piston to valve clearance

jonboy

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Basically there isn't any...

I did a major topend overhaul...new cam, valves as necessary, guides, head skimmed.....put it all back together and a bout a year 4k kms later i blew a hose and overheated, blew the head gasket....
So, off with the head and to my horror there is a very small crescent shaped, fingernail paring sized mark on the top of every piston crown, and a corresponding polished mark on the circumference of each inlet valve, maybe 1cm.... the valve is obviously fouling the crown, but the mark on the crown, though visible, you can barely feel, more in the carbon really, but definitely there's contact.

I'm thinking I'm not going to be able to skim the head again,
and so what are my options do you think...the engine was running fine after I'd replaced the water hose, though only a minute or two and before I realised the head gasket was chuffing, and no horrible noises so no obvious serious structural damage....

how about machining the top of the valve heads, adjusting the tappets to open the valves less, thicker or double head gaskets??

all advice gratefully appreciated Thanks JB
 

m5bb

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Wow, that hurts.

Sounds like the head has been shaved too much and with that maybe the block.
I would be worried that the valves may be bent and therefore would never seal properly.
About the only good solution is a thick head gasket.

This company makes thick gaskets. Never used one but have heard of others that did.
http://www.cometic.com/
 

jonboy

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well not such bad news as I feared....talking to the machinist who is an old guy who used to race e9s in the 70s against Capris and suchlike.....he spotted straight away a crack between the combustion chamber and a water passage...his opinion that the pressure and heat going into the cooling circuit set the guage into the red, blew the weak hose and tho it overheated it was a wet heat and the motor probably didn't reach catastrophic temperatures. it doesn't seem to need skimming again but he wants to mike it up after the weld repair has been done and the head is bare ....then we'll see if the valves all seat ok and as to the piston crown he reckoned there was plenty of meat on top to just lap away the marks and a bit more.... he pointed out also the mark on the soot of the valve was a mm or so from the circumference edge of the valve and not near the bit that seats against the seat, if you see what I mean....like its not the edge hitting the piston but the top of the mushroom where there is the possibility of machining off a thou or two . Thanks for the input, but the cometic lot don't seem to do an E9 , and buying stuff from the states is a pain with delays and arbitrary customs dues and hold ups...I'll see what Walloth and Nesch have to say or one of the UK specialists
 

sfdon

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How thick is the head?
What date stamp is the head?
How many thousandths above the block are the pistons?
 

jonboy

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HI ...I'm no engineer but with a reasonably good vernier caliper, the piston edge is proud of the block at tdc by 0.8mm, possibly a tad more as I can't be sure its absolutely TDC. (on 1 and 6...)

the block thickness is 129mm dead at each end but because of the angle I can't get a good reading amidships..

the date stamp is 79 with about about 4 o'clock on the pips on the quadrant circle
part number 1 250 0199

The block is original '75 according to BMW Archiv and the engine number and documents so the head is later...perhaps not even E9 ???

Thanx for your interest..
ps i've just got the cam and all the valves out and there seems no problem...all seating nicely, rotating in the guides, no binding, so I'm guessing nothing bent there...
 
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Honolulu

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Depending on your local resources, another used good Big Six head may be more valuable at this point than having to weld and ( I think) shave that head you have, plus getting a thicker head gasket, or shaving pistons.
 

sfdon

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I'm with Honolulu on this.
Wouldn't think of using a cracked/welded 1979 head.
The goal is a good head with a date of 1982 or later.
 

jonboy

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in a perfect world ....!.....but there is less than a handful of this model in Portugal and not many sedans or the others with the same engine either...don't just popdown to the local scrappie and find one, i'm sorry to say.....

And as labour per hour is relatively cheap and there's a really strong mentality of fix rather than replace, i'll wait and see what happens after the welding... I took the head in this morning and the only issue was the valve seat has to come out and will be wrecked in doing so, so the only new parts so far in this episode will be a new valve seat and necessary head gasket....unless it the whole head turns out to be stuffed anyway...and the rectification costs.
I'll keep you posted
regards JB
 

hans3

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During that initial topend overhaul, why wasn't the head pressure checked for cracks--the one that ultimatley caused the overheat?
 

jonboy

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maybe the head wasn't cracked then....? don't know honestly, but can you pressure test a head off the engine,? as the first top end overhaul was in the machinist's and the car was at home....he's a great machinist but definitely old school....I'm guessing pressure testing requires diagnostic kit at the dyno level, and my man is a miller turner, ear to the block with a screwdriver kind of guy.
 

jonboy

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UH oh !! Don't tell me I didn't suss this one out sooner... and no one else spotted it!!! because M5bb I've just seen post no 6 on the 1972 3.5 rebuild posted in restorations and projects by you, the photos of those sexy pistons..

That photo's my piston crown dings !! is that little crescent standard on the 3 litre as well, or are we talking high comp 3.5 pistons only and I do still have a certain little kiss going on ??
let me know !! JB
 

hans3

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maybe the head wasn't cracked then....? don't know honestly, but can you pressure test a head off the engine,? as the first top end overhaul was in the machinist's and the car was at home....he's a great machinist but definitely old school....I'm guessing pressure testing requires diagnostic kit at the dyno level, and my man is a miller turner, ear to the block with a screwdriver kind of guy.


Machine shops can quickly and cost effectively pressure check a cylinder head once it's off the car. The head coolant passages are blocked off so that air can be pumped in (~40 psi). Then the whole head is submerged in water to check for air bubbles. I recently paid ~$45 for this to be done to my cyl head.

https://www.youtube.com/watch?v=3aiBdZyqhy4
 
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