rebuild question- pistons and cylinder

[sbaugz]

So I have a 81 special with 4000 miles on it. It was running when I bought it. I checked compression and it was 160 PSI on both cylinders.
I proceeded to take valve cover off to inspect things inside and I broke the cylinder head seal which basically forced me to remove jugs. While I was there ,I cleaned and inspected everything and bought new standard size piston rings for an Acura/ Honda.

My question is this. I don't have the tools to check the cylinders and pistons, etc for tolerances. I would assume that with 160 PSI, that I should not need to overbore and go up a size. I am trying to do this build for as little expenditure as I can so is it safe to assume I can just put the same piston and rings back in and expect similar 160 PSI numbers? I know the "right" way would be to take it to a shop and get everything checked and measures to see if an overbore is necessary but I don't really want to do that. Thoughts?

 

[5twins]

Yes, with mileage that low and compression that good, I would say you could just re-assemble with all the original parts. I would just clean all the carbon out.

 

[Dandan]

no offense but with 160 compression why in the hell did u take the motor apart? especially when it was running when u bought it.

 

[sbaugz]

no offense but with 160 compression why in the hell did u take the motor apart? especially when it was running when u bought it.

none taken. Here is my explanation. As for the "running" statement- this is according to the PO. When I purchased it, the bike was not able to be run because of numerous issues including that he had taken apart the carbs, etc. The real reason I took the engine apart was because the PO had basically taken some masking tape and very crudely painted the engine and part of the frame with silver spray paint while the bike was still completely together. I am not even sure whether he used high temp paint of not because it was totally peeling off.

It was a filthy mess of grime, dirt, peeling paint, etc. The only way to get it clean and stripped was to remove the cylinder head. While I was at it, I figured I would do the full cosmetic treatment and have the jugs and head sent off for media blasting. So that's the long answer as to why I took apart an engine with 160 PSI. It probably would run well but it looked like total shit.

 

[angus67]

none taken. Here is my explanation. As for the "running" statement- this is according to the PO. When I purchased it, the bike was not able to be run because of numerous issues including that he had taken apart the carbs, etc. The real reason I took the engine apart was because the PO had basically taken some masking tape and very crudely painted the engine and part of the frame with silver spray paint while the bike was still completely together. I am not even sure whether he used high temp paint of not because it was totally peeling off.

It was a filthy mess of grime, dirt, peeling paint, etc. The only way to get it clean and stripped was to remove the cylinder head. While I was at it, I figured I would do the full cosmetic treatment and have the jugs and head sent off for media blasting. So that's the long answer as to why I took apart an engine with 160 PSI. It probably would run well but it looked like total shit.

undersood. But im wondering if you should at least run a stone hone(?) not sure of the name, to get the crosshatch back on the cylinder walls so the rings will reseat evenly?
This thing
http://www.zorotools.com/g/00065230/k-G1905504?utm_source=google_shopping&utm_medium=cpc&utm_campaign=Google_Shopping_Feed&kw={keyword}&gclid=CMmGvIumn7QCFQhyQgod-SwANw
this one in the link is not for a 650, i just yanked a picture from the web.

 

[5twins]

If you don't change the rings, you don't need to hone.

 

[angus67]

If you don't change the rings, you don't need to hone.

therefore, if you DO replace the rings, you Need to hone. thought he was considering honda rings.

 

[Dandan]

A quick masking and a nice soda blast prolly woulda done the trick but they are right, if its already torn apart then u might as well rehone and new rings to get the motor back in te 180s. 160 is not far from new condition for the age. Mine has 125 which I find good enough to not tear apart yet.

 

[retiredgentleman]

I don't understand what sbaugz is thinking. He has an engine with very low mileage/excellent compression, but decides to take the top end apart. He also says he wants to "do his build for as little expenditure as I can" but then buys some new Honda rings, which aren't needed.

There's certainly no need to hone cylinders or install new rings...............the engine is barely broken in.

 

[5twins]

Exactly, just clean everything up really good, assemble with new gaskets, and go riding. I've done this in the past several times on low mileage engines that still had in-spec ring end gaps. They came apart for oil leaks and/or decarbonizing and ran as good or better afterwards (and no longer leaked oil).

 

[scabber]

Just because the speedometer said 4000 miles doesn't mean that the milage is correct. If it has all that grime and dirt I would have the bore and pistons checked. A few pictures of the bores and pistons would help.

 

[sbaugz]

I don't understand what sbaugz is thinking. He has an engine with very low mileage/excellent compression, but decides to take the top end apart. He also says he wants to "do his build for as little expenditure as I can" but then buys some new Honda rings, which aren't needed.

There's certainly no need to hone cylinders or install new rings...............the engine is barely broken in.

Read my post
I didn't intend to do all this. By mistake, I turned over the engine by hand while the
Cylinder head was off to inspect things and I broke the lower gasket seal. It was my understanding that once the seal is broken you have to replace. That is why I went to all this trouble. Believe me, I would have loved to be riding right now instead of doing all this unneeded work

 

[scabber]

A quick masking and a nice soda blast prolly woulda done the trick but they are right, if its already torn apart then u might as well rehone and new rings to get the motor back in te 180s. 160 is not far from new condition for the age. Mine has 125 which I find good enough to not tear apart yet.

Normal is 140 lbs new condition. 160 lbs. is from carbon build up. You will never see 180 lbs. normal.

 

[Dandan]

I'm used to building car motors. Even so some seafoam woulda helped clear a lot of that up

 

[650Skull]

some things don't add up.

Pics of your engine show an oil leak between the head and barrels, the condition of the bike looks a bit rough for 6000 miles, 160lb compression on a cold engine seems to high.

Pics of the barrel and pistons would be good.

You should weigh up the cost of any work you might get a mechanic to do against buying the tools If you intend to do your own work