Yes, You are right Douglas..it would protrude much further with an M21 crank. About 9mm in fact.
Clearance between the top of the piston and the underside of the head should be between .030" and .040", with everything assembled and the gasket fully compressed. This puts the squish area within correct tolerances but avoids the possibility of the piston hitting the head due to crank flex when the motor is running.
There are three possible solutions to your problem depending on how much the piston protrudes above the barrel and the first thing you should do is put an accurate figure on that.
1) Add another, or a thicker barrel base gasket to lift the barrel slightly.
2)Replace the composite gasket (which I would recommend anyway) with a solid copper one of the required thickness to achieve the clearance previously mentioned..Copper gaskets are suject to very little compression when fitted.
3) Machine the crown of the piston..A small amount of material can safely be removed from the piston crown. Crown thickness after machining should not be less than .150" at the thinnest point...Ian
email (option): ian@wright52.plus.com
Hi Arthur...I have found replacement composite gaskets to be very variable in performance..some last well, others fail very quickly. Also they require a specific tightening and torquing procedure until they are fully 'settled'.
For that reason I no longer use them preferring the more reliable, and stable, solid copper type...and an added bonus with these is they can be reused after cylinder head removal.
Application of a sealant is optional...I don't use any but others use welseal or similar as a 'belt and braces' measure.
The key thing is that the gasket is properly annealed prior to fitting and correctly torqued down.
Torque down following the tightening sequence in the BSA worksheets to 28-30 ft/lbs.
Regarding the 'squish' area over the piston it is often not appreciated that this is an important dimension to be taken account of during a rebuild.
Fuel/air mixture is 'squeezed' out of this area over the piston at top dead center, concentrating the gas into the remainder of the cylinder head volume.
This creates a smaller and more concentrated combustion chamber in effect, thus providing a shorter flame path at combustion and reducing the chances of unburnt pockets of gas and an uneven burn, both of which can cause 'pinking'.
It is generally accepted that the squish clearance should be set to the minimum possible figure and that at values beyond .040" it does not function as intended.. In modern engines of more rigid construction this clearance can be just a few thou.
In the case of British singles with built up flywheel assemblies that are not so rigid the crank flexes under load, reducing the squish clearance.
If the clearance is set at .040" maximum with the engine static this will reduce when the engine is running to well within the recommended limits.
In actual fact it can be set statically slightly below .030" as a minimum figure..and I do that in some OHV tuned engines...but in that case it may be necessary to remove the head after running to ensure there is no piston to head contact.
Exploring these limits on an M20 is rather a waste of effort so a .030"-.040" figure ensures the clearance is within the limits for correct function yet guarantees there won't be any problems with clearance when running...Ian
email (option): ian@wright52.plus.com
Hi Kalle..Yes, Aluminium does work. I had some made as an experiment...in fact I think I may still have a couple...Ian
email (option): ian@wright52.plus.com
Thats absolutely right Arthur...however, as the bolts bottom out on the shoulder they seem to lock pretty well. When I have made the bolts I leave them undrilled and have never lost one! Glad to hear you have got the clearances sorted...Ian
email (option): ian@wright52.plus.com