52 replies to this topic

[giz]

hi its me again!

after re fitting the standard rockers to my new engine i took the car up the m42 last night.
after about 6 miles there was a knocking noise coming from the engine so i called breakdown rescue and got pulled home.
i've taken the rocker cover off and found the problem, a snapped rocker. it's broken clean off the shaft at the pushrod end.
is this uncommon or is there a good reason for this happening before i put another set on?

engine spec
CAM4180 head skimmed too much for class1 auto grass! with inlet and exhaust opened up to match gasket & manifolds, chamber heart tips rounded a little
standard forged rockers
genuine cooper 3 into 1 exhaust manifold
kent 276 cam
good gear box from a low mileage 1275 allegro engine
3.1 diff

[minidave1991-89]

Hi mate, sorry to hear about your rocker.
I have a full rocker set for sale if your interested for £26.00 with postage if your interested?

[100ev8]

i assume you set the valve clearances ? and made sure all the rockers were sat and seated well in the pushrods ?

[minikidx14]

Could the valve of hit the piston?

[tiger99]

You do mot mention which valve springs you are using. A broken rocker is often due to the springs becoming solid when compressed. Easy to check, once the clearances have been set, just turn the engine over by hand and confirm that there is always some space between the coils. If they are getting close to becoming solid, you have an unsuitable combination of parts, i.e. too much valve lift, set by cam and rocker, or too thick a spring.

[giz]

well......... there was plenty of gap between the spring coils and i had a good luck around the pistons and valves with a camera rather than take the head off.
i put another set of rockers on in case there was a fracture or weakness in the rocker.
it started up and idled fine i drove it 2 miles if that and bang snapped in exactly the same place.

[AndyMiniMad.]

Any pictures? How did you check the pistons without taking the head off? do you have a fancy camera that looks through the spark plug hole?

The problem could be a bent valve stem. If it jams in the guide then it could cause the problem you describe. but god knows how you got it runing again with just new rockers..

Edited by AndyMiniMad., 31 December 2011 - 05:07 PM.

[tiger99]

If the springs have clearance, could the collets be bottoming on the end of the valve guide? I can't see what else could prevent the valve opening fully, which is about the only thing that would rapidly overstress the rocker, except, as you suspect, valves hitting pistons.

As for hitting the piston, well you could take the head off, which is not a very big job, put some plasticine on the top of each piston, put the head back, with the old gasket for now, check that the clearances are correct and adjust if necessary, and turn it over a couple of times. The plasticine will tell you how much clearance you have.

[giz]

this is my daily car but i have lost the enthusiasm to get it running again so it's going to a mechanic in the new year.
i presume this mixture of cam and head has been done before? i'm not sure how much has been skimmed off the head and whether simply putting an un skimmed cam4810 will cure the problem.
i can't afford to spend more money on a new head, as suggested in a previous topic, such as a 295 or 202 etc and there's only so many times i'm prepared to put my head under the bonnet doing the same things over and over again just to have the rockers snap again.

[minidaves]

angle of rockers to valves could be the problem with a skimed down head

[giz]

i don't know how much has been taken off, all i know is that it was .010" too much for class 1 auto grass

[bmcecosse]

Are you SURE they were 'forged' rockers ?? Usual cause of a broken rocker would be coil-bound springs - but the sintered rockers are relatively weak, and who knows what life they may have had before you got them? A picture of the 'forged' rocker would help us tell you what happened. The tiny valves on that head cannot hit the pistons.

[giz]

the first set of rockers i used look like the mini sport 1.5 high lift rockers after looking through images on google, these caused the springs to compress a lot more and bent the pushrods (all of them) witht he gap set at 15 thou.
i then used 2 sets of the later a+ standard rockers which both snapped at cylinder 3 exhaust valve

[bmcecosse]

Ahh -so none of them 'forged' rockers.. You obviously have either springs going coil bound - or possibly the exhaust valve hitting the block - although I would have expected it to go down the bore........ Or indeed - a valve that is seriously seized in the guide !

[giz]

so what you are saying is that either the head is skimmed way too much for this head or one of the valves is suspect?
so in theory another un skimmed head should solve it?