How do rockers snap??

[charlE30]

Hi all, just finished replacing two brocken rockers (cylinder 1) in my 325i and was wondering what can cause this to happen?

Just over a year ago the previous owner had spent Ԛ£1100 on head work replacing... yep you guessed it a brocken rocker (cylinder 2)!!

I'm now thinking that there could be a serious problem with the engine as this surely shouldn't happen this often.

Your views on this one much appreciated

[low325icab]

hi mate ive had this happen to me twice now on 5 and 6 so luckly the head did not have to come off both times, as for why they brake, i dont really no but both times id been thrashing the car real hard

[d6dph]

you always thrash your car ryan

[Ziggy]

Incorrect clearances?

[E30Adam]

Ziggy is spot on here, rockers will take a hell of a pounding as long as the clearances are set correctly.

If the clearances are set too loose then the rocker won't glide over the cam lobe, instead it will impact it causing stress to the rocker and it will eventually fracture and break.

I would also only ever use genuine rocker as well because I've seen the damage a failed rocker can cause.

Bouncing off the limiter for extended periods can cause this too.

[hairypete]

aaah, its not just me then! lmao, oh well, i know you shouldnt thrash them too much, but when the six's are screaming so nicely its almost criminal not to feed them another set of teeth and keep them going!

does anyone know of a way to get hold of some stronger ones? where from and all that, cause i cant help worryin that when i get another zone chip and take the limiter up another thousand that it would be nice to have the peace of mind and all that.

[dsio]

The bloke that did the headwork on our E34 M20 2.5 offered a choice of titanium rockers and valvesprings (not sure how loosely they use that word, so could be just coated), or OEM parts, or machined OEM parts. He mentioned that the underside of the rockers in early M10s were extremely rough, with cracks and crevices in them from when they were made.

After a while they started to machine the underside of the M10 rockers, and then did the same thing to the M20 rockers in the E30s/E34s, but it still wasnt very good.

Supposedly, there is still some ridging and uneven-ness to the underside of the rockers, which usually gives cracks a place to start from through wear.

It is said (and to me at least it makes sense) that if there is a perfect finish, and not an uneven area or ridge, there would be less chance of cracking, as the link in the chain principle would apply.

He did a very carefull job smoothing them, to the point where they were mirror finish, before installation. Only 6 months on, but I guess they arnt broke, so they cant be too bad. He said he has been doing it on every M20 he has worked on since the E34 came out, and has never had to replace one of his own.

Perhaps someone with more mechanical knowledge could verify that?

[E30Adam]

That's a waste of time, they come with a good finish from the factory and don't break in this area anyway. Sounds like your guy's trying to make a quick buck.

[dsio]

I'll keep that in mind then. Was $30 to have it done so its no big loss, just made sense at the time. They wernt exactly expensive things to replace though, its more just the labour involved in having it done.

[charlE30]

I did think about the valve adjustment (after they snapped) and have made sure that there now set at the correct 0.25/0.10 when cold.
It's now had two timing belts and two headgasket replacements in the last 8000 miles so it should be good for a few more miles yet!!

I quite enjoyed stripping the engine and rebuilding it, just a pity it'll be coming back out to make way for my M30 3.5 conversion

Cheers for the feedback guys

[Turbo-Brown]

Also worth considering in the 'why do rockers break' debate is the fact that they're made from aluminium which has no fatigue limit, unlike steel which does.

Think about the cylces a rocker goes through, if you're cruising about at a reasonable 3000rpm, each rocker will rack up over half a million cycles in about 6 hours.

Just think about the number of cycles completed over the last 20 years (in the case of my car).

I'd say that rocker failure is almost inevitable, although overloading them with incorrect clearances can't help

[tomstickland]

I've known people to snap rockers on the Vauxhall 8v engines. Never happened to me though, even at 200K on the original items. Could just be a fact of life, but I'd expect there to be some cause.