Piston machining

[abit]

I believe many who reads technical forums, maybe won't read projects page, but I really need help, so I will re-post my question here

I'm trying to build m20b30 engine, using 90mm crank, 135mm rods and m54b25 pistons.

Of course, first test assembly, then measuring :D

bolted in one piston, aand it's not below deck, but above, fail...
later done some measurements and that actually should be like that :D
45 + 135 + 27.8 = 207.8 (if we believe that m20 block deck height is 206.7 or 206.2?)
means about 1.1 or 1.6mm over deck, in reality something closer to 2mm's, of course that's not precise measuring, but for test fitting, it's ok :D


so there is two alternatives:
1. shave off material from piston
2. 130mm rods

as we know these are one of thinnest 84mm pistons, so there in not much material from top compression ring to piston head..
what could be the smallest distance from the piston top to 1st compression ring groove? so it won't overheat top rings and piston won't crack details out :/
i believe 1mm off, could be enough for me
if now its something about 5mm, what I think already is really low


I don't tried second (130mm rods) thought but i believe that its not actually possible because of crank counterweights could hit piston skirts and m20 rods maybe won't rotate in block

aaand so big stroke and short rod on 84mm bore could be not best idea, right?


some comparison
m54b25 m52b28 late m20b25

[Supratreo]

I wouldn't shave the piston. Can you go with a thicker head gasket?

[abit]

2,05 is thickest oem spec head gasket if i remember right, and Im going to use it, aftermarket gaskets are out of my budget :/

[digger]

my guess is youll need to cut new valve pockets, which can limit how short the first ring land can be. 3.5 to 4mm should be fine though with 288 and standard intake size on a NA m20

the deck height will most likely be approx 206.2mm if its b25 block

[abit]

my guess is youll need to cut new valve pockets, which can limit how short the first ring land can be. 3.5 to 4mm should be fine though with 288 and standard intake size on a NA m20

the deck height will most likely be approx 206.2mm if its b25 block

Thank's.

you mean, that maybe i will need so deep pockets that they will go on side so deep that valve pockets meets with ring groove? (how little distance to ring can be in those valve pocket side places actually? because its clear that here distance is smaller than in other places)

And that would be mayor issue not, how close ring will be to piston deck? (how close really can be 1st piston groove top to top of piston?)
Because still, I need to get CR down :D

and these 3.5-4mm you mean distance valve to piston?

I already have pockets for valves, but i'm not tested yet, how close to valve they goes with this setup. first I need to modify front belt gear wheel, so it fits on crank.

[digger]

Thank's.

you mean, that maybe i will need so deep pockets that they will go on side so deep that valve pockets meets with ring groove? (how little distance to ring can be in those valve pocket side places actually? because its clear that here distance is smaller than in other places)

And that would be mayor issue not, how close ring will be to piston deck? (how close really can be 1st piston groove top to top of piston?)
Because still, I need to get CR down :D

and these 3.5-4mm you mean distance valve to piston?

I already have pockets for valves, but i'm not tested yet, how close to valve they goes with this setup. first I need to modify front belt gear wheel, so it fits on crank.

with pockets for a 288 i think it should be ok but in general with oversized valves and deep reliefs on stock 84mm bore its possible to get very close to the top ring groove with big overlap cam

3.5-4mm for the top ring land is still fine for NA street m20

the depth of pocket id say 3mm for schrick 288 which is about 1-1.5mm deeper than stock