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Broken rocker arm.

E28 technical advice asked and given! Troubleshooting, modifications and more.
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bm0p700f
Posts: 413
Joined: Oct 23, 2007 3:00 PM
Location: UK

Broken rocker arm.

Post by bm0p700f »

Can anyone one advise on this.

M30b35 with 20k miles after a full rebuild. Full i mean everything internal new. Only castings,  crank and conrods areÿÿ original. There is a piper cam in there its higher lift and 280/276 degrees. Im.kn the u.k so availability of non standard rocker arms is poo

One febi rocker arm on no4 intake valve broke. 
Its been sounding like a bag of nails for the last couple  hundred miles. Had thought it was electrical. Changed the ignition system and that was worn (it came from the old engine was was older but while the car idle and responsiveness improved it wasn't right.

1) could there be any damage to the engine.  Since it runs well I dont think so. I suppose the only to find out is pull the head and inspect everything. 
2) is there any way to remove the intake rocker shaft without pulling the head? Doubt it given it no 4. 
3) while I get i need to check all the rockers is it safe to assume if they are all not cracked i only need replace one.
4) how does one check if a rocker is cracked apart from looking at it and if i remove all of them I might as well chmage them all.

I assume as its a fresh build i can avoid skimming the head and lapping the valves and machine valve seats. It was only done 20k miles ago. 

I just want to avoid this again (might need to get a rev limiter programmed back in as i might have taken it too far amd check the valve clearances as maybe these were set incorrectly) but dont want to spend money needlessly. Also I did have an eccentric nut work loose a few months back. Perhaps that did damage to the rocker.

Fortunately I have a neighbour who is handy as while I get what to do I have never done more than oil chnages spark plus ignition system stuff before.
 
turbodan
Posts: 9366
Joined: Jan 09, 2007 10:19 PM

Re: Broken rocker arm.

Post by turbodan »

A rev limiter isn't even really an option, you need that.

High revs are a common cause of broken rockers. This will be exacerbated by the more aggressive camshaft. The stock cam has a more conservative profile, the ramp up from the base circle is relatively long and the lobes aren't as steep. Aftermarket cams trade some of this off. In addition to the more aggressive lobe profile, the increased lift increases the spring pressure the rockers are subjected to. This is the nature of NA engine building. Tradeoffs are inevitable. This is one way turbo e28s really come out ahead, you can make 400+ rwhp with a bone stock camshaft and valvetrain. 

Rockers won't crack, you'll see no advance notice before failure. I know a guy who used to work in radiology that once x-rayed his rockers to ensure there were no casting defects. Since most of us don't have the means to do so we just have to hope for the best.

 
bm0p700f
Posts: 413
Joined: Oct 23, 2007 3:00 PM
Location: UK

Re: Broken rocker arm.

Post by bm0p700f »

Id probably best replace them all and get a 6000 rpm rev limiter programmed in. 

In fact I used to bounce the old engine of the 6500 rpm stock limiter all the time. I suppose that the difference the cam makes 
bm0p700f
Posts: 413
Joined: Oct 23, 2007 3:00 PM
Location: UK

Re: Broken rocker arm.

Post by bm0p700f »

Well head off and found a broken chain tensioner (one tab missing of the piston) and the engine tining was out by 1t.

So parts ordered now so it will go back together right.

All rocker shafts and arms are being replaced along with new gaskets ofc. 

Got to drop the sump too to find this tab..

 
jayjaya29
Posts: 208
Joined: Jul 30, 2014 12:53 AM
Location: Atlanta, GA

Re: Broken rocker arm.

Post by jayjaya29 »

The tensioner failing is probably what caused the broken rocker. 
turbodan
Posts: 9366
Joined: Jan 09, 2007 10:19 PM

Re: Broken rocker arm.

Post by turbodan »

Maybe, but it's more likely that it would be an exhaust rocker if it was a matter of timing and valve interference. 
gadget73
Posts: 1308
Joined: Nov 22, 2017 10:30 PM
Location: New Jersey

Re: Broken rocker arm.

Post by gadget73 »

a straight edge across the tops of the valves will tell you if one is bent and stuck partly open. With rockers out all of the exhaust should be at the same height, same with the intake.  If one is low, you have a problem.  Compression test will confirm. 
bm0p700f
Posts: 413
Joined: Oct 23, 2007 3:00 PM
Location: UK

Re: Broken rocker arm.

Post by bm0p700f »

There no way to put a straight edge across the valves. They are in the head and the top of the valves is below the head separating walls. For each cyclinder.

an excdntric nut came loose after the rebuild so i suspect this had something to do with the failure also the valve lash for the cam im using is much larger than how it was set by the engine bullder so that also didnt help. So rebuilding myself correctly and it should be fine. Dont think anything is bent. The head is flat and each valve has a good seal.

Ill test compression once the head ks on.

 
bm0p700f
Posts: 413
Joined: Oct 23, 2007 3:00 PM
Location: UK

Re: Broken rocker arm.

Post by bm0p700f »

Its fixed along with a couple of other issues the engine builder got wrong. Runs better than ever.

New rockers new shafts. New no4 inlet valve as it was too short. CM timing set properly and bingo one lovely sweet m30b35

 
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