ztwntyn8 said:
I have a 1971 Pontiac 400 which I have replaced the cam and bearings in twice now. The first time for more power and the 2nd because a lifter mushroomed and took a lobe with it. I did notice that the original cam had a nearly flat exhaust lobe on one cylinder. The last cam died the same way but faster. Both times I used clevite bearings. The first time a street/strip (from what butler performance said) and the other bearings I got from Summit and were a stock replacement.
When I first replaced the cam and bearings I noticed that the stock bearing fit really snug on the cam's bearing surface whereas the new clevite bearings fit on really sloppy. I'd say that there is about 3 times the clearance. I know this is dropping oil psi. and causing my valvetrain problems but not sure how to solve it.
I have been told by a local well known engine builder that there may be 3 different cam bearing sizes for 400 ci. Pontiac engines. He wasn't sure which engines had what or what year etc. I have never heard of such a thing and have read everything Pontiac related that I could in the last 20+ years.
Has anyone ever heard of such a thing? One other thing I notice about the engine is when I replaced the timing chain (which is the only part of the engine that I could tell had been apart before I took it apart) the new one was just as sloppy as the old one. I have been told that some blocks are machined to closer tolerances than others explaining my slop in the chain.
So, I'm confused. The bearings seem to be consistently the same size and cams bearing diameter seem to be consistent.... even in comparison to the original cam which I still have. I'm about rob some bearings out of another engine I have sitting around. I'm kidding but I'm about that frustrated. Hate half assing things. Thanks, Nate.
Are the cam bearings being installed with the correct orientation which is the oil hole in the bearing shell matches the oil hole in the block which would put the oil hole almost in the 6 O'clock position. If this is off, the cam bearing will starve for oil. Probably the reason Pontiacs and Oldsmobiles are know to spin these bearings on rebuilt motors.
If your concern that the clearance is too much you can use the same Moroso restrictors that were talked about here on an Oldsmobile thread a few weeks ago. But, be careful at the number 1 main that the restrictor goes into the cam bearing feed hole not the passage that feeds the opposite side's lifters. Also be careful that restrictors are well seated so they don't obstruct the transfer passage which would reduce oil flow to the mains.
It is not likely that oil flow to the cam bearings would affect lobe and lifter life. Unfortunately flat tappet cams are suffering these days with low quantities of ZDDP in the oil. This is a big disadvantage to engines running fast lift rate cams and high spring pressures or even incorrectly light pressures for the cam being used as this can cause the spring to loose control of the valve train resulting in the lifter slamming down on the lobe with considerable and unusually high force. Most cam lobe to lifter lubrication comes from crank throw-off. Therefore, one also needs to maintain an idle speed high enough to keep plenty of oil flying into the cam.
Bogie
