The exhaust spring pair I removed yesterday has inner length 1.4" with outer 1.55". RGM supply 1.5" and 1.65" respectively. So both have lost 0.1".
I've already ordered and paid for new ones, but now I'm in two minds as usual about fitting them. I doubt if I'll be taking it to 6000rpm, and higher spring pressure will just add to valve gear loads. What would you do on a grey December day?
Morning David, let’s change grey to Norton silver....
The Garrett book gives free lengths in the table as attached. For performance use it’s the poundage derived from the wire size that controls the valve. The length ( providing your not coil bound on maximum lift), can be compensated for with shim or thermal insulator.
A drill press and weigh scale under a spring on the work table, depressing the spring to a working length will tell you how worn your springs are compared to the new ones.
Thanks Jonathan. I looked back at previous correspondence here. It seems the books say the outer should be 1.7" for my 88SS, but 1.65 is what is now sold for 1962-on twins (downdraft head). Someone wrote he thought it was to protect Combat head owners from becoming coil bound because the same springs apply to all. Since they are on order, I might as well use them I suppose.
Another observation...the valve seats don't look worn, but they are dead black. It might be valve guides could do with replacing, but frankly life is too short...
I ground the seats on the Atlas but then realised that one of the valves was too long and that if I replaced the inlet guides with commando items I could fit seals . At this point I took a critical look at the seats and realised that they were a bit pocketed but the geometry still looked ok. I got my local wizard engineer to do some additional seat cuts and it all came out well.I probably lost a bit of compression ratio ,but that was what I needed too and helped with kickstarting .The time to stop is while I'm winning.
Oddly enough, last time I was in there was when I replaced inlet guides because one was loose. I made an oversize pair by turning down 850 Commando guides to match the oversize I needed. And I fitted seals. Today the seals fell out in hard black lumps...perhaps they were OK before they were disturbed, but they aren't any good now.