Thomas Olivaux

Hello M5 fanatics.

I'm not an M5 driver (I do love this car!) but I bought a broken S38B36 and I'm starting to work on it. Is that the right forum to discuss about that?

When I bought the M5, with about 240 000 km on the odo, It was running on five cylinders only. The spark plug of the 1st cylinder was bent. I was thinking about a dead valve or piston... Last week-end, I managed to open the engine to discover the real problem : the 1st piston is dead and, execept some marks of aluminium to remove, the head seems ok

(phone pics for those two pics, sorry)




1st question :
Since I have some marks on the cylinder, I must bore it. Since I must bore the block and buy a new piston set, I'd like to overbore my engine. That's why I'm simply thinking of passing from 93.5 mm to 96.4 mm, like the S38B38. Do you know if I can use stock specs S38B38 pistons? I'll buy some new forged piston, but I would be less expensive buying B38 pistons than custom made pistons. Do you have the specs of B36 and B38 pistons, is it possible to fixe the B38 piston on the B36 rod and does the lenght between rod fixation and top of the piston the same?

Unfortunalty, when I stated to wash the head... AAAAAARRRGL, I saw a small crack... and two... ... and a total of five small cracks! All are located between the two exaust valves, valve seats not damaged.







I have this king of crack on five cylinders. Looking to the color of exaust valves and the blown piston... and knowing that the Motronic were opened at least one time before I bought it... no doubt about a "wrong" chip used with this engine...

I was about to drop the head but a friend of mine talked to his local aluminium specialist and it seems I could repair my cylinder head. For a very good price, he can :
- test the compression (won't continu if not tight)
- remove the valve seats
- repair the five cracks with "argon welding"
- install new valve seats (24 of them, by the wat)
- plane the head (if needed) and miror finish of the gasket plane

2nd question :
Do you think it's a good idea or am I totally dreaming about this king of repair?

++

Thomas Olivaux


PS : please excuse my english, hope it was clear for everyone :p

Jahangeer

The repair might be good, but then again it might not, are you prepared to go through the same thing again?
If so - go for the Argon welding, but personally I would look for another head.
Am really sceptical about crack repairing on heads, the integrity of the metal has already been compromised, then its going to go back in and again be subjected to a relatively streesful environment.

Maybe I am underestimating the Argon welding skills of your technician...

Do it right, do it once, will most likely be the advice you get from the other nutters here.

__________________
Regards
J

farrell

Hi
The repair is possible.
Its exactly where I would not want to have to repair a head
I would agree with Jahangeer.
5 repairs on 5 cylinders across the exhaust valves is something I would not
consider.
The thermal stress generated here is immense.
I fear that the massive extremes of co-efficient of expansion between the new materal added by the welding process to existing aluminium cylinder head will be too much.

I would seriously weigh up the cost & risk.
Look for another cylinder head would be my first suggestion.


Farrell