Another Question for sanborn

[340duster1]

Thanks for all the help so far as far as connecting rods. Next question is what should be the minimum clearance from the piston to the head? Do you risk detonation around the outer edge of the piston if this is too tight? I experienced this on a two stroke snowmobile engine. Also what should I use for a minimum skirt/underside of pin boss to couterweight clearance?

I plan to use the closed chamber magnum heads in a 360 magnum engine. I plan to use standard bore 340 pistons (bore block to .040 o/s). The piston will need to be worked on both the skirt side and the top. We are still considering a 327 piston as well.

Thanks again...

[sanborn]

For what you are probably using(rods, pistons, etc.) I would use .035" piston to head clearance WITH head gasket. Said another way, if you use the Fel Pro type sandwich type gasket, If you remove the gasket between the block/heads and everything turns over OK, then you will have .035" clearance when you install the gasket.

Why so much clearance? Several factors; piston rock, piston growth under heat, rod stretch(yes, it does stretch slightly under high RPM)

You can use less clearance than that; but you need to use tight cylinder wall clearances, super duty race pistons and serious race rods. Some Nascar Engines use as little as .020" clearance for qualifying (in the old days) and you can get by with that on our Super Late Model dirt engines. We actually use about .028" on our engines-but we are about at the limit.

How much piston skirt to crankshaft counterweight clearance? In theory you only need enough to clear(plus piston growth, rock, etc.). But, I like a little more than that so the piston skirt doesn't wipe oil off the crankshaft counterweight. I would want at least .100" clearance but even there you will probably get some oil wiped on the skirt. A crankshaft oil scraper really helps here and if you fabricate one, you don't need any more than .030" clearance between the scraper and crank.

Do you plan to use stock 340 pistons? If you are, be careful to maintain a minimum piston head thickness of .200"(and that is pushing it close), .250" would be better.

[340duster1]

Thanks again for the detailed info. I plan to use a stock 72-73 340 piston.. the low compression one. I plan to do a mock up in the next month to see how much we will have to cut the piston and what the comp ratio will be. If we have to go under .200", I will look at a 327 piston as we discussed earlier.

Where should I put the crank scraper and what is the best method of attachment? Would I still use the factory windage tray?