harpo57's engine fixing.

Ok, it was suggested by a member to start a new thread dedicated to the subject to keep the conversation orginized. So, I had a quick look at the damage on the basket case engine. The hole in the piston may not be burned but broken by impact. There is a crack next to the hole. Perhaps by someone striking the piston in an attempt to unsieze it. ( or burned and beat on...) So... a metal piece in crankcase somewhere maybe. My thought is to find a replacement standard size piston, (a good used?) new rings, gaskets etc. So what piston am I looking for? Its a '79. I've been researching, have seen part #'s for early and later years.. Are they truly different thru the years? Ie; weight, height, shape etc... ? Also learning to be careful of what gasket set to get for the different years. Thank you.

Yes, they are different. Early years used a larger diameter wrist pin. You would want a "447" piston, which came along in '74. They're all the same after that. You'll want to try and find the size number stamped on top of the old piston (if the hole didn't "remove" it, lol) and try to find one of a similar size.

Ok, Iv'e been searching the old threads and learning all about the 74.xxx sizing and the oversizes and the cyl. sizing.... good grief.... lol.

5twins said:

Yes, they are different. Early years used a larger diameter wrist pin. You would want a "447" piston, which came along in '74. They're all the same after that. You'll want to try and find the size number stamped on top of the old piston (if the hole didn't "remove" it, lol) and try to find one of a similar size.

Click to expand...

Yup, searched the threads, learning...

Hi folks! So I'm learning stuff here. Thank you. Todays query; pistons, where does a 74.954 size fall within the range of standard sizes? I understand that the sizes did vary and they were matched up and cyl.'s honed to fit. What is the range of standard size pistons? Thank you.

sleeve 75mm ID........ piston 74.954mm OD

harpo57 said:

...pistons, where does a 74.954 size fall within the range of standard sizes? I understand that the sizes did vary and they were matched up and cyl.'s honed to fit. What is the range of standard size pistons? Thank you.

Click to expand...

Excerpt from the 70-72 parts manual, shows (6) standard size pistons, (3) of the early high compression (p/n with -01 suffix), (3) of the later reduced dome (p/n with -02 suffix). Size increments of .95, .96, .97 mm. For your 74.954 piston, drop the final digit to get 74.95, which is the smallest offering.


Piston service bulletin showing the -01 and -02 suffixes.


I don't know if these size callouts existed for later models...

Thank you. That is the answer to my question. (.95x, .96x, .97x) If the method of identifying the sizes in the part # continued into the later years this info could be helpful.