[cobalt327]

A couple possibilities...

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.

Hi Nate, I seem to recall you've posted about this before... but in any event, the cam bearings themselves do not- generally speaking- cause cam lobe failures.

That said, if there is a problem w/the journal to bearing oil clearance, or the cam bearings are installed incorrectly, or the bearings themselves are incorrect, or the cam tunnel diameter is out of spec, or the tunnel not straight there can be problems.

Are you installing the bearings yourself? If so, do you have the correct tool that locates off the tunnel so the bearings are started straight? Installing the tapered edge (if present) inward? Holes aligned right?

Is the cam tunnel straight? Personally I've not had any that were out far enough to require honing, but I HAVE had to carefully clearance the installed bearings (only a couple times, but still... ) to get the cam to turn as freely as I wanted it to. Needing honing is certainly not unheard of, some blocks supposedly had the bearings honed to fit the cam, so installing replacement bearings could result in an out of spec tunnel.

To start with, I'd recommend you measure the cam, bearings and cam tunnel of the block to see exactly what you have going on. For instance, if the cam tunnel was line honed this can cause the bearings you have to be a loose fit.

Some dimensions you can use to check what you have (from the King bearing site):

Part Number CS 511BB STD
For 1963-79 Pontiac engines only.
Cross reference- ACL: 5C292 CLE: SH- 292S DUR: P-4 FMO: 1220M

Positions 1 through 5

• cam journal diameter- 1.8992"/ 1.8997"

• cam tunnel bore- 2.0297"/ 2.0317"

• cam bearing length- 0.6800"

• maximum wall thickness- 0.0644"

You can see from the specs above the cam bearings are basically identical, your buddy might be thinking of the SBC.

Are you sure the cam break in procedure is correct? If it isn't, no amount of working over the cam bearings will ever get the cam to break in right. More on cam break in HERE. If there was one thing that I could stress besides using break in oil/oil additives and cam break in lube on the lobes and lifter bottoms, it would be NO IDLING FOR THE FIRST 15-20 MINUTES! Next to that, you have to be sure the lifters rotate as soon as the engine fires. The lifter bores need to be cleaned of all varnish, this can take a shotgun brass bristle brush w/acetone as a solvent. Usually honing is not needed, and if the bores measure good, don't hone anyway. Do be sure there's no burrs at the top or bottom of the lifter bores or the oil gallery holes inside the bores.

Also be sure you don't have Chevy lifters. A Pontiac lifter will have the oil band beginning ~1.125" up from the foot. A Chevy lifter will be ~0.875".

The push rod length needs to be correct as well. If you are using the bottle neck studs and nuts torqued to 20 ft/lbs, this is especially important because if the p-rod length isn't correct you can't adjust the preload unless you have straight studs and polylocs or adjustable nuts on the bottle neck studs (something I personally never recommend).

If you're using 1.65 ratio rockers, the pushrod hole in the head likely needs to be opened up, otherwise the p-rod can bind.

The valve springs need to be correct for the cam, if the cam is radical enough to require double springs, removing the inner spring during break in is a good idea. HERE is a list of valve train points to check.

There is more but this ought to give you a start.

EDIT- If the crank saddles were line bored and honed, the distance between the c/l of the cam and crank can be closer than it was originally. This requires a different timing set to compensate if it's severe enough.

 

[cobalt327]

I agree that I have"D" port heads although, a RAM AIR III engine is a "D" port engine so, not sure why you say they aren't RA III heads. Enough of the semantics.

The only evidence of the engine being apart before I took it apart was marks on the timing cover where the chain had been replaced.

I have stock rockers, push rods and valve springs. I used a summit blueprint HO or 068 cam the first time.

I just don't get why the all of the bearings that I have bought feel so much sloppier than the original bearings that I removed when I have them in my hands and place them on the cam which is also in my hand and that is the stock cam and the summit cam. So, the camshaft's bearing surface is the same diameter stock vs. summit. The bearing it's self has a much larger diameter in the replacement Clevite SH-292S vs. the original bearings that I removed when the engine was all stock.

Any oldschool hotrodder that I ask says they are surprised that I have any oil psi. I was reluctant to put the new bearings in long ago when I did the first cam swap.


Thanks, Nate

Measure the oil clearance of the cam journal to cam bearing by fitting the bearing onto the block, then measure the cam bearing ID then compare this to the cam journal OD. You want at least 0.002" to 0.003" (0.004" max, high performance use) clearance w/the cam turning easily.

Because aluminum alloy bearings are harder and less forgiving to can tunnel misalignment than babbitt overlay bearings, there is a possibility the aluminum alloy cam bearings are intentionally larger.

 

[cobalt327]

As far as writing the measurements, you can leave off the second decimal point, it's not needed. The place of the number to the right of the first decimal point gives the value for that digits place, i.e. tenths, hundredths, thousandths, ten thousandths (sometimes called "tenths" when taking precision measurements, i.e. 0.0003 would be "three tenths" to machinists, etc. who understand that it doesn't mean 0.3).

It's common to call 0.0055 "five and one half thousandths" instead of "55 ten thousandths", or 0.0053 is "five point three thousandths" instead of "53 ten thousandths" though (even though either is correct, technically speaking), and maybe that's what he was trying to get across.

About the cam bearings, I have been told that Pontiac block cam tunnels were drilled first from one end, then from the other end, meeting somewhere in the middle. If that's really the case (I never saw a factory Pontiac block machining line or talked to anyone who worked on one to know for sure), there could easily be a mismatch. What I was led to believe is that the cam bearings themselves were align bored/honed after they were installed to get a straight cam tunnel. That would mean the original bearings could be thinner on one side than the other to compensate for any misalignment, if you follow me. And THAT would mean in cases where the misalignment was more than the oil clearance, there could be contact between the cam bearing journal and the cam bearings if the bearings were concentric (same thickness all the way around).

This could also account for the bearings being made "loose"; in order to compensate for the possibility there was a misalignment, extra ID was made into them rather than having the bearings "tight" and risk the cam bearings spinning or rapidly wearing in service.

 

[cobalt327]

Forgot to mention, your guy is right- many manufacturers will use crank bearings +/- several tenths from standard when building a new engine. They fit the bearing to the journal, not the journal to the bearing in other words. So on a new production crank, there could be problems if "standard" bearings were used w/o first measuring the journals.

 

[cobalt327]

Nate, speaking only for myself, this is the first time a loose cam bearing condition has come up. I've (as I think I mentioned before) have had tight Pontiac cam bearings that had to be clearanced in situ, but not loose bearings.

I'm sure I'm not the only one following this, so yes, by all means update as you go forward. And thanks for doing that this far, too.

"Errors and then epiphanies" would make a good chapter in a book on engine building!

 

[cobalt327]

Oh, what all is involved in honing bearings? That seems to be my next step if the new tool is no help.

The way I've done this in the past looks crude, but I've never had a failure...

I insert a cam and spin it so the bearings will have shiny witness marks where the bearings are tight. Then I take a bearing knife (three sided scraper tool) to clearance them in small increments until the cam spins freely.

You need good lighting that you can adjust/use w.o holding it- you want both hands free. Take your time. I recommend getting a tool, not using something else. Don't use sandpaper, the grit left behind is not good for the engine. If you want to polish the bearings, use a scotch pad.

I have heard about using an old cam w/cuts made across the journals to be used a a reamer, but have not tried this. Then there's the machine shop.