What year did they change the 305 pistons?

[Fletcha221]

Hey all bored and honed my dream 305 to .75 over. I purchased NOS .75 pistons. However, the rings I purchased (Part number 13040-266-000) are too thick for the piston. Apparently these rings are for the earlier 305's. My Dream is a 1965. I have ordered rings that should fit. When did they change the damn pistons???? Why??

[Atlas]

You have stated a very good question. I purchased a set of the 266 thick rings also and haven't found a 305 yet that could use them. I have a 60,61,63 and a 66 dream engines +one super hawk engine and those thick ring didn't fit any of them.

[conbs]

This another instance where the Honda Parts Manuals make life difficult for us Dreamers. The short answer is that ALL 305 Dream pistons (they are different from CB/CL pistons) are "266" pistons. (CB/CL pistons can be used, but I believe the dome is different and they will lower the compression when used with a CA head.)

My '64, '66 and '68 Parts Manuals show 2 numbers for 305 Dream pistons. You have to look at the last 3 digits or "classification number" of the piston to tell the difference:

13101-266-010 which is superseded and show no engine numbers for application
13101-266-020 for early C77E up to C77E-210200 ('62) and ALL CA77E. The "010" are the early and are 1.77mm thick and the "020" are the late at 1.42mm.
"266" rings go with 131XX-266-010 pistons (5 bore sizes).
"275" rings go with 131XX-266-020 pistons (5 bore sizes).

If you have a CA77/78 (product code "267" not "266") it apparently came with "020" pistons and CB/"275" rings. Unless you somehow end up with usable 010 pistons that are within specs there will never be a reason to buy "266" rings.