Motorcycle Repair: 1972 Honda SL125 Motorsport, honda sl125, needle bearings

Question
Anthony Centrella on the list below you is on vacation so hopefully you can help. I have never taken the head off an overhead cam single cylinder bike engine before...done many 2 strokers..I've removed the exhaust..carb..headbolts..cam cover..the head is loosened from the cylinder...NOW WHAT?..The reason for this repair is a frozen / rusted piston in the cylinder...soaked it in Marvel oil for 1 week it wont budge...

Answer
Mike.... you have to remove the camchain from the camshaft sprocket. Two bolts hold it together. Pull the sprocket off the end of the camshaft, then remove the chain from the sprocket. Put a piece of wire around the chain, so you can drop it down the side of the motor for easy retrieval, unless it is too worn out for replacement.

BEWARE!!!!!!  Before you spend a lot of time/money on this project, check the camshaft fit in the cylinder head. The cambearings are abbreviated and tend to wear out, causing a loose camshaft in the head issue, which dramatically alters ignition timing. There are ways of installing needle bearings in the head/camshaft to repair the wear/damage, but it all costs money. Putting the motor back together with loose camshaft clearances will be heartbreaking and frustrating to get to run correctly.

If the camshaft isn't too loose, then remove it from the head. You may have to rotate it against the valve spring pressure, so remove the little valve cover caps and loosen the valve adjusters to take the tension off the camshaft. Rotate the cam around so the lobes will come out through the openings in the head.

WARNING! Do try to remove the piston in any way other than whacking it with a hammer. I have used a steering wheel puller tool to push pistons out of the bores on bigger bikes. If you pound on the piston very much, the shock will transmit to the small needle bearings on the big end of the rod and damage them.

Other penetrating fluids to try are ATF, Diesel fuel or KROIL. If it still won't budge, clean off all the oils and flammables, then use a propane torch on the piston crown to heat it up and that may free the rings from the rust bond to the cylinder walls.

Get a shop manual for specifications and/or go here for general info to increase your 4 stroke knowledge.
http://www.dansmc.com/valve_timing.htm


Bill Silver