M20- break-in period and oil.... (Edited by Author)
I’ve fitted the engine block of my BSA M20 with a cylinder bored to 2nd oversize (-0.040), a new piston and rings, new valves, and ground valve seats and valve guides. I normally use only SAE oil. . Yesterday, as a test, filled the oil tank halfway with SAE 50, enjoyed the aprilsun and drove 2 km very carefully and all went well.....Now I need to break in the engine. My question is, first of all, what oil should I use for the break-in period? Also, I’d like to know at what schedule you use for speed and mileages and when or, even better, at what interval you typically change the break-in oil and when I finally can switch back to the trusted SAE 50.
I know there is a lot of experience on this forum, could you guys help me out with this quest ?
thank you again.
Arnold
I don't worry about break in oil. The piston, rings and cylinder are all assembled dry (no oil at all) and break in quickly. The motor gets 40W or 50W depending upon the time of year. After 100 miles the oil is changed.
I don't use antique crap Hepolite piston rings. Modern rings don't care what oil you use. Even when broken in, the Hepolite rings will always burn a little oil.
Personally, I tighten down the head gasket at 100 miles or so and give it a quick oil change. Another oil change at 500 miles and after that progressively open things up according to manufacturers intervals.
No-one has ever wrecked an engine by changing oil too often.
So only an oil change at 100 miles and that'll be all?
Arnold[/quote]No, 100 miles is the first oil change after a rebuild. Every 1000 miles after that. On a M20 rebuild the only part that needs breaking in is the piston and rings. The bearings don't break in and unless you have replaced the cams and lifters, the piston is it.
If you are using modern piston rings with a three piece oil control ring, then break in oil is not needed.
if using old rings with a one piece oil control ring, which are crap but what everyone used 50 years ago, then a 30W non detergent oil is a good choice. But I have found, putting the piston in dry will make the rings start breaking in as soon as the motor starts and before the oil gets to the rings.
New cars and motorcycles of today, don't use break in oil. They use the same oil they will use for the life of the motor.
Re: M20- break-in period and oil.... (Edited by Author)
'On a M20 rebuild the only part that needs breaking in is the piston and rings...'
Unless you have replaced bushes, valves/ guides or gearbox bushes....I change the oil at about 75 miles, again at 250, 750 and 1000 miles..Like Rik says you can't really change it too much and any accidental contamination during the rebuild and the initial metal particles from running in should be taken out with the oil changes...If I've rebushed the gearbox I'll do that too at least three times during the (1000 mile) running in period...
The best advice I could give is to take it gently for the first 100 miles and pretty easily for the first 250 miles..Don't over rev the engine during acceleration and don't slog it up hills at low revs in a high gear..Use the box on hills but bear in mind revs more than speed and run it on a 'rolled off' throttle on level going...Gradually increase the loads in short bursts as you progress through the 1000 miles reaching the upper end of the performance range after 750 miles...That's worked well for me for 55 years without too many problems over many engine rebuilds...After running in I change oils every 2000 miles but keep a close eye on the gearbox oil level if you have a military M20 without a sealed bearing fitted in the box..In that case I would check the oil level every 150 miles or so...The oil can leak out very quickly and insifficient gearbox oil is a 'bush killer'.....Like Bruce I don't use running in oil at all...
