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Strange oil pressure loss

PostPosted: December 9th, 2009, 8:54 pm
by yatsey63
Hi, just posting a question about an oil pressure loss I am having trouble figuring out. I have a '77 350 small block that I have just rebuilt. It had new pistons, rings, all bearings including cam bearings which the machine shop put in, new Crow cam, lifters, new standard volume oil pump, timing chain set. Basically everything. When it was started up everything seemed fine with OK oil pressure. Once it is warm the pressure drops to 10 PSI or less at idle. At revs it seems OK. I put a calibrated gauge on to make sure the reading was correct.
I pulled the engine out again and checked all the bearing clearances. 1.5 " on the big ends and 1.5 to 2" on the mains apart from the rear which was 3" so no real problems there. I decided to pressurise the oil system with air to see if that would show up a problem. I blanked the oil pump outlet and installed a pressure supply to the oil pressure gauge fitting. I found air escaping at a great volume from the holes in the rear cam journal. I assume these holes are to let oil from the rear side of the bearing return to the sump and not build up pressure that would blow the rear cam gallery plug out? I plugged these up tightly with some paper towel and the leakage stopped. Well, a little bit from the mains and big ends as these were not torqued up, but otherwise normal. The cam bearing is the narrow style and appears to be in the right position.
What I can't figure out is how the air (oil) can possibly get to the back of this cam journal so it can then escape via the holes in a high volume. I measured the journal size and the bearing diameter and it is very close, although it is hard to get a good bore gauge reading. The only thing I haven't done yet is remove the rear cam gallery plug to check from the rear of the motor.
Does any one have any ideas on what I could look at next???? I really am a bit confused now.

Re: Strange oil pressure loss

PostPosted: December 10th, 2009, 10:33 am
by grumpyvette
the distributor forms one wall of your oil passages when its in place, if you checked oil pressure with it removed that explains your testing of the rear bearing leakage.
your oil pressure is a measure of the resistance to oil flow rates, you can either increase the oil flow or increase the oil viscosity or decrease the oil leakage

read thru these threads



btw while were talking about weird oil pressure, many guys don,t know that the 1965-early 1966 BBC engines REQUIRE, the rear bearing journal on the cam to be GROOVED or the oil will not flow correctly
If you get the correct cam bearings for the motor 65-66 blocks use Sh615S cam bearings and you must machine the rear cam journal . 3/16 wide by 7/64 deep You can tell 65-66 blocks by them not having a grove in the rear cam housing
396 clevite SH615S 65-66 only(grooved cams)
clevite SH616S 67-93 (non-grooved cams)
1967-93 cam bearings don,t require the grooved bearing or grooved cam rear journal ... al&f=false

Re: Strange oil pressure loss

PostPosted: December 11th, 2009, 6:52 pm
by yatsey63
Thanks for the reply. I had thought of the distributor. I finally decided the cam and the cam plug at the rear of the block had to come out. I found the rear cam bearing had not been pushed in far enough by the engine shop that did the rebore. It was allowing the oil groove to very slightly become uncovered underneath the bearing shell!! Very hard to see this but yeah, it had a leakage path all around the bearing shell. It looked like it was positioned centrally on the bearing mounting, but it actually needed to be positioned further rearward by about 2.5 mm (sorry about the metrics, I'm in Australia). I have just had it pushed in a bit more, put the cam, crank, lifters, distributor etc all back in and pressurised it with air again. No leakage!!!!! I would advise anyone rebuiding a small block to check this before they put the rear cam tunnel plug in! Thanks once again for the quick reply and advice. I hope someone else can use this experience to solve a problem of their own. I had never heard of anything like this anywhere before.

печи для бани производители

PostPosted: February 21st, 2016, 7:37 am
by Thomasot

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PostPosted: February 21st, 2016, 7:41 am
by Thomasot