AboutBill Silver Expertise Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.
Experience I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.
Organizations VJMC (Vintage Japanese Motorcycle Club) of North America
Publications VJMC newsletter, as editor for two years and as contributing editor currently.
Question I am learning so be patient. I acquired a couple of ct70 engines. One has a standard piston and the other has a 1.00mm. I have not measured the piston clearance yet. Should I put a 0.25 oversize in the standard if the clearance is not good? Do you always start out and work up with piston sizes? Does the size affect power and maybe that is why they used a 1.00.Thanks
Answer Shirley! Nice to have you back... :>)
Honda makes pistons in 5 sizes.
STD.
.25
.50
.75
1.00
The come from the factory with STD pistons. The .25 increments are fractions of a millimeter, which is about .040" of an inch (1mm).
As engines wear or have piston damage from seizures or whatever, then you have the option of boring the cylinder to the next size that will make a clean straight cylinder again. Heavily worn/seized pistons may require going from STD to .50 to clean up the wear and damage. A machinist has to check the condition of the cylinder, measure the taper, roundness and ridge at the top to determine how much the cylinder needs to be bored to clean it up. If it won't clean up at 1.00mm oversize, then you get to buy a new cylinder and start with STD pistons again.
Pistons don't wear evenly in the cylinders, so you can't just jam a .25 oversized piston in a worn STD bore. There are too many dimensions to consider, as mentioned above.
Boring from STD to 1.00mm on a CT70 wouldn't really have any noticeable effect on power and will probably raise the actual cc size by 1 or 2, at most.
