The Stovebolt Site Forums The Shop Area Engine Shop What is proper break in procedure for GMC 302?

Are there any specific break in procedures for a 1954 GMC straight 6? Rebuild kit was from Kanter. I assume steel rings and aluminum pistons but I'm not sure. It's an Ebay hand me down.

I have receipts from Kantor and the machine shop so I know this motor was professionally rebuit.

I have seen various break in suggestions on Stovebolt as follows:

Run at 2000 RPM for 10-20 minutes with John Deere "run in oil". Others say that this is a mistake as your rings will never seat.

or run 20W non-detergent for 500 miles then switch to 30W detergent.

or Rev up to 40 then back off to 20 a few times to get the rings to seat. This must be done within the first 20 miles. (question here is what gear to use, 3rd may not go to 40 and 20 in 4th would be lugging)

or drive about 700 mile at varying RPM's without lugging or hard acceleration using. (should this be with detergent or non-detergent?)

I've put about 12 miles on this motor using the last technique and want to be sure I'm doing it right. I haven't noticed any smoke as some posters describe during break-in.

The US Army used the following post-rebuild break-in procedure for the GMC 270 (CCKW and DUKW). It assumes you are using a dynamometer, but may be helpful for general guidance. The military wanted the engine to be fully broke in before putting the truck in service for obvious reasons:
1. Run for 15 minutes at 700 rpm with no load.
2. Adjust valves to 0.012".
3. Run for 30 minutes at 800 rpm at 10-15 hp.
4. Run for 30 minutes at 1000 rpm at 25-30 hp.
5. Run for 30 minutes at 1400 rpm at 50-60 hp.
6. Run for 30 minutes at wide open throttle with load adjusted to maintain 1800 rpm.
7. Run for 30 minutes at wide open throttle with load adjusted to maintain 2400 rpm.
8. Idle engine (no load) for 15 minutes.
9. Tune engine.
10. Change oil.

Taken from War Dept. Technical Manual TM9-1802A

dukw45

Wow! Great information.

The run-in at full throttle step is interesting. I wonder how many $ were spent developing that procedure. I guess it kind of illustrates the difference between the
GMC (war) and Chevy (civilian) engine development in those years.

It's probably too late for me to do anything different now, since I've put on about 100 miles.
I did do the accelerate to 40 then deccelerate to 20 technique that one poster suggested. Other that that I guess I'll just drive it conservatively for about 700 miles and hope for the best.

I appreciate the reply

You're already past it, but here's what you should have done. It's the same for most any engined.

Prelube engine by spinning oil pump with a drill until oil reaches the valve train. Put distributor in, start and immediately run at 2,000 rpm for 20 minutes, minimum. Keep an eye on the temp gauge, if it gets too hot have helper start misting the radiator to cool down. Don't slow down the engine unless you have problems such as loss of oil pressure or severe overheating. After that, reset your valves, torque the head bolts, adjust carb, points and timing. Then you're ready to drive. Follow my advice on up and down throttle to seat the rings, it works. For the first 1,000 miles go easy, no high revs, heavy loads, burnouts and the like.

I would recommend the break-in oil, the manufacturers put it in these vehicles when they were new. If not, go 10w-30. Change after 300 miles, then after 1,000 miles and again after another 2,000.

Fred T

Do you know the reason behind the recommendation for the 2000 rpm break-in? I know it has a lot of proponents, but have never heard what its advantages are over the progressively-higher-load approach. Most of my 40s and 50s manuals recommend the latter approach, but admittedly they are all for truck engines with full pressure lubrication like the 270 and 302. Is the higher speed of your method to ensure proper dip-and-spray lubrication, ring seating, or ???

It's for breaking in the cam and lifters. Otherwise, it's possible that you won't get sufficent oil flow and the lifters don't turn properly, which causes a wear spot on the lifter that flattens the cam lobes. Never let an engine with a new cam idle below 2,000 rpm. I've been told that for the first 200 or so miles not to let it idle for more than a minute or so. I did ruin a cam on a 250 once by not doing this break-in step.

Makes sense. Thanks.

Fred

Thanks for the break in info. You're right, it is too late since I've put on about 100 miles. I guess I'll just drive it moderately for about 1000 miles and call it good. Unfortunately, it has probably been idled too much.

I would have responded earlier but I was out of town.

Thanks again

just for the heck of it, how is the cam lubricated in an I6? Is it splash lubricated by oil draining back down from the valve train? Or does it have galleys for pressurized oil to get to it directly? Or something else?

I've got a lot more experience with smallblock "twin 4s" and the cam there is splash lubricated, hence the need to keep the rpms up during break in. Or so the commonly held belief states. I spose you could start an argument about that with just about any group of Chevy folks.

Thanks,

I can't answer for the stovebolt, but the GMC 270 and 302 6 cylinders have pressurized flow to the camshaft bearings. The lobes are lubricated by spray from the cam bearings.

I was always told to break these guys in you have to drink a lot and drive fast. Just joking.

Done alot of engines never broke one in any special way. Never had a poblem, just made sure plenty of oil pressre before fuel and fire got to it

The cam lobes on a Chevy six are lubricated by splash from the crankshaft and drip from the lifters. The cam bearings basically drip right down into the pan. The lifters don't really drip very much, hence the 2000 rpm to get lots of oil from crankshaft windage.