Valve/motor diagnosis

[Bill R.]

was oil starvation, if it is , it will be easy to spot because the rocker arm will be galled to the rocker shaft.. I've seen it before on other cars. However if the rocker can rotate and isn't seized to the shaft then how could oil starvation cause it to break where it did... If the bulletin i posted doesn't apply or has already been done, then it looks to me like valve float, meaning that the rpm was actually a ways south of 5k and the valves floated, When this happens i've seen it break both rockers on the same cylinder quite frequently.. along with bending valves and touching pistons.... Now if its valve float then i would definitely replace the springs when you replace the rockers and whatever bent valves you have. All the springs. John you may not have seen this before cuz desmos can't float the valves

[Martin in Bellevue]

Ireland has the 284 (& hairier 292) regrind & recommends their valve caps or their oversized eccentrics for the 292. Based on the marginal loss of low end torque from their 284, I'd be scared of a stock m30 with the hairier 292 cam. The 284 regrind doesn't lose its breath as much as the stock cam did, to 6k rpm.

https://secure3.nexternal.com/shared...unt2=496101021

It is Saturday. Pull that head off & take some pictures. Take care with those exhaust manifold studs to the downpipes. Definitely get Don Gale's banjo bolts. They should be stock on these things.

This is an opportunity to add a little something to the performance of yer 535,
Martin

[Dave535Phoenix]

may want to contact bmw about it. You might at least get some rocker arms out of the deal...
Rocker Arms

My chassis number is 13070 - in the range given in the bulletin. There's no paint spot I can see on the oil pressure switch stub, and there's no dealer sticker on the B pillar.

BMW might be making this repair....thanks man. I'll let you know how it works out.

[John B.]

If the bulletin i posted doesn't apply or has already been done, then it looks to me like valve float, meaning that the rpm was actually a ways south of 5k and the valves floated, When this happens i've seen it break both rockers on the same cylinder quite frequently.. along with bending valves and touching pistons....

Bill - What RPM would you have to hit to float the valves on the M30 engine? 7K? or more?

[Bill R.]

design criteria for the valve springs, rocker assy was probably well in excess of 7.5 k rpm however (here's the LBV part) if your familiar with valve float and valve spring design you probably already know this. The valve spring has a resonant frequency that occurs at x rpm.. meaning that when you compress and uncompress that valve spring at a certain speed it loses its spring tension and can't hold the valve shut. This is known info and the designers use alloys and spring tensions to make sure that this frequency doesn't occur in a rpm that the engine will be seeing normally. On a number of engines they use an inner and an outer spring on the same valve, since these springs are different diameters, spring pressures and or alloys they have their harmonic or resonant frequency occur at different rpms, so the outer spring may turn to silly putty and 7k but the inner will still be providing spring pressure at that rpm, the inners harmonic will be at a different rpm so they overlap and you have constant spring pressure to keep the valves closed throughout the engines designed rpm range. Now this is all fine and dandy and the single spring on the m30 works throughout the designed rpm range. But as springs age the metal fatigues and changes its characteristics. And typically the rpm at which they may float will change too... A quick definition of float is the rpm at which the springs aren't strong enough to keep the valve following the cam lobe and the valve can bounce open when this happens, IF the valve happens to be hangine open when the piston comes up, the piston smacks it HARD and slams the valve closed, the valve hits the rocker arm HARD and since the rocker arm is usually in the process of coming up towards the valve at the time , the weakest link gives and thats usually the rocker in this case.. At any rate when the valve springs get old the rpm at which they float can change and get unpredictable.. Thats why its a good idea to have a valve job done on a head any time you have it off for a head gasket, any machine shop worth their salt will check and set spring pressures when they do a valve job...

[John B.]

IF the valve happens to be hangine open when the piston comes up, the piston smacks it HARD and slams the valve closed, the valve hits the rocker arm HARD and since the rocker arm is usually in the process of coming up towards the valve at the time , the weakest link gives and thats usually the rocker in this case.. At any rate when the valve springs get old the rpm at which they float can change and get unpredictable.. .

This is what I was thinking when I saw the pics. It looked like the valves hit something hard (piston) & snapped the rockers. Popping the head off will answer that question.
So it might not be such a good idea to explore the extended RPM range of my EAT chip on a regular basis at 142K?

[George M]

Nice explanation Bill and you may indeed be correct if the rockers aren't frozen or bound to the rocker shafts. Or could have been weak rockers per the bulletin you provided. Dave do you ever recall over-reving the engine?
In any event all bases are covered as to possible root cause :-) Bill how do spring pressures get adjusted on a big six short of swapping out springs?..not aware of any different shims available to increase seat pre-load.
And Martin agree with ya...the grind you are running sounds perfect for a stock or lightly modded M-30 street motor and might add I like your philospohy as well. When something breaks always look upon it as an opportunity to make it better than it was...a great perspective.
George

[Bill R.]

Say at an average engine speed of 2k rpm the springs open and close
1000 times per minute
Now lets say that corresponds to 50 miles per hour at 2k rpm
Thats 60,000 times a hour and 120,000 time per hundred miles
or 1,200,000 times for every 1k miles..
or 12,000,000 every 10k miles
or 120 million times every 100k miles
So you've got to be hitting the 150 million times mark for squeezing those poor little bastard valve springs on your car about now..... Its a wonder they last this long...











QUOTE=John B.]This is what I was thinking when I saw the pics. It looked like the valves hit something hard (piston) & snapped the rockers. Popping the head off will answer that question.
So it might not be such a good idea to explore the extended RPM range of my EAT chip on a regular basis at 142K?[/QUOTE]

[Martin in Bellevue]

I forget which issue last fall, but Hot Rod had an intertesting article about how these springs were lighter & stronger, at least for the ubiquitous chevy small block. The springs required smaller retainers up top. They were shown to reduce valve float to much higher rpms. If a machinist could be persuaded to give 'em a try, they might be fun.

[Bill R.]

shims, the machine shops have standard shims in various thickness's and diameters for adjusting spring pressures... The thing that i have been wondering is that i can't find a spring pressure spec for the bmw anywhere.. I guess you have to take a new one or 10 and measure the installed height pressure to come up with an accurate number for what the spring pressure should be on them. The only numbers i have are for older cars 87 and older and that was 64lbs pressure at 1.48 height.

Nice explanation Bill and you may indeed be correct if the rockers aren't frozen or bound to the rocker shafts. Or could have been weak rockers per the bulletin you provided. Dave do you ever recall over-reving the engine?
In any event all bases are covered as to possible root cause :-) Bill how do spring pressures get adjusted on a big six short of swapping out springs?..not aware of any different shims available to increase seat pre-load.
And Martin agree with ya...the grind you are running sounds perfect for a stock or lightly modded M-30 street motor and might add I like your philospohy as well. When something breaks always look upon it as an opportunity to make it better than it was...a great perspective.
George