Broken rockers

[blackbetty]

Hey all, I'm hoping someone might have come across this problem and might have a solution. i have a mildly worked grey with about 1200 miles on it, During this time it has broken three rockers on number one inlet valve. When the engine was built the rocker shaft was machined and the bushes had good tolerence. The push rod is the correct lenght of 259mm. Tappet clearance was set hot was 8-9 thou. When turned over we couldn't see the valve spring binding which sort of makes me think that it's a bent or sticking valve. The most confusing part of it all is the push rod stays straight yet the the back of the rocker is nearly ripped off. Any Ideas?????

[bootlegger]

Ive got a grey race engine in a boat. To reinforce the rockers they crimped metal over the rockers. Im not sure if it helps but i havent broken any.

[blackbetty]

Ive got a grey race engine in a boat. To reinforce the rockers they crimped metal over the rockers. Im not sure if it helps but i havent broken any.

Sound interesting, but what do you mean crimped? Any chance of a photo.?

[Harv]

Mmmm.... either that or the rocker is binding under load. Might be time to recheck bush clearance again just in case.

In any case, funny its not bending pushrods or valves. I remember there are left hand and right hand rockers, but havn't got the drawing beside me (or a clear enough head) to think through what would happen of you swapped a left for a right.

I assume no sign of lack of lubrication (scorching, blueing or coking) on the inside of each of the busted rockers?

Cheers,
Harv.

[blackbetty]

Thanks for the thoughts guys, I'm reassembling and setting the tappets tonight and might try your advice Tim. I am replacing the rocker shaft and all rockers and just for good measure the pushrod too. As for cam specs I have no idea but at idle it will settle down at about 900 rpm with twin strommies so I don't think it too big. Upon dismantling the last two time there was no sign of heat fatigue or bluing and when the rocker cover is removed all tappets are nice and oiled. Will let you guys know the outcome. Once again many thanks.

[blackbetty]

for your viewing pleasure. BTW Tim tried the bigger valve tolerances and the engine seems to be easier to start and have a little bit more poke, but i think only time will tell.

[bootlegger]

Never seen one break like that.

[Harv]

Man thats a wierd spot for a rocker to break - that part of the rocker is under constant compression load (albiet cyclic... TIm's fatigue theory sounds right).

Did they all break at that point, or closer to the shaft (where they come under bending/shear load)?

Cheers,
Harv

[blackbetty]

Tim, originally I set inlet 8-9 thou and exhaust 12 thou. After speaking to you guys inlet is not 11thou and exhaust is 14thou, I'm going to let it run like that for a week or two and see how she is. I have broken number 1 inlet three times now and they all broke in the same way. Each rocker has lasted approx 400 miles. the craziest thing about it is i'm not revving the car that hard because it's a hyrdamatic car and I've changed the diff ratio 3.36. the secon one broke whe I was doing about 50mph on a cruise. Any way will keep you guys up to date with any news.

[Craig Allardyce]

How much valve lift are you getting with your camshaft? i.e. you might need to check valve to piston clearance, especially if the head has been shaved a fair bit. If you get valve to piston contact you might not bend a valve as they are dead vertical to the piston, not like red motors which are slightly angled. The impact of the valve to the piston might be getting transfered to the rockers if they are the weakest point. Also as the engine warms up and the valve expands it will get closer to the piston and a slight over rev might be enough to do the same
Also might pay to check your valve spring tensions if the head is modified. Maybe the spring tension is too heavy causing excessive load on the rockers. The springs could also be too light if a larger valve is used and you would get valve bounce at lower Rpm's and possibly some piston contact due to inertia of the heavier than standard valve.

[vmx-mxr]

How much valve lift are you getting with your camshaft? i.e. you might need to check valve to piston clearance, especially if the head has been shaved a fair bit. If you get valve to piston contact you might not bend a valve as they are dead vertical to the piston, not like red motors which are slightly angled. The impact of the valve to the piston might be getting transfered to the rockers if they are the weakest point. Also as the engine warms up and the valve expands it will get closer to the piston and a slight over rev might be enough to do the same
Also might pay to check your valve spring tensions if the head is modified. Maybe the spring tension is too heavy causing excessive load on the rockers. The springs could also be too light if a larger valve is used and you would get valve bounce at lower Rpm's and possibly some piston contact.

It's an old trick but one worth trying I reckon.

Try poking a length of thin lead solder wire through the plug hole in the direction of no1 inlet (the one that keeps breaking rockers) and hand rotate through full stroke. If it binds (ie the 'clearance' is less than the solder diameter) no harm done just back off and if it goes full rotation the valve isn't contacting the piston or alternately the solder may compress and show the clearance. Then increase solder diameter and repeat. Compare with unaffected cylinders too. Just don't force anything (doubt it would bend a valve but not worth the risk)

Dave Mac

[fingers]

It's an old trick but one worth trying I reckon.

Try poking a length of thin lead solder wire through the plug hole in the direction of no1 inlet (the one that keeps breaking rockers) and hand rotate through full stroke. If it binds (ie the 'clearance' is less than the solder diameter) no harm done just back off and if it goes full rotation the valve isn't contacting the piston or alternately the solder may compress and show the clearance. Then increase solder diameter and repeat. Compare with unaffected cylinders too. Just don't force anything (doubt it would bend a valve but not worth the risk)

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Great idea Dave .........Ya learn something every day on this Forum ......and I like ya thinking mate its seems like the logical fault ...Fingers

[hagar]

The cam would have to have huge lift to contact the valves in a grey motor and unless it has huge overlap the valves should be well out of the way when the piston is at TDC.The most common cause for rocker breakage is coil bind ,get the rocker on full lift and check you have at least .040" between coils. Cheers Gary

[blackbetty]

A bit of feedback for everyone, after reassembling and inspecting for valve bind and other issues, we have determined that the rocker was shifting along the shaft towards the back of the car causing the push rod twist the rocker as it opened. I have just purchased a very good set of rockers so if it happens again I'll be putting solid bushes in. Fingers crossed it doesn't happen again though.

[Harv]

Hmmm... from memory, the only thing stopping the rockers shifting sideways are the springs on the shaft (that and the tightness of the busing tolerance, which you checked). Were the springs crook (compressed, soft)? I'm trying to think what would load the rockers sideways... valve stem not square ground at top?

Cheers,
Harv