Got a skoda favorit running on cbr1000 bike carbs, ported head and fast road cam.

We have snapped two rocker arms now...

Dont know whether we are getting contact with the pistons or if there is something else??

Dont think there should be any contact unless the head was skimmed too much, but then we would get them breaking constantly would we not?..

Can anyone help us?

Thanks

It does sound like you are getting piston to valve contact,how much was skimmed off the head? can you fit a thicker head gasket? Was it the same rocker that broke a 2nd time or a different one?

Odd; the rev limitting factors on these engines are usually the pistons or the pushrods (which bend if you go much past 7_000 rpm; original source John Haugland).

I've seen these break before, it was caused by the head being cooked up previously, when it was skimmed to make it flat again it ended up with one side being completely flat and the other side was like a banana, it happens a lot on the vauxhall ohc engines too.

It does sound like you are getting piston to valve contact,how much was skimmed off the head? can you fit a thicker head gasket? Was it the same rocker that broke a 2nd time or a different one?

Its a different rocker that broke each time.

We think the valve springs are compressing fully but the rocker is still trying to compress them more so are breaking????? The valve stems sit lower on the two that broke so the spring is already slightly compressed before the rocker pushes it down. We have now ground the ends of the springs down slightly so that the overall compressed size is slightly less. This seems to have worked on the first one that broke, time will tell if it works on the other one.

Hopefully its sorted and no more break. We'll let yous know how it goes.

The head has been skimmed quite a lot as we are running about 11.5:1 compression. I think standard is about 10:1.

Er, changing the free length of a valve spring changes the spring rate.

Yeah i know what ur saying but the springs are uprated ones anyway so dont think its too big an issue grinding them down slightly

How "slightly"?

How "slightly"?

Less than a mm. Seems to have made all the difference though. Stops the springs compressing fully so reducing the strain on the rocker arms.

Well, yeah, it would do - going coil bound means an immense load on the valve train, so I'd check for bent pushrods too. That won't make much difference if any to the spring rate, although you may have been able to shim them differently - I've seen various thickness shims below the springs on different heads. Sounds like the cam you have is higher lift than the one on the engine that head was fitted to before.

sounds like you need to check what thickness steel washers are under the springs too and also the amount of lift you actually have, maybe there is a problem with the followers binding too (they can kick sideways a bit if the block is worn).

an exworks gpA motor we had here had shims under the rocker posts to get a better angle on the rockers thruout the range of movement too as the more you skim off the head the worse the rocker goemetry gets due to the "longer" pushrod.

what are you revving it to?