Cylinder & Pistons
(see also the cylinder mk2 page)


Nice'n clean after decarb.

Soaking the parts in white spirit or a degreaser overnight makes the decarb job go like a dream, with much less need of force or hard tools to get the carbon off. (Same applies to the combustion chamber\top\valves). Mark the pistons inside with FL & FR immediately upon removing, as they should go back were they came from. These are the old pistons types (6.5:1?) without split skirt, works fine in this machine. Only the rings were changed. Careful while removing the rings, they have a tendency to suddenly break (how do I know that?). So do not rush, and they can easily be bent just enough to slip over the pistons. If to be refitted remember their position in regard to piston and top\bottom. To check the rings' end gap, carefully insert one in the cylinder, and push it with the ringless piston until it's approx. in the middle of the cylinder. With a feeler see that it's within specs, compression rings 0.013 inch, oil ring 0.011. If in doubt, a little too big a gap is much safer than too tight. Do this with all rings. Check the pistons' ring grooves for deposits, wear and side clearance, 0.002 to 0.003 inch. Polish the piston tops with Autosol & clean thoroughly.

Honing the cylinder

Lapping (or honing) the bore is possible provided wear or damage is not too pronounced. It's an easy job to perform with a relapping tool, available from car parts stores. It can give the cylinder & existing pistons a new lease of life. It will improve compression & oil distribution on the cylinder walls, especially with new piston rings. The wear at the piston rings top position can be seen & felt with a finger, it will show as a dark ring after some lapping. Use a slowly revolving drill and lubricate the lapping stones with some thin oil or similar. Move the drill with even & quick up-down movement trough the whole barrel. Do not extend the process longer than strictly necessary. Careful while inserting & removing the lapping tool. Clean very carefully several times when finished. While not expecting to do any magic (this would require a rebore + new pistons etc) this process will smooth things out, postponing a rebore & keeping the pistons going for some 1000's of miles.When done insert a piston ring & check the end gap again at various positions in the bore. After the lapping, treat the engine as after a rebore, with a careful running-in period & oilchange after 500 miles or less.

Wear at top

My cylinder has it's original bore, 70 mm. The rings top position of travel is where the cylinderwear is most pronounced. Ideally there should be no ridge at all, but one do not have to rebore too soon either. Always use good gaskets when reassembling, & tighten all bolts at the right torque (the old A10 service sheets says "real tight" for the head bolts, whatever that means). There is a tightening sequence, do it right or you may end up with leaks & may bend the top end. Some say one should retighten the bolts after the gaskets etc have settled.

Small end falling out

New small ends were needed, one was actually loose. Check for play & wear. New ones must be lapped to the exact internal diameter. Also check the cam followers for wear & free movement. Clean out the oilways going from top to bottom of the cylinder & from the bottom of the cam follower housing. Clean the cylinder & give it a coat of black heat resistant paint. Before removing or fitting the gudgeon pin heat the piston with a hot water soaked rag. When all is done be sure to loose a circlip and some nuts into the crankcase :-) One may avoid this extra entertainment by stuffing the crankcase opening with a rag around the conrods.

Dinner's ready, Dear!

If you are impatient, a kitchen oven is great for newly painted parts. Use minimum heat & keep parts inside for a couple of hours, with the oven door open & good aeration. WARNING: DO NOT INITIATE THIS PROCESS UNLESS THE WIFE HAS GRANTED A WRITTEN CONSENT IN PRESENCE OF TWO WITNESSES.