Very low compression, what to look for?

deafanddiabetic · Post by **deafanddiabetic** » Fri Jul 04, 2014 7:08 pm

If it helps, the piston measures 52mm around, and I had 1.8mm top and middle rings before, and the 2.8mm oil ring fit just fine (minus breaking when I tried to slip it on).

I think 1.5mm rings will fit just fine, but all I can find on Ebay is people listing them as bored over .75 or 1.00 or the piston rings listed as 54mm size. There was no rattling of them in the cylinders when I checked, so I presume the actual pistons are in okay shape.

G-Man · Post by **G-Man** » Sat Jul 05, 2014 1:42 am

Standard CA72 pistons start at 54mm and then go up in sizes in 0.25mm increments. You need to either measure the pistons accurately or clean the tops gently and look for markings. Often the "oversize" will be marked. i.e. +0.25, +0.50 etc...

See my picture below. You can make out the 0.75 on the crown.

The rings you need will then be decided by the size / markings. Early ring sets had the thicker compression rings, later rings were thinner. You need to look out for CB72 rings with a later revision code to get the thinner rings.

You can see here that Dream (-259-) started to use CB72 (-268-) rings after sometime in '63

http://www.cmsnl.com/honda-c72_model743 ... ml#results

G

deafanddiabetic · Post by **deafanddiabetic** » Sat Jul 05, 2014 3:03 pm

That was actually really helpful, thank you. Well, I've now ordered TWO wrong ring sizes, hah. My pistons clearly show .50 on them. So I need to buy the .50 oversize rings, correct?

Any suggestions on how to get the bottom ring on without snapping it again?

G-Man · Post by **G-Man** » Sat Jul 05, 2014 4:43 pm

The normal technique is to use three thin strips of metal positioned around the piston and then you can slide the rings down over over the strips.

G

deafanddiabetic · Post by **deafanddiabetic** » Wed Jul 09, 2014 10:34 pm

So, I have the correct rings, the 268 part #. Measuring the rings and the booklet specs they seem like they should work. I got the oil rings on just fine (thanks for the metal strip method, it's PERFECT), but the middle black rings don't seem to fit down into the grooves on either piston. I remember when I took them out the first time that these specific rings were very tight in there too and didn't seem to expand or slide around (that's why I assumed they were stuck rings), and I know the top is the silver edged ring and it fits just fine in the top groove.

I have them facing upwards and in correct order. Are all three rings supposed to have some contraction and spinning ability or is that middle ring supposed to get in there tight?

Thanks again.

G-Man · Post by **G-Man** » Thu Jul 10, 2014 12:17 am

Break one of the old rings for that position and thehn use it as a tool to clear out the groove. There may be burnt oil in the groove.

G

deafanddiabetic · Post by **deafanddiabetic** » Sat Jul 12, 2014 3:43 am

Alright, got the rings on (numbers up), pistons in, clips into the pistons, arrows facing correctly, new gaskets, got the cylinders on, put the head on, got the cam chain together, put the valves back on after a light lapping with compound and a thorough clean up, then tested it with carb cleaner to see if they leaked through and no sign of it.

I set the valve clearances, put some oil in the engine and rotated it a few times.

Is there a way to check for compression now? The frame and all the parts are at the painter's right now. Can I hook up my gauge, set the engine on a stand and kick at it to see if I made at difference? I'd be devastated if I still got low numbers after all this work :(