I waited long enough to get my head sealed. Yesterday I pulled the head. I blued the head surface (You might call it something else in the UK, its a blue dye you use for lots of things in a machine shop. Sometimes its red) and put it on a surface plate with some fine sand paper. Not a low spot or high spot to be found. It removed all the blue evenly. I then re-annealed the gasket to be sure (I didn't install it originally so I need to rule that out). I will get back to this later.
First off. Someone with more experience with M20's can look at these photos and respond. The reason I pulled the head was because once hot, the bike seemed to bubble oily goo. It also would start to run a bit lean and idle up when hot.
Before I install the head I want to be sure everything looks normal. The valves look good to me. The seats look nice and shiny. My concern is that there seems to be alot of oil sitting on top of the piston. It has never smoked. It doesn't wet sump. So obviously the rings are the culprit. But is a little oil on the piston normal? Also, I have never ridden the bike longer than about 20 minutes.
I also have one head bolt that wont torque. In your experience would you run a die over this and tap the threads? Or should I skip that and helicoil?
Hope all the photos dont bother anyone. Im a very thorough person
Many Thanks,
Michael
email (option): micran1234@yahoo.com
Hi Michael..I would say you have far too much oil present on top of the piston. Often bores are honed to provide a better surface on a cylinder that really needs a rebore. Don't forget that honing may have increased the piston to bore clearance beyond serviceable limits if both bore and piston were already worn.
I would have the cylinder off the bike and measure both the bore..at various points along its length..and the piston to determine exactly where you are.
It may also be the case that the rings have been incorrectly gapped in the past or the bore is worn such that the ring gap is too large.
Valves should also be checked..particularly the exhaust.. to make sure the valve face is 100% true to the stem (it is surprising how often refacing the valve on a refacing grinder shows up this problem), as well as looking at the condition of the seats in the barrel.
The head bolt looks quite badly deformed...probably due to carbon or dirt that has been forced into the thread in the past...I would helicoil the barrel and fit a new bolt. Torque up to 28-30 ft. lbs when refitting the head and follow the tightening sequence in the manual...Ian
email (option): julie@wright52.plus.com
Thats what I was expecting. Just didn't want to admit it.
I will take it down and check everything out.
Report to follow.
Many thanks!
email (option): micran1234@yahoo.com
I pulled the cylinder and took some measurements.
Bore (measured at top of cylinder)= 82.3mm
Bore (measured at bottom edge)= 82.4 mm
Piston (measured at top edge above rings)= 82 mm
Piston (skirt)= 82.1mm and 82.2mm kept getting these two measurements.
Uppermost ring end gap = 10 thou.
Middle ring = 12 thou.
Broke bottom ring trying to remove it 
On the bottom 1/3 of the cylinder I can see all of the nice crosshatch/basket weave hone. On the top half its visible but more worn.
Whats the trick to removing the gudgeon pin? I dont wanna beat on it too much. Removed circlips. It wont tap out easily.
What do the experts think?
email (option): micran1234@yahoo.com
There is a tool, essentially a band of steel with a hole for the gudgeon pin to exit. On the opposite side is a bolt that acts as a press and pushes the pin through without any damage to the con rod or big end.....
Try Frost Auto Repairs website for specialist old style tools.
email (option): stinkypete80@hotmail.com
Hi Michael..Riks recommendation is the way to remove the pin. With the application of a little heat it will come out easily..As a matter of interest, all the headbolts are helicoiled on my M20..you won't have any problem with them...Ian
Do those measurements mean anything to you? I'm leaning towards having it bored to +60 and a new piston. I really dont know what Im doing though.
email (option): micran1234@yahoo.com
Hi Michael, 82mm is a standard bore for the M20. In determining piston to bore clearance the piston should be measured across the thrust faces of the piston skirt..ie below the rings across the front and back of the piston. Piston to cylinder clearance should be .004"-.006" for a freshly bored cylinder.
Piston ring gap should be .008"-.012" for all three rings.
Niether the Army standards book or BSA manuals are totally clear about the total amount of combined piston and cylinder wear that is acceptable.
I take the mid limit figure for the piston to bore clearance in a new engine(.005") as the optimum. If the piton to bore clearance is at or exceeds .015" in a worn engine I will rebore it.
Your engine is on a standard bore so the next rebore would be to +020" assuming your bore is currently under that size...Ian
Hi Michael, I won't go into all the figures for bore and piston wear detailed in the Standards book and BSA manuals as it can get a bit confusing. Suffice to say that one possible interpretation of the figures allows for a higher degree of wear than I outlined in the last posting. Taking the total possible bore clearance based on minimum possible piston size and maximum possible bore size as outlined in the standards book results, in my opinion, in an excessive running clearance.
The books state a maximium bore size and a minimum piston size but do not specifically state whether it is acceptable to run the two extremes together.
If for instance you had the max. bore size but with a new piston fitted that would be completely a different clearance to the same bore with the minimum size piston fitted to it.
Wear limits as indicated in my last posting are therefore what I have concluded are acceptable maximums over the years.
Based on that you may get away with a new set of correctly gapped rings.
Before fitting new rings carefully remove any carbon deposits that have built up at the top of the bore and remove any wear ridge (if present.)
Bear in mind as well that it can take up to a couple of hundred miles for the new rings to bed in fully and seal correctly.
I noted in your previous posting you mentioned you broke a ring removing it...was that the oil scraper ring?..and was it tight in the piston?...if it was this may have been at least a partial cause of the excessive oiling....Ian
I agree, that was a useful 'dodge' I learnt many years ago as a teenager from an 'old boy' who lived near me.
BSA pistons go up in increments of .020" (std, +.020", +.040") In Michaels case he is on a standard bore so if an 'aftermarket' ring set from the likes of Hepolite can be sourced in a +.010" size all the better. Many of the aftermarket manufactures of the day produced pistons and rings in these 'in between' sizes (+.010"/+.030")and also went beyond the BSA maximum of +.040" to +.060"...Ian
Hi Michael..Some directly side to side play is required...about .010"-.012"...but they will run ok with more. You should not be able to detect ANY up and down play..but getting the 'feel' for this can take some practice if you are not familiar with it.
The amount of 'tilt' is fairly minimal when a big end is newly fitted but is always apparent when the engine has covered some miles and is quite normal. I would go with a standard set of rings initially...Ian
email (option): ian@wright52.plus.com
Hi Michael..I sent you an e mail regarding ring sets...Ian
email (option): ian@wright52.plus.com