'75 Vette with crate 383, 5k miles on engine, spark plugs oily, carbony from beginning, using oil-worse than 1 qt. per 1000 miles but forget exactly how much (part of that from leaking pan and valve covers, though).

I've pulled the heads, and intake valves are gunked up with carbon deposits, wet oil on them; exhaust valves clean but white as though got too hot. I was told to check if valve stems are loose in guides and whether seals are sealing.

What I found is the valve stem FEELS like a good, snug fit in guide: no obvious slop. Perhaps there is a more proper way to measure this than by feel?

However, I'm suspect the seals are giving problems. The exhaust seals are using an "umbrella" style, if I understand things correctly. The upper end of the seal fits around the valve stem using a spring around its circumference to hold it to the stem; directly below the spring the seal steps out and becomes a boot that fits loosely over the guide boss. Whole thing is about 1" long and is a soft rubber. There is nothing to keep the boot from pulling up off the boss, which the seals have done, as I can see between the valve spring coils and can tell the bottom of the seals are about 1/4" above the guide boss. I suspect it should be staying down over the boss, but if so, how is its design supposed to work to keep it there? As soon as the valve opens the seal will ride up it and away from the boss.

The intakes are using a whole different style. The seal is pushed down over guide boss and is on pretty hard using 2 springs acting like elastic bands. It's a cylindrical-shaped seal, about 5/8" long perhaps, with its upper end just barely sticking out above the end of the valve guide boss. I assume its design is meant to seal against the stem but stay put on the boss. What I don't understand, is if you look into the seal, the top end of the boss can be seen and it is just a tiny big smaller in diameter than the seal's inner diameter. It seems, then, that the seal would fit against the valve stem looser than the guide itself, thus not sealing the oil out, but perhaps I am misunderstanding its design...?

No O-rings at top of valve stem, such as I have read about.

So I wonder whether the seals are what's causing the oil burning. I should also mention the motor performed flawlessly, and there was no apparent oil burning except a very quick puff of smoke upon start-up which would immediately clear out.

Should I convert to O-rings and eliminate the present seals, or should I merely add O-rings, or should I change out the present seals or is anything even wrong with them? Thanks for any help!

The original o-ring style (original) ride on the valve stem and don't seal anything. Rather they cause the oil to splash off from the movement of the valve stem. This works well.
The next type uses a spring to hold them on to the boss where the valve guide goes through the head. They hold tight in place. The top of these also have a spring loaded seal that fits snug on the valve stem, and seals to the moving stem. Any loosness between the seal and stem will leak.
The umbrella seals also work well. They stop oil from running down the stem from the top above the valve spring. They must fit snug on the stem, or they will leak. Usually the valve rocker holds them in place. If not they are worn, or not the correct ones.
Valve stem play can be roughly checked by wobbling, but is more accurate with a dial indicator on the side of the valve. This must be done with the spring removed.

If you have a valve spring remover, use the new seals that clip to the head boss, and seal on the stem (these can be a clearence problem with high lift cams and high ratio rockers), or the o-ring. If not get a new set of umbrellas and use them. All work well when they are correct for the car. It sounds like yours are the wrong ones, or just shot.

How is the choke?

Ken Styer

|UPDATED|3/7/2004 8:44:43 AM (AZT)|/UPDATED|

Thanks for your description of how the valve stem seals are supposed to work. Regarding the choke, I do not have a tester or meter, so I think what I'll do is have my brother look things over. He is quite handy with electrical stuff, and I'm sure he can check the items you mentioned in the posting. I hope it's something simple that's wrong!

Thanks, Patricia

Is this a GM crate motor? There were problems with valve seals on Target motors a few years back. never heard of any problems with 383's tho. If it is a GM crate motor, this should be covered under warranty. Check with your dealer. You might get a bit of a runaround from them, and if so, call GM customer relations. getting them involved usually solves any problem the dealer may give you (if at all). I was a service advisor for Chevy for many years, and many dealers are not well informed on crate engine warranty.
Check it out and good luck!
Reid 72 t-top

All you really need to check the choke is a test light. You can get an inexpensive one at AutoZone for about 5.00

Ken Styer

I ran across a similiar situation two years ago, where the diameter of the umbrella seal was just about the same size as the ID of the valve spring, so when the spring would compress it would grab the seal and pull it up and allow oil to run down the stem to be burned. On another engine, we had it making so much vacuum at the intake that it was pulling oil through the breather cap through the vacuum tube connected at the base of the carb. When we pulled the carb off we saw all the oil inside the intake and on the bottom of the carb. We corrected it by putting in oil baffles inside the valve covers and installing breather cap extensions. Fixed the problem.