AJS & Matchless Owners Club

Before I go to the trouble and expense of removing and replacing the tappet blocks and tappets from my 1949 model 18 AJS I would like some advice on acceptable wear on these items. I understand that 5 to 7 thou is acceptable for the gap between the tappet and the guide block, but can find nothing on wear to the foot. Hopefully there are two photographs attached showing the inlet and exhaust tappets. The inlet has some marks but feels quite smooth, but the exhaust has a circular depression about 0.005" deep. I bought the bike as a box of bits and have no idea how the engine was running before it was taken appart. I have no idea on the mileage it had done. Should I rebuild the engine leaving them as they are, or bite the bullet and replace them?

Page 9 of the army inspection standards manual just says to replace at 0.0750" for the thickness of the head.
TBH provided that the tappet face isn't roughened/pitted I wouldn't bother with the hassle (special tool etc) to replace it even if it is dished a bit. If it has only worn that much in 75 years it'll see you out . Replacements are currently unavailable from club spares (and in 40 years involved with AMC bikes I don't know anyone who has replaced theirs).

Army standards manual here:

http://archives.jampot.dk/book/Workshop ... ndards.pdf

The tappet must be free to rotate because the cam contacts it off-centre and turns it to even out the wear.

I recall Sprid's advise echoes that provided by Ken Bryant many times.

Johnny B

I had T&L replace a pair in a 48 18 that were heavily pitted due to corrosion. If your valve lifter is in the engine chest like mine I could not source the correct exhaust tappet, Russell's had just sold their last one (was 49 the year it moved to the rockerbox but still had an iron head?). T&L machined a new one down, you will also need new tappet guide blocks, it starts getting expensive.

Exactly so, John. Ken Bryant (aka ITMA on the Forums) worked at Plumstead and later had an AMC dealership in S. London. He was a mine of experience and pragmatic advice. We're still in touch from time to time, putting the world to rights (Ken does it rather more bluntly than me as anyone who knew him via his Forum posts would know ).
Incidentally, he wrote several really excellent articles for Classic Mechanics magazine, one of them on inspecting and rebuilding the H/W single engine which can be found here:

http://archives.jampot.dk/technical/Spe ... hanics.pdf