> I do not have the equipment to measure accurately, but I'm puzzled by
> the descriptions in the relevant catalogs of 3/8 to 5/16 conversion
> guides, if that is what happened previously.
TR2-early TR4 (CT21470E according to Moss) originally had exhaust valves with
3/8" diameter stems. Both the inside and outside diameter of the exhaust guide
was larger than the 5/16" intake, although as I recall, the outside difference
wasn't as much as the inside difference.
After that point (approximately), the exhaust valves had 5/16" stems and the
intake and exhaust guides were the same. Although the head casting didn't
(necessarily) change, the holes drilled in it for the exhaust guides got
smaller.
The smaller stem valves flow better, and weigh less (so less prone to float), so
fitting them to the early head is a popular upgrade. The "conversion" guides
have the right od for the early heads, and the right id for the 5/16" valves.
Not certain, but ISTR someone telling me the factory even offered these.
I can't quite imagine someone not noticing the huge clearance; but I also can't
explain the oversize guide any other way. The mistake might have been at the
factory, though.
Randall
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