hi - i'm told that on a standard yb cosworth engine the pistons protrude form the block slightly? would this mean then, that in the event of cam belt snapping or incorectly set up cams that the pistons would hit the valves?

the reason i ask is cos i'm building an engine to 7.2 cr using flat top pistons which will need machining to get cr correct and calculations show that pocketing the pistons instaed of simply facing them would be possible and would accomodate such an event.

any ideas???

 

Yes mate unfortunatly this is 1 of the downsides of cossie ownership

If u can account for having it pocted 2 then get it done as its worth doing even on a normal spec engine let alone a higer spec one.

 

does the use of different cams increase the potential for contact because of higher lift, or does a different open / close time increase it? by sitting the head on the block with a used gasket i can lift the inlet valve 4.2mm at tdc before contact. havent yet machined pistons but have 72.5cc total volume using 1mm gasket (looking for 81cc = 7.2cr). it would be pointless to pocket pistons if unconditional clearance could not be achieved but if each pocket was bigger than 81-72.5 / 4 = 2.1cc, cr would be out, then i'd need to skim block to recover cr and this would mean that the pockets should have been deeper!!!!! does anyone know the reqiured mathematics to calculate pockets or can i get away with a simple skim of the piston if i'm careful with cam set up etc

 

OK ur getting too techincal for me

Bsically in a normal YB cossie if the belt snaps thne you get piston/valve contact unless ur very very lucky. With bigger cams then ur deff likely to get it which is why running bigger than standard is always advisable 2 pocket the pistons, as i say if u can do it even for standard cams its worth it IMO to save bending any valves if the belt goes.

 

Very few have been lucky BTW. 16 new valves for me
And no idea about the maths. Sorry.