Mk3 Timing cover to Oil Pump Shimming

mdt-son said:

Greg, I doubt Viton was ever considered by NV due to cost and general acceptance back then. Invented in the late 50's and gaining popularity in the 90s, it's more likely NV kept what Norton chose in the 50's, and I guess it's NBR. Same story as with valve stem seals, probably.

- Knut

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Agreed but I was not talking about then, talking about AN today.

@Greg: My impression is that AN's policy is rather conservative when it comes to product improvements (their billet crankshaft and cam followers are notable exceptions). I'd be excited to find that AN has changed composition of the oil pump gasket and valve stem seals.

@Slick: NBR is superior to Neoprene wrt. max. operating temperature and chemical resistance. Neoprene is mostly used for low temperature applications, dust seals, and the like. I sincerely hope the oil pump seal or valve stem seal isn't made of Neoprene, as the material would deteriorate quickly.
Wrt. your comment "these worked adequately well then, and would likely be adequate now", I beg to disagree. There are components in synthetic oils which require a premium grade of NBR (>36% Acrylonitrile), of course not considered in the 70's. The operating temperature limit you quoted is probably optimistic - I have seen a 100 'C operating temperature limit quoted for continuous operation, probably depending on composition. Lastly - increased ambient temperature and operating mode (esp. for tuned or big bore engines) implies higher internal temperatures, and may render previously accepted composition seals less suitable.

- Knut

I replaced my failed, original AMR mod O-ring with a silicon one 2 years ago.. Still good.

Yakatak said:

I replaced my failed, original AMR mod O-ring with a silicon one 2 years ago.. Still good.

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When it comes to rubber products for sealing, choosing quality brands is crucial. I guess you are referring to the O-rings AMR put inside the oil pump? Quality silicon O-rings will behave neutral up to about 125 'C (257 'F). However, when exposed to motor oil at 150 'C / 302 'F (>15 days), the material gets softer, elasticity is greatly reduced, and the cross-section deforms by creep.

- Knut

This is the first mention of shimming the seal I've ever seen. There's no mention in the MKIII manual. I've had two seals split noticed by the loss of oil pressure, thus my advocacy of an oil pressure gauge. The return of oil to the tank does not indicate the seal is good. The seal I last replaced has been fine for some years and perhaps is made from better material than the former ones. I recall the failed conical seals had what I'd consider normal compression marks.

mdt-son said:

Greg, I doubt Viton was ever considered by NV due to cost and general acceptance back then. Invented in the late 50's and gaining popularity in the 90s, it's more likely NV kept what Norton chose in the 50's, and I guess it's NBR. Same story as with valve stem seals, probably.

- Knut

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Got the answer:

MK3 Oil Pump Seal, 06.6190, is NBR (Nitrile Butadiene Rubber, aka Buna-N)
Pre-MK3 Oil Pump Seal, NMT272, is CR (Chloroprene Rubber, aka Neoprene)

Both were available when the Commandos were being built so you are probably right.