Jump to content

1931 Pontiac 6 jumped timing


Recommended Posts

I have a 1931 Pontiac split head 6. It was running like the well old machine it is (27k original miles). One day great, next day, tried to start and heard an awful metalic grind. Now the timing is WAY out and there is no compression at the spark plug holes. I have pulled the harmonic balancer and looked at the timing chain and gears and they are all still perfect and not very much play in the chain. The distributer gears are also good. The flywheel (which you can see the teeth through the timing hole) all look good also.

I am afraid if I just reset the timing that there may be more severe damage some else in the engine but not sure what would make the timing jump out so far but everything I described above still looks perfect? Appreciate any help!

Edited by WayBro (see edit history)
Link to post
Share on other sites

If you have no compression in any cylinder you must have a broken camshaft. All the pistons wouldn't break at once so it has to be valves. All the valves wouldn't go bad at the same time so it must be camshaft or else the timing gear but you say they are perfect. Do you have oil pressure when you crank the engine with the starter?

You say you have the timing cover off (well you didn't say that but you said the gears are okay). In that case I would pull the timing chain (three capscrews on the camshaft gear and the two gears and chain will come off easily). Then turn the engine over with the starter. Should be no noise. Take the side covers off then and see what happens when you turn the camshaft by hand.

Note the timing marks (small circles) line up with a straightedge placed across the cam and crank centers.

One other very remote possibility, had the engine been recently overhauled? Most people don't realize that the bottom ring on the original piston comes partly out of the cylinder at the bottom of the stroke. If the original one piece oil rings are replaced three piece ones more commonly used now the bottom scraper comes out of the cylinder and when the piston goes back up it breaks the bottom off.

Link to post
Share on other sites

The engine has never been opened (original family car passed down now to me) so not overhauled. If the camshaft was broken, it would not turn at the front of the engine correct? If I take the side cover off, what will I be able to see? I have looked for an engine manual with diagrams but no luck.

Link to post
Share on other sites

On a flathead engine like this, you should be able to see the camshaft when you take the side cover off. Turn the engine by hand and see if any of the valves operate. You say there is no compression on any cylinder which indicates to me that the camshaft is broken very close to the gear. A jumped timing chain would cause a drop in compression and serious lack of power but the engine should still run albeit poorly.

Terry

Link to post
Share on other sites

What you see turning at the front of the engine is the timing gears. This does not tell you whether the camshaft is turning or not. You also mention the flywheel teeth this also has nothing to do with the camshaft. Since you already have the timing cover off why not take 4 more cap screws off and see what is happening in the valve chamber. You would be able to see the valves going up and down (if the camshaft is turning).

Link to post
Share on other sites
What you see turning at the front of the engine is the timing gears. This does not tell you whether the camshaft is turning or not. You also mention the flywheel teeth this also has nothing to do with the camshaft. Since you already have the timing cover off why not take 4 more cap screws off and see what is happening in the valve chamber. You would be able to see the valves going up and down (if the camshaft is turning).

Thanks. The only thing changed from last running great was added a little aluminum radiator stop leak. Is it possible this caused the water pump to seize and jump the timing or is it just a coincidence?

Link to post
Share on other sites

Al Brass On the Pontiac L head engine the camshafts gear is driven by a steel gear on the crank via a Morse timing chain type 766. 1 1/4" wide with a 5" degree pitch of 44 links or 22" l0ng.

In the first post he says there is no compression on any cylinder therefore the cam must not be turning. He says the gear and chain are fine. Therefore the cam must be broken.

WayBro The water pump could be seized solid and it could/would not cause a problem with the timing. You really need to take the side covers off. If you have the harmonic balance and the timing cover off you must have the hood and rad off so you are half way to being able to pull the whole engine.

PM me if you wish. Good luck.

I was thinking with such low mileage perhaps the oil pump got clogged, seized and broke the camshaft in the middle but in that case you would have compression on the front three cylinders.

Link to post
Share on other sites

I am not an expert on Pontiac engines. But my thought is, even if the cam broke there would still be compression in at least two cylinders. No compression anywhere means the pistons are not going up and down.

Link to post
Share on other sites
I am not an expert on Pontiac engines. But my thought is, even if the cam broke there would still be compression in at least two cylinders. No compression anywhere means the pistons are not going up and down.

This begs the question WayBro. Did you do a proper compression check? I think everyone here has been assuming you did. The engine needs to be turning to do it.

Terry

Link to post
Share on other sites



Thanks everyone for the advice. As you can see, one valve for 5 and one for 6 is not cooperating. Any leads on parts for rebuild? I will pull the top but want to know where I can get parts depending on what I find. Edited by WayBro (see edit history)
Link to post
Share on other sites

Thanks - will do - pulled the top and the two valves are extremely hard to move They look clean what I can see of them. If I tap on them to go back down they stick again on the up stroke. Tried lube but no luck. Does the bottom of the spring have a normal keeper on it?

Edited by WayBro (see edit history)
Link to post
Share on other sites

Doesn't look broken,,,,,just very stickey valves,,

all holding off the seats,,by gooey gumm,, Buy a

reamer or small hone,,an' maybee practice on

some junk head,,,I remember small spring hones

for this job,,Ben

Link to post
Share on other sites

So if the cam shaft is broken why did those two middle valves move up on the first crank over, and why are those same lifters rotating on subsequent crank overs ?

Note the two centre valves moved up but didn't return; as odd as it sounds, I think the two centre valve springs are broken.

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...