Does a Broken Rocker Arm Automatically Mean a Bent Valve?

ber911 · 03:22 AM

Trying to learn as I go along. Buzzed my 2.0 race motor a couple of times at a previous race meet on downshifts getting used to the 901 box. Had a full days racing on Saturday, car seemed to be running good and then 20 mins into the last race of the day the motor suddenly went off song. I thought I had run out of fuel as I knew I was getting pretty low.
Filled up the tank on Sunday morning, but the car still ran the same. Pulled off the top rocker covers and part of the rocker arm on No.5 intake fell out. Then did a compression test - the other 5 cylinders were between 155 and 165 psi, No.5 was about 30 psi. Would this be because the intake valve stays shut and therefore the cylinder cannot draw in any air to build up compression or more likely because the valve is bent?
Will do a leak down test as soon as I can borrow a leak down tester and compressor but wondering in the meantime if I have any chance that the valve is not bent?
If the leak down test turns out good, can the rocker shaft be changed with the engine in the car?
Any advice and information appreciated.

Rob

Dan J · 09-29-2014, 03:31 AM

Leak it You never know
If the valve reliefs are correctly cut you might be ok

robfike · robfike's Garage

I miss-shifted at Road America and broke one rocker arm and bent another.
This was in my 2.2 race engine.
The rockers were not turbo units, just standard.
I just had to replace the rockers, no valve damage was done.
Lucky I know.
Rob

billybek · billybek's Garage

With the rocker broken and valve closed I don't think you would have had an unfortunate meeting of parts.
You are probably onto something with your analysis. With the valve closed you are just re expanding the left over air in the cylinder.

ber911 · 09-29-2014, 04:05 AM

Sounds like at least I have some hope of getting away without a bent valve(s).
What would cause the rockers to break after an over rev if the valves did not hit the pistons?

Rob

Dan J · 09-29-2014, 05:09 AM

The valve did hit the piston It just hit it square

john walker's workshop · 09-29-2014, 06:12 AM

put a rocker back in and see what the compression and leakdown is. broke one in my 930 when the clutch slipped under boost and the valve didn't bend.

Tippy · 09-29-2014, 06:13 AM

If you bent the valve enough, it won't move at all. BTDT!

I'd say put another rocker on, gently turn engine over by hand, and turns over ok, redo compression check.

T77911S · 09-29-2014, 06:26 AM

leakdown test.

Lapkritis · Lapkritis's Garage

The force is sheer against the stem even with the relief cut in the piston... To have contact while running but not bend the valve is luck. I have seen it before though in other interference engines with a lower angle difference and timing belt failures. A slight bend of the valve head on the stem is fairly common and will manifest as lost compression as the valve doesn't contact well on the seat with the bend. A valve with a weakened stem is spooky... have seen plenty of valve heads snapped off an forcefully mounted into piston crowns. Every time it returns to the seat it's pounding away on a weakened point.

theiceman · 01:07 PM

if the arm broke of its own accord you will be fine as the valve retracts. into the head. If the valve punched it i think you are going to have bigger issues.

Can it be done in the car ?
I would thinking pulling the engine would be a whole ot easier thats for sure, but yes it certainly can be done.

ber911 · 09-30-2014, 12:50 AM

Maybe I got lucky - leakdown on No.5 is 10%, which is pretty much the same as the other cylinders. Next step is to replace the rocker arm, do a compression test and if all looks good, run the motor.

Rob

Walt Fricke · 09-30-2014, 09:45 PM

I'd say if it leaks like the others, you should be OK. If leakdown is good, compression should be too.

I assume that you did not hear air leaking out the intake, and that all that 10% was by the rings?

And that you have looked around to make sure all of the other 11 rockers are still OK? You can see and push on just enough of the exhaust rockers that you can confirm this through the intake side.

I bent all six exhausts once - 1st instead of 3d. Engine didn't run very well after that, but with some fiddling it seemed back to normal. I was transitioning from a VW bus, so of course the 911 felt peppy. Ran for over a year that way. The exhaust valves, whose stems were sort of S shaped, sawed a groove into the guides so that the heads pretty much seated.

Hell of it is, after I had the heads off and guides and valves replaced, within a year the head of an exhaust broke off and fell in, which did a number on the motor for sure. I suppose that could easily have happened while my bent valves were wearing themselves more or less back into a weird sort of functionality.

Still, I've replaced a broken rocker at the track, and there have been no ill aftereffects. It is certainly luck.

ber911 · 10-01-2014, 02:00 AM

Walt - there was definitely some leakage through the intake valve, but not much different to the other cylinders.

Replaced the rocker arm, set the valve clearance and did a compression test cold only on no.5 - 125 psi vs 155 psi on the others when I tested them hot previously.
Started the motor and it sounded all good, but then heard a clatter in the vicinity of No.5 when I revving it above about 2000 rpm. Rechecked the valve clearance - was maybe a touch too tight and tried again, but no change in the noise. Removed the rocker arm and ran it again - obviously was running rough on 5 cylinders, but no untoward noise when revving the motor up.

I noticed that the rocker arm which I took out of a 1972 motor was different to the broken one, which I suspect was from a 1970 motor, but I cannot see the rocker arm being the cause of the clattering.

So what about the possibility of a broken valve spring. Could this cause a clattering noise and could the valve spring breaking cause the rocker arm to break?

Rob

911pcars · 10-05-2014, 11:14 AM

Quote:

Originally Posted by robfike

I miss-shifted at Road America and broke one rocker arm and bent another.
This was in my 2.2 race engine.
The rockers were not turbo units, just standard.
I just had to replace the rockers, no valve damage was done.
Lucky I know.
Rob

911 rocker arms. 64-94. No difference IMHO.

Sherwood

Walt Fricke · 10-07-2014, 10:43 PM

Rockers for these years, after the end of the unbushed rockers, and not counting the RSR rockers which adjust with lash caps and are forged instead of cast, can have several minor variations in their castings. To my knowledge, though, none of those differences matter.

One guess is you bent the #5 exhaust. Exhausts are more prone to bending during a mechanical overrev. It may make more noise seating, or have a kind of dynamic extra lash - the bent part of the valve shaft abrading its way into the guide. I'd do a leakdown on this cylinder again, or just put in compressed air, and listen for an exhaust leak compared with the other exhausts. I'd pull the headers so I could isolate things. You could also spray Windex or the like up behind a closed exhaust, and pressurize the cylinder as if doing a leakdown. Any bubbles at all are bubbles too many. Taking the intakes off is quite a bit more work - but you probably have carbs, so that is not a big deal. Do the Windex (or soapy water) test on the intake also.

The fact that one rocker broke and its companion did not doesn't necessarily indicate that only the one valve (here an intake) is damaged. Could luck out with the intake, but not the more prone to such problems exhaust.

I've broken a fair number of valve springs, but always just one, inner or outer. Never made any extra noise. Either one is good to maybe 7,600 rpm before it floats and feels like a soft rev limiter (on a big cam race motor), the more so depending on how many of the valves have one broken.