Help Badly Needed

Ken Mason · 10-31-07, 07:23 AM

I am having huge problems with the Chevy engine in my GT40 after a rebuild. Everytime its started the engine runs fine for a few minutes and then it either bends a pushrod or pushes it right through the rocker arm!

My friend John, who did the rebuild, has set the tappets up according to the manual i.e. on each cylinder find where both valves are closed, wind the adjusting nut down till there is no play (can't turn the pushrod) and then give it another three quarters of a turn.

We have tried loosening things off a bit but the same thing keeps happening. One suggestion is a new set of hydraulic lifters which I will try next week but does anyone have any other ideas ? (Apart from get a Ford...)

It's all very frustrating as the car is ready to go and when the engine does run, it sounds superb with great oil pressure.

Paul Thompson 'Hooligan' · 10-31-07, 07:39 AM

When you say 'rebuild' - what was done? or replaced?..

i.e. are the camshaft, pushrods, valvetrain, springs etc all 'as was'? did it run fine before the rebuild or was the rebuild required following a problem?..

When you say 'cant turn the pushrod', do you mean that you cannot phyiscally turn them?, or that the lifter plunger has just moved off its retainer?

Also - have the heads changed? or guide plates been changed?

Ken Mason · 10-31-07, 09:06 AM

Paul,

Thanks for your input.

The rebuild was done because the oil pressure was very low although the engine ran fine. We just replaced the main bearing shells as the bores were OK. Nothing else was done to the engine other than cleaning up the heads and installing new valves.
By turning the pushrod what I meant was, according to the manual, just just nip down the adjusting nut until you can no longer turn the push rod round with your fingers ie the rocker is just tight enough on top of the pushrod to stop it turning.

Chas · 10-31-07, 09:17 AM

Ken,
Possible causes: pushrods too long, the "clean-up" on the heads-were they decked excessively? Check valve train geometry and pushrod length with checking springs and an adjustable length pushrod. Lifter type and condition?
Good luck.

Big-Foot · 10-31-07, 10:33 AM

Your valve lash is too tight for one. I'm presuming we are talking about hydraulic lifters.

You must set each cylinder in order - I use Firing order.

Tighten the nut down just to the point where you can no longer move the pushrod up and down (slack is out) - note, you will still be able to turn the pushrod.

Then tighten it no more than 3/4 turn. I personally prefer 1/4 turn.

If the heads were rebuilt, you may also have valve guides set too tight. When they get hot, they will seize up - bending push-rods.

I really think it's a problem with setting the lash..

Andy Green · 10-31-07, 10:33 AM

Hi Ken
you previously told me it has hydraulic tappets. As the engine had low oil pressure before the rebuild, they may have de-primed. Once rebuilt you now have full oil pressure and it is fully priming them and extending them. Also they be sticking slightly caused by the built up of laquer from oil deposits over the years.
The one that done the latest damage, measure the overall length and see how it compares to the others. I'd then run the engine with excessive play in the rockers until all the tappets prime up

Regards
Andy

Jim · 10-31-07, 10:42 AM

CraneCams

Or maybe your GT40 is begging for a FORD Engine!!!

Doug Sainlar · 10-31-07, 11:10 AM

I guess I'd be a bit worried that the problem might be spring bind as the engine heats up and things expand. Did you change the valve springs when you did the rebuild? Did you change from single springs to double springs? Did you put a higher ratio rocker arm on when you rebuilt the heads?

You say nothing other than the main bearnings was changed in the short block, so I assume the cam was not changed, but did you change the lifters (logic would say "no", but many people change out the lifters/push rods when they rebuild the upper end, too).

Other than that, I agree with BigFoot--when I adjust a set of hydraulic lifters, I do as he says, just tighten the rocker nut till the free play is out of the system (you should have no "slop" between the push rod and the rocker arm), but the push rod can still be freely rotated between the thumb and fingers. If your friend is tightening things down until the push rod cannot be rotated, chances are the plunger in your lifter is bottoming out when the engine warms up and it becomes in essence a "solid" lifter, one running without any "lash". That might explain it.

Of course, it's also possible your 40 might be saying "Feed me Ford food, not Chebbie".

Doug

Steve26 · 10-31-07, 11:40 AM

Unless I missed something your problem is the way your adjusting the rockers. On a Chevy the engine must be running when you adjust the valves so the lifters are fully pumped up. This is why people who work Chevys have a valve cover with the top cut out to try to contain the oil while adjusting the valves, it is a messy job. I am of course assuming you are not using the LT1 solid lifters. If you look in a Motors manual the proceadure is to set the rockers loose and start the engine. Slowly tighen each lifter until it stops clicking and then turn the nut 1/2 to 3/4 of a turn very slowly. The amount of rotation will depend in the engine you have, if you can't find the spec let me know what year the engine is and the heads you are using and I will find it for you. Hope this helps.

Steve

MReid · 10-31-07, 11:45 AM

I'm with Doug and Randy. When adjusting the valves, you don't want to depress the plunger at all when taking the slack out (it is held by a spring, so it won't matter whether they are primed or not during static adjustment). Make sure each is on the baseline circle of the cam when adjusted. Once the slack is out (can't move the pushrod up or down, but can spin it with your fingers with a slight drag), tighten it another 1/4 to 1/2 turn to preload the lifter. If there are geometry issues, they should be verified with a light weight valve spring and adjustable pushrod to test the rocker tip movement across the end of the valve and to determine exact pushrod length.

Ron Earp · 10-31-07, 12:08 PM

Seems I remember a spec about pre-loading the lifters about 0.050". So if the plunger is depressed that amount it'd be correct.

Ron

BruceLambo · 10-31-07, 04:40 PM

From what you've said, you've changed the valves in the head, maybe one is a wee bit too tight?
Is this happening on 1 or more rocker arms? From what you said, "tighten it down till it cant turn, then a bit more" makes me think your friend has wound the lifter plunger into the base of the lifter!
I agee with what Big-Foot has said.

John · 10-31-07, 05:34 PM

Quote:

Originally Posted by Ken Mason

I am having huge problems with the Chevy engine in my GT40 after a rebuild. Everytime its started the engine runs fine for a few minutes and then it either bends a pushrod or pushes it right through the rocker arm!

My friend John, who did the rebuild, has set the tappets up according to the manual i.e. on each cylinder find where both valves are closed, wind the adjusting nut down till there is no play (can't turn the pushrod) and then give it another three quarters of a turn.

We have tried loosening things off a bit but the same thing keeps happening. One suggestion is a new set of hydraulic lifters which I will try next week but does anyone have any other ideas ? (Apart from get a Ford...)

It's all very frustrating as the car is ready to go and when the engine does run, it sounds superb with great oil pressure.

Ken,

I don't want to ask the obvious - but are you sure you have hydralic lifters. Solid lifters would probably do that when they get hot and expand as there is no gap between the push rods and the tappets when you adjust them that tight!

John

Paul Thompson 'Hooligan' · 10-31-07, 05:53 PM

Looks like its all been said.... are the valves the same length? - either way would affect valve geometry, but shorter might lead to the springs binding. Turning the motor over by hand should allow you to check with the rocker covers off. You want enough preload on a hydraulic lifter, when on the heel of the cam to keep the middle plunger (touching the pushrod) just off its retaining clip.

Let us all know what you find....

Ken Mason · 11-01-07, 06:51 AM

Thanks to everyone for their help and suggestions. There are a lot of things to try and I will keep you posted. Fingers crossed the solution is there somewhere!

Tony Hunt · 11-01-07, 08:07 AM

Ken,
Your problem is not a tappet clearance one. If you close tappets up too tight, the worst that can happen is loss of compression or valves touching pistons, both of which would be immediately apparent.
So, the problem must lie in the rockers or the valves themselves. A seized rocker? Seized valve? Binding spring?
Good luck,

Tony Hunt

Ken Mason · 02-15-08, 07:11 AM

Good news! The engine started and ran beautifully yesterday. After pulling it out and checking everything it seems Andy from just down the road from me had the right answer. It was jammed hydraulic lifters. They had well and truly gummed up. A new set seems to have solved the problem and she sounds gorgeous! Thanks to everyone for their advice and help.

I hope to get her MOT'd in the next few weeks so roll on the summer!

J. Salmon · 02-15-08, 08:21 AM

Excellent! Thanks for the follow up. Glad to hear you got it sorted.

The car was obviously NOT craving a bulky Ford motor

Andy Green · 02-20-08, 04:03 AM

That's really good news Ken and I'm glad I was able to help out

Looking forward to meeting you and your beast

Regards
Andy