'73 Honda SL125 top end rebuild questions

I'm just about finished restoring my '73 Honda SL125.  Everything looks great, but the engine only is pushing about 70 PSI.  Only has 2k miles on it, but the most recent miles were in the desert with no air filter :unsure: .  I have adjusted the valves more than a few times, just to be sure it wasnt that, but it hasnt changed anything.  Bike fires up decent enough, but does have a mid-range miss and has a hard time mantaining a constant idle (new points, condensor, battery and plug, just to be sure it wasnt electrical).


To me it sounds like its time to put a new piston and rings at it.  I have rebuilt single cylinder OHV engines, but never an overhead cam.  I figure I will pull the engine out again and put it on the bench, so that will help.  I know that there are timing marks on the stator which will help, but I imagine I will have to make a few of my own on the top end.  Will I have to tear the engine completly down, or can I get the head and cylinder off by itself?  I also am not sure how the timing chain comes off.


I havent been able to find any OEM pistons and rings, but have found a few aftermarket piston and ring sets on eBay, so figure I will grab one of them.  Figure I will go with a 1.0mm OS piston and have the OE cylinder bored and honed at a machine shop here.  That should bump me from the stock 122cc spec stamped on the cylinder to just over 126cc's.  Is 4cc's a worthwhile upgrade (and machining expense), or should I save time and effort and just get a stock piston and rings and hone it out myself?


Looks like I can also find NOS aftermarket piston kits that will push it to 3.0mm oversized, which would put it to 135cc's, which would be a nice bump.  Honestly, it really doesnt do badly now, and hauls me around fine around town.  It does need to rev to do its thing, but a 3-tooth larger front sprocket keeps the RPM's to around 5k at just over 30 MPH.  Gets a little buzzy around 45 MPH, but I dont think that more cc's would fix that.  Not really sure if more cc's would give me much of a benefit.  I dont plan on running it on the dirt, have too much into it.


To give you an idea, here is a picture of it a couple months ago, before I got the muffler back from body work:





Edited by Caddy Kid

1.00 over works fine and the 3mm is nice also. When you have cylinder being bored you should have head checked. I build these for a living. Have every part on shelf including handmade stainless and even titanium valves. You just need guides and valves checked/decent valvejob and a mild port touchup wont hurt. I am very very reasonable on cost and headwork is one week and back in mail to you.

I actually just took the easy out and sold it as is.  Ran fine and all, and I knew that if I started on the engine rebuild that it would only give me a small gain for a lot of cost and work.  Thanks though!  If you have a chance, PM me your contact info as I may have a need for it in the future, you never know!

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