how smooth should it be?

[gerald ross]

i just had the engine rebuild on my 55 convertible farmed out (by my local mechanic whom i do trust)  to a reputable, local garage whose owner it was said knew these cars way back when.  now with about 600mi on the motor i took it back  b/c of a rough idle.  fearing that it was a bad ring the engine builder took the car back saying that ya just never know if the rings are good or not until you put em in and see.  two weeks later the car  was returned to me (today) with the mechanic saying that it wasnt a bad ring but a stuck valve secondary to a weakened spring which is now replaced with a stiffer one.  a compression check showed 140 psi in all cylinders except the affected one which showed 110 today - "still good enough" i was told as i picked her up.  on firing up the engine i told em at the shop that she still sounds rough and shes definitely not cadillac smooth, nor quiet, idling at about seven -eight hundred (est). i was told to drive her around this weekend and wed see if she smooths out after a few miles.  if this doesnt get the car purring like the standard of the world should the shop owner said wed have to get new lifters.  does this sound like 1) a plausible explanation and 2) a sound course of action?   and just how quiet and smooth should a well tuned 331 be?
thanks,
jerry ross

[John Washburn]

Gerald,

To answer your questons: No, No, and Real Quiet.

If the drop between cylinders, in compression, is from 140 to 110 then you got valve problems, or it could be the cylinder and rings. I assume you put a new or reground cam and new lifters in this 55. Oh make sure that the one of the lifters is not bent...

So (my guess) I would check the valves for this cylinder, the seats for both valves in this cylinder (to make sure one is not cracked), because on a new engine the compression should be real close. On an old engine this is still out of limits.

Remember this is just a hobby.

John Washburn
CLC #1067

[John Washburn]

Gerald,

Not lifters, it was supposed to say check the push rods to see if either of them might be bent.

The dummy
John Washburn

[Bruce Reynolds # 18992]

Further, I would have thought that the cranking pressures would have been in the 150 to 180 ballpark.

One way to isolate the valves from the rings would be to do a "wet" compression check.

That is to do the normal compression check first when the engine is cold.

Then do a further check when the engine is hot.

Then, squirt a couple of squirts of oil from an oil can into each cylinder, and do the compression check again.

This way, if the pressures go up after the oil is added to the cylinders, then the Compression Rings are at fault, but, if they stay the same, then the Valves are the culprit.

Dont forget to disconnect the coil lead at the coil, and hold the throttle wide open when performing the compression checking, and keep it open all the time, or you will be squirting a bit of petrol into the cylinders every time the pedal is pushed down.

Plus, write down all the readings as you get them, and estimate just how many revolutions it takes to eventually get to a steady pressure.

Bruce,
The Tassie Devil(le),
60 CDV

[Dave CLC #20768]

Jerry,

What were the symptoms that led you to the rebuild?  Was this a complete rebuild including everytthing new in the short block, heads, and carburetor, or was it some portion of the engine?    Cylinders honed?  Heads magnafluxed and/or pressure tested?  New valves and seats?  Any evidence of coolant in the oil pan?

You might be have more leads to the problem depending on the above answers.

Dave

[gerald ross]

this was a complete rebuild, with a very sl. bore over.  the carb was also rebuilt.  im not sure if the heads were magnafluxed or not.  the valves were new and before this present (mis)diagnosis i did take it back for the same rough idle and was told that one valve stem was too long and had to be ground down to correct height.  this was apparently done, and the car returned to me at which point i refused to accept her saying that the problem had not been corrected.  thats when the engine rebuilder then said that it could be a bad ring.  he took off the heads saw no problems with the rings but did replace the valve spring with a stiffer one...to no better result.
as you are probably now asking yourself:  why do i keep using this guy?  well, he does have ~$2400 of my money and id like to get the job done right! nevertheless, im losing faith at a rapid rate and may just bite the bullet and take her elsewhere.  however, that "elsewhere" is 500mi away in a city to which i will be moving next spring, near where a reputable rebuilder/restorer resides.  my next question then is:  can i safely drive this car, even with these issues, 500 mi (at 50-65mph) or would that be asking for even more trouble?
frustrated,
jeery

[Dave CLC #20768]

I agree with John and Bruce regarding your compression readings.  For 1957 the spec is 165-185 psi.  I would be surprised if 1955 is much different.  Compression on all cylinders should be within about 10percent of each other.  140 sounds low to me.

I would suggest you perform your own compression test so that you can be sure of what your have.  Bruce outlined a good sequence of conditions for the test.  If you really have a cylinder reading 30 psi less than the others, something is not right in that cylinder.

If your own compression test shows OK, then make sure you do not have a vacum leak in or around your carb, hoses, connections, or intake.  I think you have a bakelite insulator under your carburetor.  They tend to be brittle.  Are you absolutely sure it is not cracked?  Are you absolutely sure you have the correct gaskets in and under the carburetor.  There are only slight differences in the various 4GC gaskets that come in many kits, but big differences in operation if you install the wrong one.  Many carburetor rebuild kits are almost universal.  I have screwed up by pulling the wrong one from the box.

Good luck,
Dave

[Bill Sullivan]

I dont have much to add to this thread except that my experience has been that a rough idle after a rebuild is usually NOT due to bad rings, lifters, valves, camshafts, etc.  I mean all that stuff was supposedly rebuilt. And most engines will still idle very smoothly even with bad rings, valves, etc.  Assuming the compression is OK (if you have 140 on 7 and 110 on 1 that is not OK and should not happen with a freshly rebuilt engine), you need to look for vacuum leaks, distributor problems, and carburetor problems.  There is plenty of room to find problems in these areas, and rough idling is often the symptom. I would look thoroughly there before worrying about rings and valves.

How did it run before the rebuild?  It should run at least as well now and even better if everything was done properly.

I also dont think your engine is going to get better simply by driving it more.  There is little about breaking in an engine that would improve the idle.

[John Washburn]

Gerald,

You dont grind down the valve stems on a overhead engine, the lifters, and such take up the slack to zero clearance. It might be that the rocker arm for this cylinder is bad, but generally if they are they just clatter.

You need to find someone who can fix this, not the guy you are using. I would think it is a valve problem, bent push rod, bad seat, cracked head, or something they did wrong.

But check it out. Put about three squirts of oil in the cylinder then check the compression. If it goes back to the reading of the other cylinders then you have bad rings.

Good Luck

John Washburn
CLC #1067
Elizabeth, Colorado

[gerald ross]

thank you all for the thoughtful and excellent advice.  ill get on this just as soon as the work week resumes and report back.
jerry