[68grandprix]

I took apart my two Cl175 motors (one ran but smoked a ton and fouled the right plug, and the other motor was a spare), and one has what I think are standard pistons, and the other has larger pistons. Both say HONDA. The smaller pistons have the numbers 313 and 3 and the larger pistons say 313 7. Does this denote the oversize?
I need rings for one of the larger pistons(oil ring was missing a chunk that I found in the bottom of the engine). The good news is that the bore wasn't harmed (somehow) and still shows a nice crosshatch from the rebuild. The bad news is that I took a piston from the other motor and installed it in this motor, which is now in the bike, before I realized they were different sizes. Luckily I found out before I completely installed the motor and ran it (I only ran it for about a second on the bench). At least I'm getting my mistakes out of the way on these little motors before I attempt the big bore and rebuild of my Cb750. Jimmy

 

[jbranson]

Jimmy,
Honda (Art) pistons should have a marking on the top of the piston if they are oversize....it will be .25, .50, 1.00 etc which denotes how many mm oversize they are...for instance .50 would be 1/2 mm. The markings are usually etched pretty lightly into the dome of the piston and would be hard to see unless the piston was cleaned very well. You could measure them with a set of calipers. Stock size is 52mm which would mic out to about 51.8 - 51.9mm perpendicular to the wrist pin on the piston skirt.

To do it right the cheapest possible way would be replacing both ring sets on the larger pistons...you can't get single rings. The cylinder bore would have to be in very good shape..no scoring, etc. The ring broke for some reason...usually those reasons also mess up the cylinder. You need to check piston to cylinderwall clearance...should be about .0025 to .003". Meaning the piston should be that much smaller than the cylinder (again, measure the piston as the skirt, perpendicular to the wrist pin). If the clearance is less than .002" you'll need to get the cylinder honed out properly. If the clearance is more than about .005" then you'll need a new set of oversize pistons and have the cylinders bored to match.

I might even have the pistons and rings you need in my collection. Invest in some measuring tools if you don't already have them. Cheapo $30 6" dial calipers are good enough for measuring the pistons, and a set of expanding bore gauges ("T" shaped things), $15 will do the trick if you use them carefully.

Just ask if you have any questions....I know those engines inside and out.
JohnnyB

 

[68grandprix]

The bigger pistons are the 4th oversize (I couldn't find any markings, so I used a friend's digital caliper), or 1mm larger. Doesn't this give me 199ccs? The bike did seem pretty peppy, even smoking like a two stroke. I still haven't torn the engine back down to measure the bores. I'll post again when I do. Thanks, Jimmy.

 

[jbranson]

Jimmy,
4th oversize gives you 53mm pistons....about 181cc. You'd need about 56mm pistons to get 199cc. 56mm is the limit you can bore a 175 without going to larger cylinder liners and lots of machine work.

I'll look around and see if I have any parts in that size in case you need some.
JohnnyB

 

[68grandprix]

Yeah I was thinking of 4th oversize pistons for a four cylinder bike. I remembered that a 1mm overbore on my Cb750 gained around 25 ccs. I forgot I'd get less increase with two fewer cylinders. Thanks, Jimmy.

 

[jbranson]

Jimmy,
It also varies with bore size...meaning a 1mm increase on a 70mm bore gives a bigger displacement increase than a 1mm increase on a 50mm bore.
If you run windows I wrote a neat little program for calculating displacement and a few other things if you know bore and stroke. I can email it in a zip file if you ever need it.
JohnnyB