Compression Test on Engine Stand revisited

[lonesouth]

This has already been covered, sort of.

I have a 69 351 on an engine stand. When it was in the bronco, it smoked like crazy. Knowing now that this is (likely) either bad rings or bad valves, I want to run a compression test to determine which is more likely the case. I've read a bunch of explanations on how to run the test and figure on just bolting a starter to the sucker and going nuts with a spare battery and starter solenoid.

The question is, with the engine on the stand, not warm, will the results be enough to help me determine the guilty party? Or, could I just take off the valve covers and heads and be able to determine from looking at it what needs to be done? I'm hoping for valves, but fear rings...

 

[SOUTHLINE]

With an engine on a stand I do a leak down test. You don't have to spin the engine with the starter and you can usually determine source of the problem.

 

[wildbill]

Since you have the motor out I would just pop the heads off and look at the rings mike the bore and if you are under .006 then you can hone and rering. I would also pull the valves and ck. the guide's and replace all the seals. This is a low buck fix or the best just rebuild it then you now what you have good luck and have fun. Bill %) ;D

 

[lonesouth]

YEa, i would love to have it rebuilt, but the wife just went back to school and am on a budget. Can I hone and rering at home, or replace valve seals etc? Guess on the time to do those things and cost? Not looking to build a hipo engine, just want to get the 351 in without smoking or too much $$

 

[Rustytruck]

You can buy the tool to replace the valve stem seals on the engine without taking anything apart. You probably use your local tool loan at the auto parts. Your talking about a 20.00 fix for that problem. Anything else the engine comes apart and is going to cost mucho dollars. If the engine is over 100,000 miles then there will be alot of bore taper with these old style engines running cooler thermostats. You could rering it and buy some time but there would be no guarentee that will solve the problem for anything longer than short term use. Once you pull the heads you start the might as wells and the costs go up greatly. Most smokes alot is ring failure due to bore taper. The rings cant expand and contract fast enough and keep up with the taper during the piston stroke. This causes a chain of events plugging the ring lands sticking the rings further and everything goes down hill from there. You could also have a broken ring too. What I would do is just use compressed air in the spark plug hole with the piston down in the bore and both valves closed and see if the air is blowing out the valve covers badly. if it is the rings are shot. Move the piston to the top with both valves closed and if it leaks air out the valve cover both the rings or the bore could be bad. if it leaks out the intake manifold or exhaust manifold then you have valve issues. If it leaks big time through the valve cover then there is a holed piston. If your running an automatic transmission make sure the vacuum line to the modulator doesnt have oil in it from a leaking modulator. Alot of old engines smoke due to restricted oil drain back holes under the valve covers this allows the oil to pool up reaching the valve stem seals which are probably bad anyway and then the valves suck in much more oil than they normally would with just bad seals alone.

 

[lonesouth]

seals == seats?

 

[Steve]

seals == seats?

No. The valve seats are in the head where the valve opens and closes in the cylinder head. The seals between the cylinder and the top of the head. The seals surround the valve stem.

I would throw a starter on it on the stand and do a compression test. Cold readings are what you really want anyway, not hot.