Question about pistons/bores
Question about pistons/boresDo Honda twins need to have pistons replaced in matched pairs? Can a "std" piston/bore live next to a +.25 or even .50 oversize piston? Patient on the bench is only a 175, but I figure this board gets lots more looks than the "other twins" page.
Well, the fact that it wasn't applied to yet, has possibly something to do with the question.
The answer is no, obviously, and more correct, no, not a very wise thing to do. I will not answer the question "why not", maybe you need to collect some information what kind of part a piston is and what is does for a living. assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Let me give you a little nicer answer to a reasonable question but I will use the CB77 bore and stroke for calculation.
The displacement of a cylinder bored to standard bore is 3.0 * 3.0 * pi * 5.4 = 152.7 cc. If it is bored to 1 mm diameter (0.5 mm radius) oversize the displacement is 3.05 * 3.05 * pi * 5.4 = 157.8 cc. The difference is 5.1 cc or 3.3% which is not huge so I suspect that an engine set up this way would run well. The difference between standard and first oversize is only 1.3 cc or 0.8% -- probably unnoticeable. (Note that with the smaller bore of the 175 (whatever it is) the differences will be greater.) However, you would, upon assembly, have one new cylinder and one worn cylinder and the worn cylinder will require service before the other one. Assuming that rings and pistons are still available at reasonable prices and given that a rebuild for one side is almost as much work and cost as it is for two sides, why would you ever do this? You would be creating work for yourself (or the unfortunate subsequent owner) later with very little benefit now. So, jensey is right. It would be unwise. Also, you should replace the piston pins and circlips as well and perhaps have a look at the valves and ...
I agree with Tim, but one more thing to consider: If you have one damaged cylinder, find out why before proceeding with a rebuild. History may repeat itself and the damage occur again if the underlying cause is not remedied.
"If the women don't find you handsome, they should at least find you handy." Red Green
I do appreciate the replies. This is my first tear down of a twin cylinder engine, my experience has been limited to singles, generally smaller two strokes, and a TL125.
The reason for the strip of this 175 twin is to fix damage from sitting in a barn for decades with one intake valve open, humidity caused the sleeve and piston to fuse together quite firmly...it did come apart with cycles of penetrating oil and heat. I'm off to a machinist to measure what I'm left with before I source parts. Visually, the top end will require redoing while the trans is clean, just some rust on the shifter forks which live in the top half of the case and not sitting in oil like the gearset. And Tim, the second part of your answer should have been obvious to me. Not to worry, it won't be my last stupid question! I'm be reading up on balancing engine components tonight.
I have taught at the high school and university levels and one of the first things that I say to my new students is that any question asked in good faith to obtain information or clarify something is not a stupid question. It may seem so once you know the answer but hindsight is always better, isn't it?
So lets allow the whole damn world to join.
Not supply any information asked at sign up. THEN ASK QUESTION AFTER QUESTION. Usually make no attempt to understand how or why these machines work and if they do not even to LOOK for faults. (then at least they could offer what they did on their own) Can a Cam Chain Tensioner have the shaft move or should I fix it? Give me a brake! What kind of average mechanic would even consider asking for that kind of help? Thank goodness I had the energy to figure out how these HONDA'S worked and knew to get manuals and especially Parts Books because 1962 didn't have forums or places other than the HONDA dealer to get parts and have work done. Imagine I take my bike to a dealer and ask his (PAID) mechanic to watch me so I don't screw up. I'll end with even some on the forum with many entries have us help them through a problem sell the bike, get another and off we go again. (still asking the same questions) To me these types are lazy but want the money these HONDA'S bring today. If your one of those and don't like this message SO BE IT!. ....................lm
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