There are not published cam journal to cam bearing clearance specifications, due to the fact that this clearance cannot be adjusted reliably. Main thing is to check the cam, utilizing v-blocks, dial indicator and accurate surface plate to verify that the cam is straight. Install the camshaft using engine oil or assembly lube and verify that the camshaft will rotate smoothly by hand.

 

There is a post by the guy that does 496 BBC engines with oval port cast heads that shows him taking the larger OD bearing, #1 position, and grinding the OD to fit other positions to bring clearances down. He documents that #5 position is usually worse clearance. Checking for cam straightness is one more thing to check. Thanks. I was more concerned with lower oil pressure caused by extra clearance.and throwing more oil around for the rings to control. Keith Jones, Total Seal, mentioned that to me as we we were talking about oil control on a Vortec 454 shallow oil ring groove situation.

 

What you're really asking is how tight can you run them, I've run them as tight as .001" without issues. This was when I was building offshore 496 BBC engines (Bowtie blocks) 27 years ago and was after camshaft stability with oil control as a good side effect. Our early builds were hard to control cam bearing clearance due to factory production tolerances but we as we ramped up our engine volumes we were able to get unfinished blocks machined to our specs and I obtained some unfinished large diameter cam cores ground to my specs enabling whatever I wanted.

We spun these 7200-7600 rpm with 300/700 seat/open pressures. Durabond bearings and the larger cam cores were the only variations I remember running that tight but cam bearings are very forgiving. We had a run of new blocks that was missing the oil feed hole for the rear cam bearing and one block got past us without being corrected at build time - so that bearing got Briggs & Stratton oiling. It ran dyno time and a whole race and looked perfect when I caught it on the refresh, there is a lot of oil sloshing around down there. I purposely gave up some power on those engines with extra oil flow in some areas for durability reasons.

 

0.008". Yikes! From my Sealed Power catalog, clearance is 0.0006 to 0.0046"

 

IMO if you're still running the oil hole straight on the bottom, 99% of the oiling to the cam is coming from splash/windage/drain back and not the pressure side anyway. Even with mild valve springs you've got multiple springs at 150# or more continually pushing down on the camshaft and somehow ~60-80psi of oil pressure is supposed to squeeze in there at the bottom and follow it all the way back around to the load side at the bottom again.

If you want to see an improvement in cam bearings and ability to run them tight for oil control, they need a back-band and the oil hole to be around 3-4 o'clock thusly able to form a proper oil wedge at the load surface in the bottom.

 

When I started building pulling truck engines with a lot of spring pressure, I started to see the importance of checking cam bearing clearances. I now mic/bore gauge the bearings just like I would a main or rod bearing and adjust the clearance by polishing/grinding on the cam journals if necessary. I try to run .001" of clearance per inch of journal, just like I would mains/rods, and increase the clearance based on discretion.

I will also say that some of the Dart blocks I use have grooves behind the cam bearings and the bearings have 3 holes in them. I'll usually try to place one hole at around 4:00. However, there are a lot of other engines that I build, such as S/O FE's, where there is no choice but to put the feed hole in line with the hole in the main saddle and I see no differences in how the bearings look, even with 250-275 lbs of seat pressure and 700 lbs of open.

 

The Dart cam bearings with the 3 holes were supposedly because Dick Maskins thought people were not smart enough to locate the single hole in the best spot. Durabond makes a 1 hole bearing to fit the dart blocks. I have used them without issue.

I can get a .004" inch feeler gauge between the bearing and cam before bearing is installed. Not sure how much this will be reduced by the press fit into the block. The cam OD to block cam journal will result in a .0035" press fit on the block I am doing now. I will be checking that diameter once installed. Nice to see others are up on this often overlooked part of engine building.

 

The Dart bearings having three holes just makes for extra leak paths. Their design was simply for installation purposes, not having to be picky about where they were put in at. - We run single hole bearings and cut the back band ourselves on stock diameter stuff, on the current 55mm cam combo I used bearings from Straub that are single hole with the band already machined in.

My stuff is 340-380# seat and 1100+ open, 8k+ rpm. The back band and clocked hole made noticeable difference in the bearing wear as well as the benefits of added oil control. I'm now toying with restricting the feed passage to them above each main.

Between notching the lifters for continuous oiling, then restricting the lifter gallies, and the cam bearings, we've been able to lower oil viscosity and slow the pump down/run a smaller pressure section (dry sump, on the previous wet sump we went to a smaller pump) while still keeping the bearings very happy. Similar peak oil pressure to what we've always run but lower idle pressure and much more linear curve. The dyno & the time slips have shown value in the better refined oil system & subsequent windage.

- I'm sure to the higher end engine guys, this isn't news.