Tuesday, February 19, 2008

Knuckle Head Leaks

There are a lot of places for a Knuckle to leak oil. Here is a pretty easy and basic tip for assembling Knuckle heads that may help eliminate one.

One thing I see a lot when I dissasemble Knuck heads is that once you remove the rocker boxes, often the lower spring covers (cups) will be loose. Since said cups are held in place by the valve guides, it is an indication that the guide has moved slightly. Obviously if that cup is loose there is nothing to keep oil from running out the bottom.

Lip on guide

What is not so apparent is that the replacement parts available today can be part of the problem! A stock OEM valve guide had a shoulder that measured approximatley .103". The stock OEM lower spring collar had a counterbore in the bottom of it that was approximately .093" deep. That means (if my math skills are still intact) the valve spring would put full spring pressure on the guide, holding it tight down on the cup, and in turn the cup tight down on the head.

Counterbore on lower collar

Unfortunatley, the likelyhood of finding a stock OEM guide is about the same as you stumbling on Elvis' personal Harley (signed under the seat: with love, Prissila). Besides, even if you found the guides, they wouldn't really be the ideal material for use with todays fuels. So it is a given that you will be using reproduction guides.

Here's the problem, though. A typical reproduction valve guide will have a lip that measures only approximately .092". Those of you with higher math skills (or a calculator handy) already see the problem. That's right. The lower collar will no longer hold the guide down, so if the guide moves at all there is nothing to keep the oil from seeping under the lip of the guide and through the guide hole in the cup. True there is still spring pressure on the cup (since the lower collar is now setting on the cup instead of the guide) but that presssure is not concentrated as it should be.

My solution is to machine abut .010" off the bottom of the lower collar. I also use aircraft grade Permatex on both sides of the gaskets that go under the cups, as well as Three-Bond sealer under the lip of the guide itself. While it may be next to impossible to prevent every bit of oil seepage from your Knuckle top end, everything you can do to prevent it is a step in the right direction!


Anonymous said...

What is the real reason the exhaust tins do not have a vent hole?

St. Lee said...

The idea is that there needs to be a source of high pressure in the tin so that the oil will be "sucked" into the return lines which are timed via the breather gear to the crankcase when the upward movement of the pistons provides a low pressure. The intakes get the hole to provide a source for atmospheric pressure (which is higher than the timed crankcase vacuum), while the exhausts get their high pressure from "blow-by" between the valve and guide. For that reason, many builders will add a hole to the exhaust tins if using valve guide seals.