C2Dude

66 L-79. Mileage: Reads 43,060 but no idea what it actually is and no documentation if PO's rebuilt the motor. So the analysis comments talk of the lead levels being elevated but stable but since the last analysis 3,400 miles ago. They indicate that the lead would be from the bearings and advise to look for such things as low oil pressure. I had re curved my distributor and performed jetting and adjustments detailed in Lars paper on Street Avenger carbs this past winter. That work permitted me to lower the idle, which is now stable, from 900 rpm down to about 750rpm. Was out driving yesterday and did notice that once at operating temp (60F ambient) oil pressure dropped to about 5 psi at idle. A few hundred rpm increase and she swings back to normal typical pressure. 10W40 high zinc oil in her. So this lower pressure at idle "may" be from the idle now being 150 rpm lower than I am used to or is it a sign I may have some engine work in my future. What say you?

C2Dude

Let me clarify the idle psi number. 5psi is a guess as the oil pressure gauge is not marked that finely. The needle swings to 3/16"? of zero, maybe a 1/4".

C2Dude

Yeh it's been a morning. Forgot to post the report.

67:72

The low idle pressure is probably indicative of excess bearing clearances when the engine is hot from thinner oil and expanded gaps. The only stop-gap is to use a 20w-50, but it sounds like at least new mains and rod bearings are in the near future.

thoyer

C2Dude

Thanks for the reply's. Think I figured it out. Prior to modifying my advance curves and carb jetting etc she would only idle at about 900 rpm. After the mods I set the idle at 750. Driving on the day that I noticed the low oil pressure at idle, the motor also wanted to stall at red lights. Yesterday I used my dial back timing light that has a tach function. What I saw was that 750 on the dash tach was reading about 600 on the timing tach. I increased the idle to read 800 on the dial back and now she idles fine with 30 psi pressure hot.
So the elevated lead levels probably indicate the need for bearings in the future but perhaps not now.

Westlotorn

Paul it sounds like you are getting it sorted out. I just got to read your oil report. Your Rod and Main Bearings can be either Copper Lead Tin (copper 77%) or they are Aluminum. This is for the Rods and Mains. Your Copper Lead Tin numbers are good except for the Lead which is high, Since the Mains and rods are 77% copper and your copper content is low your mains and rods are just fine and the lead is another issue.

The most common Cam Bearing is a Lead Based material aluminum cam bearings are also sold but not nearly as common.
Your cam bearings are Lead based with a smaller amount of Tin. Babbitt material basically. These cam bearings are so soft you can dig into them with a finger nail but they work well for a cam bearing as long as you don't have too much Valve spring pressure like they run for some of the very high lift camshafts. .550+ lift

You may be on to part of the cause with your low oil pressure and low idle situation.

Many engines are built with loose oil clearances which allow pressure to bleed off at low RPM, the most common bleed area will be the Main, Rod and Cam Bearing clearances, cam bearings are a common location for an internal oil leak. The GM blocks are not really that straight at the cam journals. At the factory they installed rough cam bearings and after bearing install they had a procedure to cut them to the proper size, made them true/straight ready for cam install. In our Rebuilt small blocks the factory tooling is not available so the cam bearings are installed and then the installer gets to cut or massage them till the cam slides in and spins free. Sometimes this is done with a crude Cam Spoon, a spoon with a cutting edge on a handle that is long enough to reach in from the front and cut the bearings till the cam fits. Some shops have old cams with cutting slots on the bearing journals and they slide this cam in and let it cut the bearings till the cam inserts and spins free. I state this just to let you know this part of engine building is a little rough, not precise and often you end up with an oil pressure bleed at cam bearings. It is normal and has little affect on engine life. If the bleed is large they might install a high volume oil pump to mask the bleed or leak. They do not have to carve on every engine, many are pretty straight at the cam bearing bosses but it is very common to have to carve to get a cam installed. The Lead based bearings work well for this because the lead can actually flow once the car is running, it will mold to better fit the engine but oil pressure leaks are still possible. I am not at all surprised to see the high lead content in your oil sample given your comments about slow idle and low pressure.

Although GM always said if you have more than 7 PSI at idle the engine is fine, no warranty. Even 20 PSI at hot idle makes me nervous because I know it could be better. I prefer the engine to be tight. .002 Rod and Mains will give 40 PSI + at idle normally. In a tight engine your oil pump by pass is opening already at 400 RPM. In a loose engine the by pass may not open till 1,000 RPM. The pump by pass pressure is set by the by pass spring in the pump or Pressure relief valve. This keeps the pump from over pressurizing the system. If your by pass spring is set for 46 PSI the pump tries to limit pressure to 46 PSI all the time. On cold starts you may see 60 PSI because the cold oil does not by pass well when it is cold and thick but pressure drops as the oil warms up and thins out.

What is your oil pressure at 2,500 RPM? Just curious.

With your idle now solid at 750 and 30 PSI at hot idle oil pressure it sounds like you have a good running engine. I think you can relax unless something changes.

Mark

C2Dude

Excellent tutorial mark, thank you for taking the time to write that. In 24 hours I have gone from designing a rolling gantry to facilitate lifting the engine out to looking forward to the next road trip, thank you for that. The only correction is that I have adjusted the idle up to 800 and am seeing 30 to 30+ psi. Cold she swings pretty much across the scale to 60 psi (60 psi gauge). I will check Sunday as to what she reads at 2500 at Op temp.

silver837

Mark ------ I always learn from your responses , so thank you .

Westlotorn

Thanks but I am learning that what I know is becoming obsolete. And what I know about a C2 restoration would fit in a thimble.

silver837

But are cars for the most part are obsolete ,
Your knowledge of the rebuilding of our engines and the associated parts is invaluable .