Trying to work out from a mate whether he has got low comp pistons in the block he's got, is there any way of telling just from looking at them?

He said they have got slight valve cut outs on them, and all the RST blocks he's seen have had totally flat pistons...

Any help muchly appreciated...

 

Valves hit the pistons then.




Dave.

 

lol no not that slight

 

Well I have the remains of a standard RST piston on my desk in front of me (makes a good paperweight ) There appears to be a slight recess at one side of the piston 5.5mm wide and 2.5mm in from the edge of the piston.I can't tell if there is/was one on the other side as that edge is looking a little worse for wears after a minor melting issue.

 

Std pistons.........

 

std ones ive got have got 2 valve cutouts one either side...

 

is that valve contact?!

 

Yup Outcome......

 

Looks like it to me! Low comp pistons may still have cut outs in them, if they are standard Mahles that have been skimmed, unless someone has taken a massive chunk out of the top of them, you won't know unless you remove the piston and measure the height from the centre of the gudgeon pin to the face of the piston, do it both sides and take a mean measurement.

 

blackRS, how did that sound?! lol

 

james_birch, It was my own fault, didn't do the timing up correctly. There wasn't a mark for TDC on the adjustable cam pully and as i went to start it, it went tick bling bling bling bling tick !!!

 

lmfao@ crazy frog style eh! lol!

 

Well he's told me they sit flush with the top of the block on TDC so sodesn't sound as if they are

Not good really as he was sold it as a low comp one in good faith by a "friend"

Thanks for the pics Adam, exactly what I was after

 

The standard pistons protrude the block by ~1 to 1.5 mm when at TDC, so low comp pistons would be around level with the block.

Not directly related to this, but I've alwasy wondered why people opt for low comp pistons and then run very little boost? The standard comp seems to work very well with 30+ psi of boost.

 

On standard rods and pistons??

 

as above as a general rule standard pistons will protroude slightly from the block where as 'low comp' (i hate that term!) one will fit almost flush.
ALL rst mahles should have some valve cut-outs in them.. if they havent they must of had PURE loped of them!
are you sure they are mahles and not accralites or the like?

Jesse i think standard compression and 30+Psi on Uk fuel would make a machine gun noise come from your engine!

Adam nice 'witness mark' and unlucky about the guide... didnt you turn it over by hand first!?

 

That puts a whole different light on it then mate.... I'll get him to check where the pistons come to on TDC for definite when he finishes work