I'm Scared. My Pistons Poke Out Me Hole....

[Shoota_77]

I finished putting my bottom end together yesterday but it wasn't till I'd finished and turned the engine around to look at the top that I realised how far the peak of the pistons protrude out of the block.

Has anyone done a high comp RB and seen that amount of piston out of the block? Obviously I'll find out when I sit the head on but is there enough combustion chamber to even fit that much protrusion??

The pistons are custom CP 9.5:1.

I'd appreciate some input!

[r32-25t]

I have the cp 9:1 and they protrude out the block as well, yours being a custom piston you will want to double check piston to valve clearance just to be safe

[Shoota_77]

Ok cool. I googled images but couldn't find any decent shots showing so much protrusion.

I'll gently sit the head on when I get a chance and I'll soon know. I'll be nervous as all hell the first time I turn it over with the cam belt on!

[luke gtr]

on my 26 they looked to protrude that far.

[r33_racer]

You're just going to have to put some play dough on there and bolt it all down and turn it over and see how close the valves come to those slugs!

[Shoota_77]

Thanks for the responses guys, giving me a little bit of hope!

Thanks god it's got the valve reliefs in the pistons cause I think it's going to need that extra 1mm of clearance!

[34GeeTeeTee]

on my 26 they looked to protrude that far.

Yours came out the block for a different reason though

[Scando]

As a trial engine, I'm running RB25 pistons with the deck skimmed 30 thou to get them up flush with the deck. Also gives 9.5:1 compression and I have a similar amount of piston sticking up past the block as you do so I think you'll be fine.

To get 9.5:1 on an RB30 bottom end with an RB25/26 head you're running about the same amount of piston dome as a standard RB25/26 so you shouldn't have any issues.

[GTRPSI]

Fit the head on with a few weak mockup valve springs and the cams your running to the correct valve clearance, use a old known thickness HG, put some plasticine on the pistons, fit cam belt on and very gently turn the engine over by hand for 2 full turns feeling for binding.

You want about 60 thou or 1.5mm valve to piston minimum clearance, better 80 thou or 2mm, any less than that is very dangerous and you might have them meet at high RPM....more clearance the better to allow for rod stretch and valve float.

[XKLABA]

As a trial engine, I'm running RB25 pistons with the deck skimmed 30 thou to get them up flush with the deck. Also gives 9.5:1 compression and I have a similar amount of piston sticking up past the block as you do so I think you'll be fine. this is what I have done, RB25 14cc dome tops with 26 head and 9.53:1, no issues yet and the engine has been running since January

To get 9.5:1 on an RB30 bottom end with an RB25/26 head you're running about the same amount of piston dome as a standard RB25/26 so you shouldn't have any issues. there is actually more clearance with the 30 bottom end then the 25/26, cause 500cc getting compressed at 9.5 parts to 1 means 1 is a bigger area then 428cc getting compressed 9.5 to 1