427 390HP Low Oil Pressure? Tech question

[Neil S.]

My 69 427 was rebuilt about 5 thousand miles ago. From the first day till now, when running cold the oil pressure is about 35-40 psi. When it warms up and idling it is between 17-20. I have hooked up a seperate oil gage to make sure the stock one was working correctly and it was. I am running a stock oil pump NOT HV or HP. Also when cold it makes and always has had a very light almost like hollow thump sound and goes away long before it is warmed up. This noise has not gotten worse, if anything it seems not as noticeable as when it was rebuilt. The engine builder assured me that everything was in tolerance, New 6223 crank standard bore L2300F pistons, resized rods, etc. So my question is should I worry about this noise and oil pressure? Also the noise seems to come from the front of the engine, cannot tell if it is top or bottom. My thoughts are rod bearing, wrist pin. I am by no means a mechanic so I sure can use some expert help. I am welding a crack in the oil pan so the bottom end is exposed. Is it possible to do a Plastigage check on the rod bearings while the engine is in the car? Will it be accurate? Since the noise goes away as it warming up, would a rod noise go awy? Am I worring about nothing? Mr Williams? Mr lucia? Anyone?

[Terry M.]

Re: 427 390HP Low Oil Pressure? Tech question

Is it possible to do a Plastigage check on the rod bearings while the engine is in the car? Will it be accurate?

Yes it is possible, and given the circumstances a wise move. So long as you don't rotate the crankshaft it should be accurate.

There is also a $$$ kit from Kent Moore which will measure the rod bearing clearance using a dial indicator. This kit is far more accurate than Plastigage, but unless you can borrow one, may be out of the question. If you have a desire to pursue that kit, I have the number at home, and can post it later today.

I'll leave your other questions for more experienced folks.

[Dick W.]

Re: 427 390HP Low Oil Pressure? Tech question

Rod bearing are not normally a source of low oil pressure. Main and cam bearings are the usual cause. One question though. Are you sure that your oil pressure guage is accurate? Many a problem has been cured by verifying accuracy

[Neil S.]

Re: 427 390HP Low Oil Pressure? Tech question

The only source that I have for accuracy is the fact that I used the original and an aftermarket oil gage to test and both read within a couple pounds of each other. I have heard that 10 lbs for every 1,000 rpm is safe but this concerns me.

[Dick W.]

Re: 427 390HP Low Oil Pressure? Tech question

OK Neil you eliminated that problem. What is you normal running oil pressure and what viscousity oil are you using? The 10# per thousand rpm's is a recommendation for racing engines. You never saw that with stock oil pumps on factory engines. I believe that my L/88 would build about 50# at 6,500 rpms. Maybe 55#

[Edward B.]

Re: 427 390HP Low Oil Pressure? Tech question

Fore and aft play on the crank shaft within specification? Sounds like a problem I once had with a 454.

[Neil S.]

Re: 427 390HP Low Oil Pressure? Tech question

Oil viscosity I was using 20-50 then switched to 10-40 with hardly any difference. Cruise oil press is about 35. High rpm I really have not checked. Fore and aft play I have not measured. Getting back to the noise, what do you guys think? Can I check bearings in car with plastigage and still be accurate?

[David B.]

Re: 427 390HP Low Oil Pressure? Tech question

Neil,

While you have the pan off, you can also do the following. If the oil pump is the original, check it for wear. They can develop grooves in the gear flat surfaces over time. If it has wear, there is a procedure (in the GM Overhaul Manual if I remember correctly) for restoring it's capacity. It involves a using a pane of glass for a flat surface, and truing the flat gear surfaces on emery cloth with some light oil to remove worn surface grooves that can reduce pump output. I did it with my '69 L36 original pump during the rebuild and it carries correct pressure. My shop manuals are packed away temporarily so I can't be more specific about where it is described in them.

Also, check for proper clearance between the pickup screen and the bottom of the oil pan. Sometimes, if the connecting tube rotates, it can be a problem - or come loose. For that reason, it is common practice by many to weld the pipe in place to the pump (after temporarily removing the spring).

At the least, it would be worthwhile to make sure the pump pickup screen is clean, and the assembly is set at the proper height.

Dave

[Rob M.]

Re: 427 390HP Low Oil Pressure? Tech question

Neil,
I had similar reading on my 65 396. The oil pump was a new std vol, std pressure. I think that your numbers are OK for that pump. If you go with the higher pressure spring, you will see higher pressure as the revs build - it will not change the idle pressure.

I changed to a HV / HP pump and saw an increase in both idle and high RPM pressure. Most will tell you not to use the HV pump. My old engine seems to have excessive lifter bore wear and the HV pump was a reasonable fix.

Search the archives for big block oil pressure and you will find several threads. The only problem is that you can not search on the 3 letter words in that phrase...

[Dick W.]

Re: 427 390HP Low Oil Pressure? Tech question

I would have to think that 35# at cruising speed would be acceptable. That would even fall in the 10# per thousand rpm rule.

Have to use * to make words four letters. Example oil* pan*

[Neil S.]

Re: 427 390HP Low Oil Pressure? Tech question

What about the noise that I explained? Do you think there is anything to worry about? Should I check the bearings would plastigage be accurate enough to use while engine is in the car?

[Wayne W.]

Re: 427 390HP Low Oil Pressure? Tech question

What about the noise that I explained? Do you think there is anything to worry about? Should I check the bearings would plastigage be accurate enough to use while engine is in the car?

My guess is, that noise is piston slap or wrist pin. The described symptom is common. Probably nothing to worry about. I had a stock 427-390 that had almost no oil pressure at hot idle and about 30-35 at speed. Lasted forever with no problems. Of course it wasnt treated rough. I also had a crate LS6 that had only 5-10 at hot idle. That thing was built loose like that from GM. Never a problem with it either. I did put a .002 under rear main in it that helped a little.

[Duke W.]

Re: 427 390HP Low Oil Pressure? Tech question

GM has a hot oil pressure specification for all engines that is published in the AMA specs and maybe the CSM or maybe the owner's manual.

They do not specify hot or cold idle pressure and the "10 psi per thousand revs" is for racing engines.

The ONLY GM spec for production engines is hot oil pressure - usually at 2000 RPM.

I don't have a copy of the '69 AMA specs, but the '67 AMA specs I have specify 50-75 psi @ 2000 RPM for all 427 CID engines with a fully warmed up engine, and this spec implies an 80 psi gage is the IP.

So what is your hot oil pressure @ 2000 and at what speed does it achieve maximum. Also, what IP gage do you have? If you don't have the '69 AMA specs, maybe someone else can chime in with the data, and you can get your own copy of the '69 AMA specs by calling GM Customer Service and requesting the "restoration package" for your car. They will want the year, of course, and VIN.

The L2300F should be fit at .0015" clearance. If looser it could slap when cold.

Duke

[Paul H.]

Re: 427 390HP Low Oil Pressure? Tech question

I am running a stock oil pump in my 68 L36. My warm idle oil pressure is very close to yours and my highway cruise is about 40. My 427 is a fresh, stock rebuild that runs very strong. As for the noise, that certainly requires further invest as others have suggested.

[Neil S.]

Re: 427 390HP Low Oil Pressure? Tech question

Duke, Yes I do have a copy of the restoration package and they too list pressure at 2000 rpm as 50-75 psi. My hot oil pressure @2000 is about 40. I used the stock gage and an aftermarket gage to check. Duke, What do you mean by IP gage? This block was rebuilt at standard bore and I suppose that one of the bores could be slightly overbore and cause piston slap. That is what I am hoping. I just want to eliminate a rod bearing as the culprit. Years ago (too many years ago) I had a 49 Plymouth that had a rod knock and seems to me it wasn't as loud cold as when it was hot. Am I correct? The noise in my 427 Definately goes away as soon as it reaches roughly 120 Deg. I have seen posts here that give almost the same oil pressure values, just like Yours Paul, but thats not to say that there is nothing wrong. To me lack of pressure means something loose or an oil pump is not to spec. Something else just came to mind. When the engine was rebuilt, there was one lifter bore that was extremely out of round and a new sleeve was installed. the lifter bore was out maby 1/32, you could wiggle it side to side. Could it be that now all of my lifter bores are now slightly out of tolerance from loosing pressure? When I got the engine back from the builder I wiggled the lifters and didnt notice any slop, but I'm pretty sure there is no spec on a wiggle test, and of course the builder said all others were fine. Would putting a melling 77HV help? Duke, what would do if this was yours?