Hi all,
All 3 exhaust rockers broke on one side (4,5,6) when I started my engine for the first rime after cam and rocker re grind.
What’s the likely cause here folks?
Thank you

Lots of possibilities. None of them good.

I'll start with the obvious. Did the engine rotate freely when assembled? Did you verify proper valve lift?

Then to the less obvious.

Did you verify that valve springs aren't going into coil bind with the new and/or reground cam? Did you verify rocker shaft OD to rocker ID is proper? Did you verify that the 4,5,6, cam is timed properly prior to firing the engine?

Sort of odd that all 3 are on the same bank. Seems to suggest cam timing to me.

Did you verify proper piston to valve clearance?

I'll stop there and suggest that it doesn't really matter what anyone (including myself) speculates. Only sensible thing to do is to remove motor, tear down at least that bank and get to root cause.

Rotated freely, cam only changed from T to E profile. valves pistons etc undisturbed

At what point did they break?

Did the cam timing change from T to E?

I'm with Phil, tear into that side.

I've been told in the past that some year of T pistons do not have adequate valve pocket that would match an E piston

don't know if that is true for your motor model

but since it was all on one bank.. really points to that cam timing for that bank

all of the above.
Consider yourself a bit lucky. If you had earlier forged rockers the damage
would likely have been much worse.

Ugh what a sick feeling. My guess is wrong cam timing and could the down have been missed when timing that cam or forgot to TQ the cam nut?

Assuming the pistons hit the exhaust valves while still partially open when at top of stroke. Cam timing needs to be off quite a bit (delayed) if it hard hard enough to snap the rocker arms. By any chance did you/someone forget to insert the locking pin into the cam gear. Tighten the cam nut and it will rotate by hand and everything looks good. Under power, the cam gear slips and that's it...
If you are real lucky it didn't trash the valves or pistons.

The valve hit the piston.
Either valve timing or clearance.
You will need to pull the heads and inspect piston tops and see if any valves are bent.

So sorry about this, but hopefully you find some way to smile about this some day in the future.

I seem to think i read the same thing ,there is a reasonable increase in lift with the "E" cam over the "T" cam however shouldn't be an issue .
I'am presuming this is a original 2.0L 914/6 motor ,issue here is the 'E' cam will only make more @ the top end with higher RPM (6500) and the 2.0 and 2.2L motors made 5800 rpm due to non-counterweighted crank.
The rockers I think in this era were cast and designed to break with valve to piston contact,this was meant to prevent bent valves and it appears on an over rev or contact that's what they do,think on a race motor it may be a less than happy outcome
however .
with the comments on bad cam timing on that bank but then i also think about a collapsed chain tensioner also ...maybe,i have chain safer collars on my tensioners.
This must be an awful result for the OP.

Bad News, you bumped cast pistons....

cam timed 180 degrees out? It happens.

Like @FIxer34 suggested, missing or sheared cam pin on the RHS cam would fit the symptoms (turns freely on the bench, slipped timing under mechanical and thermal load). Interesting to see what the teardown reveals...?

- Tony

Cam timing on my 6

This is not so easy and you need a number of special tools to do it.

Bob B

Click to view attachment

Many shops have the tools ,but have done 180 out and with t piston /e cams no room

Have you checked the valve to piston clearance ? This is a step you can't miss if you swap out cams. What engine size are you working with? E cams into a T engine?
Like stated above, it sounds like the valves hit the pistons. You may want to take a look at the pistons and valves. If the rockers broke more then likely a valve or piston maybe damaged as well.

I've done cam timing on several 911 engines. By far the easiest method/tools I've found is by Stomski Racing. I use their digidix tools and I can get dead nuts results.

https://www.stomskiracing.com/products/digidix

If you're gonna just do this once the price of the tool may or may not be for you. I've used this tool on two engines and have one more to do. Watch the video on the link if you haven't already seen it.

I have lots of experience with setting the cam timing on 911 engines. I always suggest that you remove ALL the rockers on both sides, then put in just the intake rocker for the side you are currently working on. Once you have it set, then put in the one on the other side and set it.

I always use two digital dial indicators, and once I have them both set, I go back and make about 4 more rotations to verify that the valve clearances are correct and the cams are timed right.

Only then do I put the rest of the rockers in the motor.

If you do this, and are careful about not forcing the engine to turn when you get resistance, you will not bend any valves.

Just my $.02.

Clay

Same here.. Using the crankshaft position encoder I can check my work on both sides at the same crank position over and over again, back to back.

I'd expect the situation the OP has posted has ben some exhaust valves due to timing retardation.. Having things tensioned properly is a key to this. If the tension isn't correct the timing will change during engine operation, Vs what it was set with statically on the bench..