Checking M20 Rocker Arms

[douging]

Is there a way to tell if a rocker arm was recently replaced? Or if there is a tell-tale sign that the rocker(s) need replacement? Other than one separating itself...

I bought a 1987 325is for a track car that has had maintenance. There are some new-ish parts on it. I took the valve cover off and there's not a lot of gunk anywhere. The rockers look new, especially compared to the new OE BMW arms that I have on hand. And they're not broken obviously.

Should I dive in and replace them anyway?

TIA

Doug

[nates 87 325is]

No

[douging]

Hmmm, 2 of them say Febi. The rest look like the BMW logo and p/n.

I guess I'll proceed with replacing all of them.

[jlevie]

Since this is a race engine, polish the area of the rockers around the eccentric forks. And use new OE rockers. Best to adjust the valves every other race weekend, but certainly not less than every fourth weekend. And don't rev the engine past the factory (6500rpm) redline. The ideal shift point is well less than than (~5800rpm), but in a race you'll exceed that occasionally.

[douging]

Thanks Jim. Your posts and insight are always helpful (SE30 and elsewhere).

[BergerNJ]

Since this is a race engine, polish the area of the rockers around the eccentric forks. And use new OE rockers. Best to adjust the valves every other race weekend, but certainly not less than every fourth weekend. And don't rev the engine past the factory (6500rpm) redline. The ideal shift point is well less than than (~5800rpm), but in a race you'll exceed that occasionally.

Quick question Jim, what is your opinion of the IE hd rockers? I know its cheaper and probably just as effective to run the polished rockers, but I know you have to have an opinion on the subject.

[MR 325]

Quick question Jim, what is your opinion of the IE hd rockers? I know its cheaper and probably just as effective to run the polished rockers, but I know you have to have an opinion on the subject.

The polished OEM rockers will still not be as strong as the IE HD rockers. I am a big fan of them for the price.

[jlevie]

Quick question Jim, what is your opinion of the IE hd rockers? I know its cheaper and probably just as effective to run the polished rockers, but I know you have to have an opinion on the subject.

I have no experience with the IE rockers so I have no opinion as to whether they are an improvement. But nothing I've seen suggests that anything stronger than the OE rockers are needed. Well, not unless the engine has been heavily modified to make power high in the rpm band and a higher rev limit is used/justified. In that case a better rocker might be justified.

The torque curve of an M20B25 engine results in the optimal shift point being less than the redline (6100rpm for my Spec E30 engine). In a race situation, you may wind up running past that, even to the rev limiter. But generally, going past the optimal shift point is a loss of performance and is something to be avoided. The OE rockers, especially if polished, are fine for that realm.

My theory on broken rockers is that the cause is a history of poor maintenance. Specifically a lack of timely valve adjustments. Excessive valve clearance results in greater impact forces as the rocker meets the valve stem. Those repetitive impacts eventually result in cracks and the rockers break. Broken rockers on street cars with automatic transmissions aren't unknown. Those engines spent little, if any, time at the high end of the rev band. So something other than high speed operation is the cause.

The "book" calls for the valve clearance to be set to 0.010" cold. But if the head is in very good condition setting the intakes to 0.006" and the exhausts to 0.008" gains a tiny bit of performance and reduces impact forces on the rockers. This works on a built race engine because it will have a fully rebuilt head and the head will be rebuilt every few years. Those clearances might not be enough on a head with worn guides or a worn cam.