E30 Zone

what's the likelihood the the pistons will need to be machined to have deeper valve pockets on a hc m20b25 with a 284/272 cam?

Hello gromgsxr

Not worth the risk of guessing, trial fit a conrod/piston/head with cam and plasticine on top of the piston to carefully check it. explained below, cheers.

i was going to measure it before i run the engine ect, but i don't want to take it all apart if its highly unlikely that it wont fit without machining effectively wasting a head gasket set and set of head bolts .

what would be a safe min piston to valve clearance??

gromgsxr wrote:i was going to measure it before i run the engine ect, but i don't want to take it all apart if its highly unlikely that it wont fit without machining effectively wasting a head gasket set and set of head bolts .

what would be a safe min piston to valve clearance??

You don't waste a gasket and bolts - you just lightly torque it down with a used (compressed) gasket and old bolts.

its a daily car so i don't wan't to take it apart if there is no chance in hell its going to fit then having to put it back together with the stock cam exactly how it was before i started.

its a schrick so 11,4mm inlet valve lift 11 mm exhaust valve lift

Brianmoooore wrote:

gromgsxr wrote:i was going to measure it before i run the engine ect, but i don't want to take it all apart if its highly unlikely that it wont fit without machining effectively wasting a head gasket set and set of head bolts .

what would be a safe min piston to valve clearance??

You don't waste a gasket and bolts - you just lightly torque it down with a used (compressed) gasket and old bolts.

trial fitting 1 conrod/bearings/piston/rings and as Brian says old head gasket and old bolts, if you check bearing clearances as well, you can leave the piston in if the valve to piston clearance is ok. What's an extra hour or 2 verses bent valves.

I would rather have it definitely isn't going to hit instead of it's highly unlikely going to hit.

gromgsxr wrote:its a daily car so i don't wan't to take it apart if there is no chance in hell its going to fit then having to put it back together with the stock cam exactly how it was before i started.

Don't sacrifice low end drivability for top end power in a daily driver, you will spend 95% of your driving rpm below 3000rpm as a daily driver, chip it instead if you haven't already.

motormanmick wrote:

gromgsxr wrote:its a daily car so i don't wan't to take it apart if there is no chance in hell its going to fit then having to put it back together with the stock cam exactly how it was before i started.

Don't sacrifice low end drivability for top end power in a daily driver, you will spend 95% of your driving rpm below 3000rpm as a daily driver, chip it instead if you haven't already.

the car already has a megasquirt 3 fitted and will be be getting a b28 stroker soon.

and again i under stand how important it is to measure and i wouldn't fit it blindly without measuring. but i would like to know what the chances are as i don't want to pull it apart if i am just going to have to put it back together how it started.

gromgsxr wrote:

motormanmick wrote:

gromgsxr wrote:its a daily car so i don't wan't to take it apart if there is no chance in hell its going to fit then having to put it back together with the stock cam exactly how it was before i started.

Don't sacrifice low end drivability for top end power in a daily driver, you will spend 95% of your driving rpm below 3000rpm as a daily driver, chip it instead if you haven't already.

the car already has a megasquirt 3 fitted and will be be getting a b28 stroker soon.

and again i under stand how important it is to measure and i wouldn't fit it blindly without measuring. but i would like to know what the chances are as i don't want to pull it apart if i am just going to have to put it back together how it started.

No worries, below is a link to info on piston to valve clearance from Racing Head Service, these guys are doing it all day, so are pretty clued up, pdf at bottom page is a must read.

http://www.racingheadservice.com/rhs/pi ... nce-guide/

That said, I don't think anyone on here will give a yes or no on whether your valves will clear or not with your new cam, there are too many variables. To name a few, has the block been decked, has the head been skimmed, head gasket thickness needs to be considered (some will use a thinner head gasket to get a little more compression), piston manufactures may do different reliefs in their pistons in view of what their customers may be doing to their engines std verses performance builds. You will only know some of these if you where the builder.

Maybe leave the cam for the b28 when you will no doubt be building it and check it as a matter of course.

people have had issues with the 284/272, especially if the head has had some material taken off. but others have used it without issues. At TDC the 284 lobe is 2.1mm off seat compared to a stock cam of 1mm, plus the quicker ramps and you lose about 1.2-1.4mm clearance at 10* ATDC which is likely the point where contact will occur. This assumes it installs in the position that schrick says. if it goes in advanced for any reason things escalate pretty quickly

an alternate way to check is use just install the inner springs and push down by hand on the valve to check how far it moves before it hits the piston, you do this with crank at TDC and every few degrees after untill you find the minimum with a dial indicator. this only works reliably with cast OEM pistons and stock valve sizes as it only checks axial clearance not radial.

you can use a lever to press down with double springs installed but you need a way to apply a fair bit of force safely, i have done this before even with springs that have much higher seat pressure than stock.

people aim for 1.5mm inlet and 2.0mm exh but for sure people have gone closer but it depends on the application and parts used