I’ve found that 2 of my cam caps have a sliver of shim material inside. It’s got to be about .001 or .0015.
Also, one of them has two punch marks - perhaps to take up some slack.
Anyone think these might be dated from 1953? I don’t see someone rebuilding in the 60’s being quite this fussy given the selling price for these back then.
Well… that’s odd.
The cam bearing caps are normally letter/numbered to match their places on the head. Do yours all match?
Engine builders more experienced than me will perhaps comment, but I have heard that cam bearing wear is unusual even after high mileage, unless it was oil starved.
The only reason I can think of to do that would be a bubble gum method to tighten up the bearing crush if the cam bearing had spun in the journal, but there is no evidence of that. I don’t see how the shim could affect the cam bearing clearance. Would have been more appropriate to sand a thou of the cap face.
OK. I just ran the exhaust side to determine shims. I took the foil out figuring that with new caps the foil is likely not needed where they are.
I also plasti-gauged the caps… twice, with the cam turned 180*.
I got .0015 to .0025 on all four caps. I consider this good to go on the bearings…?
Yep, all 8 nicely numbered.
Make very sure that those shims did not cause the cap(s) to crack since they had to bend farther down (than designed) to meet the head as they were torqued. Also, IIRC, the clearance spec is 1 to 3 thou.
I recall reading, most likely in the E-type Haynes manual, that a “useful dodge” is to flip the cam bearings 180 degrees, since the upper shells take almost all of the load and consequent wear. I still prefer to change them out with new. They’re not expensive.
I stand to be corrected but I am under the impression that the rule of thumb is 0.001” of bearing clearance per inch of journal diameter - though that might be specific to crankshaft main and big end journals. If it applies to camshafts, which in the case of XK engines have 1.00” journals, your clearances might be a little loose. I haven’t plastigauged mine yet so have no real comparison to go by.
The manual says permissible clearance is 0.0005 to 0.0020.
The difference between .0005 and .001 on a 60 year old cam journal and a split shell in an aluminum head in a non-temperature controlled environment is not going to be detectable by anyone outside a NASA facility. Certainly not by me. As long as I don’t see anything much below or past .002 or so, I’m going with it. The shells or cam are likely out of round by that much, I’ll never find it with plastiguage, certainly not with the valves pushing back.
As nearly all of the force on the lobes pushes the cam into the upper cap, you could almost run the back 3 with no bottom cap making the need for clearance only to assure the bearing isn’t too tight and to keep it from getting pushed sideways. Of course you need to control the gap to ensure distribution of oil to the front of the cam so I suppose there is actually a function for them.
Plastigauging the cams is not that difficult. You gauge each of the unloaded bearings one at a time, removing and replacing the individual caps as you rotate the crankshaft. If your bore gauge does one inch circles then go with it instead - this will also indicate ovality.
(Edited)
A simpler–and usually effective way, given the low stress on the cams–is to install the cam, no tappets or valves, torque to spec, and feel how it spins.
That’s interesting, Wigs. What does one feel for? … how do you know from the feel what’s ok and not ok?
Smooth rotation, no “catches” (which can be a sign of a bent head), and no sensible upwards notion when you gently lever up the cam.
On L-series Datsun engines, I always bolted on the cam and cam towers, sans rockers, to perform the same check. Ditto with OHC Mercedes engines.
What he said. I would hazard a guess that binding from a warped head would be the thing most likely encountered. And you have two of them, so if one feels different it would indicate something amiss. You can temporarily swap them side for side and figure out if it’s the head or a particular cam.
Once a bent head is found, one can loosen the caps, front to back (or vice versa) one at a time to roughly determine where it’s bent.
Very cool.
Always a good thing to learn from those much more experienced.
Lay the plastigauge onto the oiled bottom shells, then bolt the cam down. If you do this with clean, dry cam journals, the plastigauge will stick to the cam and not the bottom shells. Any warpage should show up as uncompressed or over compressed plastigauge.
