Different piston sizes?

All Models and Years
WinginEngineer
Posts: 150
Joined: Sat Sep 03, 2016 3:31 pm

Different piston sizes?

Post by WinginEngineer »

So I have found something (else) odd between my two engines as I prepare for reassembly: they have two different sizes of piston.

Both engines are claimed to be '62, although the rear cover on one is from a late model (determined from an earlier post and the extensive knowledge of this forum) and one block (the one with the late cover) came from an EM with a manual and the other from an EM with a PG (according to the stampings). Both cars have a PG btw, so clearly the manual block with the LM cover is not at all original.

The heads are identical with the exception of staked valve seats in one of them (the PG block).

The rods too are identical.

The pistons, however, are very different. Same diameter, but the pistons out of the manual block are at least 1/4" longer than those out of the other. And it's not just height. The ring groves in the shorter pistons are considerably smaller on #2 and #3 groves, #1 is only slightly smaller. Stranger still, the shorter pistons seem awful tight on the wrist pins and have stampings on the pistons themselves left of the wrist pins that read "8", "15", "4", "8", and "15". There are additions stampings on top of the pistons that are all "7". The stampings are on the top of the pistons about 3/8" tall.

What the bleep am I looking at here?

The new rings I got only fit in the taller pistons, which is good cuz those are the set that did NOT spin the bearings, but the tops of those pistons have very clear indicators of impact with the valves. The PO said the valve guides were sticking, which is why he tore it down, but now I'm nervous (wouldn't be the first misinformation I've discovered).

So masters of corvair knowlegde, hit me! What is the deal here?
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
66vairguy
Posts: 4531
Joined: Tue Dec 21, 2010 3:44 pm

Re: Different piston sizes?

Post by 66vairguy »

Are both types of pistons the same distance from wrist pin center to the top? If so only the skirts are longer.

Over the years a lot of companies have made Corvair pistons.

Typically valves don't stick in the guides unless old "bad" gasoline was run through the engine. Reported on forum a few times - "I started the engine with gasoline that was in the tank (car sat for many years) and suddenly the engine stopped - valves seized in guides!"
WinginEngineer
Posts: 150
Joined: Sat Sep 03, 2016 3:31 pm

Re: Different piston sizes?

Post by WinginEngineer »

The rods are the same size. If I line up the rod bearings and wrist pins the skirts match perfectly. The height difference is in the tops.
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
66vairguy
Posts: 4531
Joined: Tue Dec 21, 2010 3:44 pm

Re: Different piston sizes?

Post by 66vairguy »

OHHH - You need to dig out the books. Off the top of my head ---- When the bigger (longer stroke) engine came out (64) they used the same rod length (but the rods were revised) so the longer stroke required less piston top to wrist pin distance. You need to investigate your block numbers. However folks have modified early blocks to accept the longer later crankshaft. It could be that someone just installed LM pistons in an EM engine, but the compression would be very low.

Nothing surprises me after some of the kludge work I've seen folks do to engines.
WinginEngineer
Posts: 150
Joined: Sat Sep 03, 2016 3:31 pm

Re: Different piston sizes?

Post by WinginEngineer »

Also discovered the smaller piston/rod combos weigh about 25 grams MORE than the larger ones. I'll measure the crank and see if the stroke is longer. Doesn't really matter as that crank is a paper weight, but at least I would have an answer to my puzzle.
Kevin - Phoenix/Mesa, AZ
1962 Corvair 700
66vairguy
Posts: 4531
Joined: Tue Dec 21, 2010 3:44 pm

Re: Different piston sizes?

Post by 66vairguy »

I recall there were three different rods. The EM rod, the EM Turbo rod (heavier), the LM rod (thicker, but thinner end caps for clearance). All the same wrist pin to crank bore center distance. So yes their weight varies and using the wrong one for a specific application can lead to a disaster.
Scott V
Posts: 707
Joined: Wed Jun 30, 2010 2:31 am

Re: Different piston sizes?

Post by Scott V »

WinginEngineer wrote: Fri Jun 23, 2017 11:02 pm So I have found something (else) odd between my two engines as I prepare for reassembly: they have two different sizes of piston.

The rods too are identical.

The pistons, however, are very different. Same diameter, but the pistons out of the manual block are at least 1/4" longer than those out of the other. And it's not just height. The ring groves in the shorter pistons are considerably smaller on #2 and #3 groves, #1 is only slightly smaller.

What the bleep am I looking at here?
the shorter pistons w/the smaller ring grooves go to a 64-9 engine. the other pistons that are taller & have the fatter grooves are 60-3 engines.

-Scott V.
Post Reply

Return to “Ask your Mechanical Questions here”