S52 pistons in otherwise stock M42 (ohh I've searched)

[M5wanaB]

Just wondering where s52 pistons will put me in terms of displacement and compression ratio in an otherwise stock M42 engine. I have a spare engine that blew a head gasket and was left to sit with water in a cylinder and It now needs a good Bore so I am going with S52 pistons for the price, availability and the need to stick with OEM BMW parts.

I just want to know Compression ratio?
Will the pistons need to be decked to make a reasonable CR? will new valve reliefs need to be cut into piston tops?
What head gasket will I have to use?
Any other pertinent info I should know before this undertaking?

The motor will be going into my '75 BMW 2002. Car only weights 2000lbs so I don't need a ridiculous amount of power.

Also some info on the VW lifter swap would be helpful as well. thanks in advance.

[wazzu70]

I think a good reference for you would be Gizmo's thread on bimmerforums. He modified euro S50 pistons to work in his, but also used a different crank.

Your best bet really is just to do the math to get proper compression ratio. Also a quick calculation will give you increased displacement.

[DesktopDave]

Wish I knew.  All this custom build stuff is way over my head.

[MLM]

I cant quite remember where i saw it and it was a bugger to find, might have been the Mahle pistons web site but there were compression heights and all other dimensions listed for factory pistons. IIRC S50b30 pistons had nigh on identical compression heights. shaving and pocketing may only be required for stroker cranks.

will see if i can hunt out the page.

[MLM]

found it

http://www.motorenteile.mahle.com:8080/eLIZA/mahle/query/engine/byParams

Try here for piston specs

[M5wanaB]

found it

http://www.motorenteile.mahle.com:8080/eLIZA/mahle/query/engine/byParams

Try here for piston specs

This is awesome, This should be a sticky, submit the link on the sticky thats already at the top of this sub-forum.


Anyone know what the piston measurements relate to? KH, VT, MT, GL,

[M5wanaB]

alrighty found all the info I needed I think.
KH = Center or wristpin bor to top of piston
VT = depth of depression in piston crown
MT = Depth of depression in piston top
GL = Total lenght of piston

           l           M42            l            S52
KH       l           31.65          l              31
VT       l      1.65/1.85          l         1.62/3
MT      l            4.5             l             4.08
GL        l          52.65           l             53

[wazzu70]

Just make sure its a US S50 not a Euro S50 spec.

[Warsteiner]

Just wondering where s52 pistons will put me in terms of displacement and compression ratio in an otherwise stock M42 engine. I have a spare engine that blew a head gasket and was left to sit with water in a cylinder and It now needs a good Bore so I am going with S52 pistons for the price, availability and the need to stick with OEM BMW parts.

I just want to know Compression ratio?
Will the pistons need to be decked to make a reasonable CR? will new valve reliefs need to be cut into piston tops?
What head gasket will I have to use?
Any other pertinent info I should know before this undertaking?

The motor will be going into my '75 BMW 2002. Car only weights 2000lbs so I don't need a ridiculous amount of power.

Also some info on the VW lifter swap would be helpful as well. thanks in advance.

You will need to trim off 1.15mm from the S50 Piston
The S52 piston won't work due to its CH of only 31mm and your looking for 31.65mm to stay with stock rods etc.

Compression all depends on where you want it to be. You can dish the piston and cut the reliefs for desired volume to give you your CR.

You have a choice of 1.74mm or 2.04mm in stock head gasket material. You can choose Cometic material and then go with more options.

Much easier to get a stroker crank and make some nice HP if you plan on going to the extreme of boring, decking the block and head, decking pistons along with cutting reliefs for cams and possibly dishing them too.

Then you have the hair pulling decision of which cams to run....and do you have enough compression to support them? To stay with hydraulic or go solid lifters? Then do you want 35mm or 33mm lifters? 6mm valves or stock 7mm? Stock TB or ITB's?:-) How will the exhaust handle all this new flow?

My point is that you need to think an engine build through from air intake to exhaust which includes everything in between. It's not that easy but it can be done. There are numerous formulas out there that work. It's only time and labor and they both equal $$.

I will be glad to help if I can.....

Good Luck.

Cheers,
~Ralph