I really should write this stuff down….although I probably have and just can’t find it now.

Today’s dilemma: with a new exhaust valve how far should the valve face protrude above the cylinder face?

The valve seats look OK to me but then I may well need new glasses.

Cheers
Pete

email (option): petercomley@web.de

Hello Pete,

This is the answer I got from Ian about exhaust valves,

John

Re: decompression valve cam connection to exhaust valve

When the valve is fitted the 'land' on the valve lip (above the valves seat face) should protrude above the gasket face (and be at least .040" wide). If the valve has be refaced too much or is badly 'pocketed' in the cylinder you should consider a new valve and seat the next time it is apart...This condition effectively reduces valve lift and performance, which an M20/21 hasn't got much of anyway.. ...Ian

email (option): john.kater@chello.nl

Cheers John!!

I'll write it down this time

Pete

email (option): petercomley@web.de

This distance is a function of the valve seat's condition; a worn seat allows the valve head to sink down to flush (or worse) vs. the cylinder gasket surface.
A low seat will increase operating temperature, since the valve's curtain area is not fully exposed to the chamber.
It's also affected by the valve's margin thickness. If the valve has already been re-faced the margin may be thinner. This causes the edge of the margin to operate at higher temperature, and IMHO if the margin is thinner than .040" the valve should be replaced.
If the valve condition is otherwise good but the seat is too low, a concentric 30° "relief cut" should be made (piloted off the guide) beginning at the outer edge of the existing seat, as wide as required to just contact the gasket surface, and forming a chamfer.
In extreme cases, where this does not clean up the seat enough, a slightly oversize valve should be used, and the seat re-cut at the original angle to match the new valve head OD. As little as .060" oversize may be enough. Due to the odd stem diameter, a smaller stem such as 11/32" may be easier to find; sleeve the guide to suit.

email (option): sales@victorylibrary.com

Valve heads in incremental sizes aren't available and it is getting increasingly hard to find valves with sufficient stem length to make M20 valves..The latest generation of lorries (a good source previously) now have much shorter valves....
BSA also used a 'bastard' size for the M20s valve stems. The problem with making modifications to the valve stem sizes and guide sizes is that the whole thing then becomes non standard, complicating replacement at a later date.
The best solution for pocketed valves in my opinion is to have the valve seat replaced. This is a 'one off' procedure, improves the performance and lifespan of the seat (particularly on the exhaust side) and with a new valve fitted restores everything to the correct position but retains standard valves and guides for the future, making replacement of these parts much easier...Ian

email (option): ian@wright52.plus.com

Ian Wright

....The best solution for pocketed valves in my opinion is to have the valve seat replaced. ...Ian

email (option): viaconsu [at] planet [dot] nl

Depending on your local machining costs, it may be cheaper than making a new valve to spec. In New York it's up to $50. including the insert if the original valve can be saved.
What I dislike about seat inserts:
1. there is always a thermal gradient between the parent casting and the insert, and both the valve and inserted seat run hotter than the original.
2. it's not easy to anticipate the amount of crush (negative clearance) when pressing the insert in place. Too loose (just a guess: -.002"?) and it comes loose the first time the engine pulls up a long grade. Too tight (-.005"?) and you risk cracking the cylinder.
The first place I'd look for an oversize valve is the H-D 45" exhaust valve. It's about 5-7/8" long, 1-5/8" head, .341" stem, 45° seat. The length can be extended with a lash cap, the head can be reduced to whatever you need.

email (option): sales@victorylibrary.com

The degree of increase in seat and valve temperatures is an unknown..but in practical terms I've been running valve seat inserts in the exhaust side of my sidevalves for at least 15 years and there has been no perceptible increase in valve failure rates...Also the valve seats themselves have proven to be far more durable than a seat cut directly into the cast iron..unsurprising I suppose as that is their purpose.
The technology is proven and running in millions? of engines world wide..
If fitted accurately by a competent engine shop, preferably one familiar with older engines, they aren't troublesome.....Ian

email (option): ian@wright52.plus.com

Don't go to a motorcycle engineers, they tend to be very expensive, best to go to a good car engine reconditioners who would probably charge about £40

email (option): davmax@ntlworld.com

check your valve lifter is set right before spending money on new valve seats maybe disconnect the lifter completely and see what happens

email (option): roger.beck@node6.com