Missed a shift, broke some rockers, what else?

[Pat S]

I was at a PCA DE at Putnam Park this weekend. Came out of turn 10 on to the straight and went from redline in third to second. Engine didn't seem to like that too much, but I did get to quickly learn what to do when something goes wrong on the track
Anyway, after I got the clutch back in the engine died. I tried to restart it, with no go. At that point I coasted off onto the grass and got a tow back to the paddock. I assumed that I bent many or all the exhaust valves. It was nice to see that the engine didn't dump oil on the track and from the outside everything looked fine.

Thankfully Jim Meyer of Meyer Motorwerks in Circleville OH was kind enough to let me use his trailer and tow vehicle to get the car back home (2.5 hrs drive).

Tonight I dropped the engine and pulled the valve covers expecting to check valve gaps and identify bent valves. I was ready to pull the cams and heads until I found 3 broken exhaust rockers (3-5-1). One broke into 3 pieces and I recovered them all. The other 2 just snapped in 2 nice and clean.



I rebuilt this engine 4 years ago (7K miles) and reused the old rockers at that time.

Could I have gotten really lucky here? Is it possible that the rockers are the only damage? I assume that a compression test will check to see if the valves are sealing or not. So I plan to go get a compression gauge tomorrow and test that.

I did place each of these cylinders in TDC and blew compressed air into them. With a stethoscope I could not hear any air leaking from the valves, but did hear air blowing into the case past the rings.

If they check out okay with a compression test, I'll also check the cam timing on each side. Is there anything else that I should be looking at? Would it be unusual to have broken rockers and suffered no other damage?

Thanks,
Pat

[EarlyPorsche]

Air past the rings shouldn't be a problem. Without the engine running and oil splashing onto the skirts you will not have as good a seal. If the valves are holding air then you may have gotten lucky. Remember, you found your engines weak link, once those gave way probably nothing else broke.

[osidak]

good luck

i once broke a rocker, bent 6 valves, cracked a head and piston.........

[MBEngineering]

HI Pat S

"Remember, you found your engines weak link"

THE other is the big end bolts you WILL have stretched them and it will only a matter of time (if you leave them) before you will have a rod through the case. Look hear on the job,

miss shift at the Glen?

regards mike

[Mo_Gearhead]

Remember what causes a rocker arm to break like that ... the valves have HIT the pistons ...hard enough to create the mangled parts you now have.

I am no expert on P-car engines, but even with a fairly good compression number, I would pull the engine apart to inspect the valves and pistons.

Alternative: There are instruments (like a bore site tool) that you may be able to go in through the spark plug hole and view?

Good luck.

[Pat S]

Good point on the rod bolts. I used stock ones on the rebuild, so they should be replaced. If there is even a small chance they could have stretched it is much cheaper to replace them now than replace the whole engine later.
Looks like I will be pulling the heads off after all and will get a closer inspection of them then.

Thanks,
Pat

[jester911]

Man, I hate to see that carnage.
For others out there that want to help avoid this I highly recommend going to a Rennshift shifter or Wevo. I have the former and I will tell I would not consider driving on the track with out it.

It makes the money shift very difficult to do.

[Early_S_Man]

Pat,

When it was discovered that the new, cost-reduced, cast iron rockers from the 1968 911T engine worked very well as a quick-acting 'fuse' in the valve train during mechanical over-revs [missed shifts] ... they were rapidly adopted across the entire 911 engine line.

The broken rockers tend to minimize the visual, aparent damage, but not eliminate it altogether from the bottom end. In the 'good old days' of the forged rockers ... many times, after a mechanical over-rev ... bent rods, and cracked/broken pistons would be found!!! I doubt that reusing the rockers four years ago was a bad decision!

So, I think it would be false economy, and risky, to just replace the rockers, now, and button it back up!

My suggestion is to tear the engine down, fully. And, check the valves for straightness. Send the heads, crank, and rods off to a qualified machine shop to have them checked for straightness to factory specs. If they are OK, or are rebuildable, then you have a re-qualified bottom end to base a rebuild upon ... otherwise, make other plans!

Good luck!

[Pat S]

Well I pulled the heads and found that all exhaust valves contacted the pistons and most are bent (one bent very bad). I also found that nearly all the guides are cracked and one was broken.
Pistons and cylinders look good from a visual inspection.
Sorry for the blurry pics.



Whew, this is going to get expensive... maybe I should upgrade to a 3.0 instead

Pat