I made it home.

Now it is torn down and the JCC pistons are scored from top to bottom. Plenty of people have told me that advancing the timing too much will do that. I had fit the pistons to .0045 so the original clearance was not the problem.

Now I have a set of .040 over pistons made by a defunct Japanese ring manufacturing company. (They still make rings but no longer make pistons.) They are cam ground way more than JCC (Emgo) pistons. The JCC pistons have about .002 taper in the skirt from the bottom to the oil ring. These have .007. The JCC pistons only have about .006 difference from the bottom of the skirt to the side perpendicular at the wrist pin. These have .046 difference there. The JCC only taper .012 from skirt to top diameter up by the rings. These taper .014 from skirt to diameter up at the rings.

In my post about "soft seize" I had fit a previous set of JCC pistons too tight. (.0025) 100% of the contact patch that caused the "soft seize" was out by the ends of the wrist pins. These new pistons have that material ground away by .046 inches. The taper in the skirt itself is greater by .005" so I am thinking that these particular pistons need to be fit tighter than the recommended .0045 for JCC pistons.

Any knowledgeable info out there?
Cheers, Dan.

motorson said:

The taper in the skirt itself is greater by .005" so I am thinking that these particular pistons need to be fit tighter than the recommended .0045 for JCC pistons.
.

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Ay yi yi.
You going for the triple set of gunged pistons ??
Fit them with EXTRA clearance if there is any doubt.

Finding what actual clearance is recommended would be a good move too,
guessing just doesn't cut it when they likely have done the homework and can suggest precisely what is needed for good long life.

I don't think you will be able to gauge the ideal bore fit without knowing the silicon content of the pistons.

Why should you bother with unknown exotic pistons? Standard Hepolite pistons are widely available again at very reasonable price!

nortonspeed said:

Why should you bother with unknown exotic pistons? Standard Hepolite pistons are widely available again at very reasonable price!

Click to expand...

Manufactured by.....JCC.

All good points above. I have had cam-ground pistons in Hondas that lasted year after year of hard running that were fit to .0015 on a 3" piston which is what made me make the mistake on my first rebuild fitting my Norton at .0025.

I realize that the cast cylinder does not expand like an aluminum finned cylinder does. But I also noticed that my seizure was out at the wrist pins where a cam ground piston would have no material to even interfere.

These may be exotic pistons but I wish at this point in Norton history it was easier to get a good "exotic" cast piston with accurate clearance specs to go on.

I am guessing that the JS forged pistons are nicely cam ground and require fitting to .005. That totally makes sense for a forged piston.

I'm still thinking on this and appreciate all the input.

Dan.

Seizure by the wrist pins suggests 4 point seizure from piston overheating from too advanced timing, so the evidence points to operator error not piston maker error. Why compound it by guessing the exotic pistons correct operating clearances and instead fit known pistons with the piston makers tolerances.

The comment about advanced timing causing vertical scratch marks on the piston and cylinder is very interesting...is it because of overheating or something else?

It was operator error for sure. The over heating is typically of the pistons only in the excessive advance situation since they are under the heat of combustion too long and the pressure gets higher as well.

Rohan said:

motorson said:

The taper in the skirt itself is greater by .005" so I am thinking that these particular pistons need to be fit tighter than the recommended .0045 for JCC pistons.
.

Click to expand...

Ay yi yi.
You going for the triple set of gunged pistons ??
Fit them with EXTRA clearance if there is any doubt.

Finding what actual clearance is recommended would be a good move too,
guessing just doesn't cut it when they likely have done the homework and can suggest precisely what is needed for good long life.

Click to expand...

Hi.
What is a good clearance for the Hepolite pistons?
0,10 mm is good?
Ciao
Piero

Piero,
The most knowledgeable guys on here are recommending .0045 inches. (.1143 mm) For those cast Hepolite (JCC) pistons.
Cheers, Dan.

motorson said:

Piero,
The most knowledgeable guys on here are recommending .0045 inches. (.1143 mm) For those cast Hepolite (JCC) pistons.
Cheers, Dan.

Click to expand...

Hi Dan,
I have always rebored the new Hepolite to .09 /.10..
The next i will go to .11
Ciao
Piero

Piero,
It sounds like you have had success at the clearances you used. .1mm is .0039 inches so that is almost .004. That is the lower end of what the same experts recommend. It looks like you are good.
Dan.

motorson said:

Piero,
It sounds like you have had success at the clearances you used. .1mm is .0039 inches so that is almost .004. That is the lower end of what the same experts recommend. It looks like you are good.
Dan.

Click to expand...

Sorry,
0,09mm = 9 centesimi;
0,10mm = 1 decimi.
Ciao
Piero