TR4A Oil Filler Cap

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Ken Stone:
Piggott states that the TR4A oil filler cap incorporated a metered valve which allowed air to be sucked in.  In pictures it looks quite different from the breather cap on the 3s and early 4s as well as different from those on the 250s and 6s (which look more like a traditional radiator cap).   I'm having a heck of a time finding one.   Moss does not carry the proper cap for the 4A and recommends purchasing the cap for the closed circuit 4 (which is, again, the same one they sell for the 250 and 6, the one that looks like a radiator cap) and then drilling a hole 1/8" or 0.125" in diameter through the cap for venting.  Has anyone ever done this?  If so, is there any blow back through the hole?  And what do people think of running the engine with a sealed, none vented cap?  What problems might develop?   My current cap is not making a good seal and after a good run, I am getting a little bit of oil pooling on the top lip of the valve cover on the cap end. ???

TR3driver:
I haven't tried it (haven't owned a TR4A), but IMO the cap is not your problem.  If you are getting oil out at the cap, then your PCV system is not keeping up with the blowby past the rings and the crankcase pressure is going positive.  The solution is to either find the problem with the PCV (if any), or reduce the blowby (new rings, perhaps new pistons &or liners).

Running a sealed cap will probably reduce the leak in that area, but increase leaks elsewhere (including the rear main seal).  The sealed cap also has the bad (IMO) effect of not allowing any fresh air into the crankcase, which increases formation of acid, sludge and varnish.  But obviously it isn't too bad, because so many TRs were built that way.

Ken Stone:
Thanks Randall - will check into that.

sleipnir:
Hi Ken,

Randall is missing the fact that the rocker arms are throwing a lot of oil around in there and that even with proper pcv, there's likely to be some oil leaking out, if the seal at the edge of the cap is bad, just due to the amount of oil hitting the cap from the rocker arms.  That being said, if the valve cover cap was the only source for air to get into the engine, having a small metered hole near the center of the cap should allow enough air in and keep any oil from coming out.  It's easy to check if the pcv valve is working.   When the engine is running (but before it gets too hot) take the valve cover cap off and cover the hole with the palm of your hand.  The vacuum should suck your palm down slightly.  If it doesn't - a: the pcv valve isn't working   b: there's an air leak into the crankcase somewhere (does your car have a road draft tube? I can't remember.)  or c: too much blowby and the pcv can't keep up.

One other thing that I've seen cause oil to leak out the valve cover/ valve cover cap is that the rocker shaft is letting too much oil past the rocker arms and there's so much oil up there that some is bound to come out somewhere.  I think that your rocker shaft is new and I know that there is plenty of oil pressure so hopefully that's not the case. 

I'd try the Moss cap without the hole first.  TR-250s, Spitfires and GT6s have the same pcv valve set up and I think they both used a sealed cap with no ill effects.  Although now that I think about it, the Strombergs pcv routing may allow air into the crankcase, whereas I know the SUs on your car wouldn't have.  If your car runs funny without the hole drill the hole like Moss suggests and see how that works.

Good luck and let me know how you make out!

Erik

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