Piston height?

[4A-GE Power]

Hi all,
ok whilst putting my bigport together, when it came to putting the rods and pistons in and bolting them in, i noticed when any of the pistons are on top dead they stick out above the block deck. They stick up about 13 thou.

Is this normal? If not what size metal head gasket will i need to negate the higher compression?

Cheers again
Dan

[Hen is a total nutcase]

I didn't check closely, but I think when I was doing the same thing recently the pistons were slightly below the block deck.

Have a word to Terry from Road and Track in Ipswich QLD. I'm sure he can steer you in the right direction and even source the right gasket for you.

Hen

[ae71]

are they std or higher comp pistons?

yes i would say thats fairly normal

these are high comp

http://img114.imageshack.us/img114/8571/3t2tghybrdqp3.jpg (http://imageshack.us)

[rthy]

heeey not a 4age piston!!! whatever it is it must have a pretty shit combustion chamber shape. From the piston alone I can tell that its limited in its hp/L

Dan: could you please tell me what that is in current measurements please?

[ae71]

yeah not a 4ag, 3t-c bottom end with cut-outs for a 2tg head :)

just using to demonstrate what a higher comp piston would look like.

[4A-GE Power]

it's .3302mm

[rthy]

that cant be right, that would reduce the squish down to less than 1mm and theres no way the factory would do that, any signs of the block being shaved?

[terryo]

on a 4AG with flat-top pistons, the crown usually sits about 030" below the deck surface. there is no way you want them protruding, especially on an old engine that has head shaved several times, otherwise your compression ratio will be too high. You can correct it with a thicker head gasket, but its not ideal. what we often do for such cases, is machine a squish ring on the piston crown, say take 020" off a small ring around the outside. .
To give you specific advice, I need the exact data on pistons, head chamber volume etc.
terry

[4A-GE Power]

Yeh i had the block and head only skimmed by my engine mob but because the engine was rebuilt in japan im not sure wat they did over there. The pistons are just standard Toyota with the valve cut-outs.

[rthy]

can you post a picture?

Terro has some excellent points, it may well be you need to get a thicker gasket to correct the piaton to squish area clearence

[4A-GE Power]

Here is a picture. Yeh thats all i thought i needed to do, just get a slightly thicker headgasket.

http://www.ae86drivingclub.com.au/dcimages/4/8/4/3/156086.jpg

[rthy]

could you please tell me what is the stock compression ratio for that engine? thanks

[4A-GE Power]

9.4:1

[rthy]

thanks

[terryo]

9.4 is now a meaningless number. it increases each time the block is bored or decked or head skimmed. when all 3 are done, it rapidly increases. this is why using thin gaskets without having accurate data, is a dumb idea.

[4A-GE Power]

But i still dont know if the pistons will hit the head or not with them sticking out thirteen thou? Thats my main question.

terryo- When you bore out a block the compression ratio decreases not increases as you a putting the same ammount of air/fuel into a bigger space?? Is this wrong?

[Delazy]

terryo- When you bore out a block the compression ratio decreases not increases as you a putting the same ammount of air/fuel into a bigger space.[/b]

ur arguing with the wrong bloke....terryo is easily one if not the most knowledgable engine builders that uses this forum regularly

[Trev084]

But i still dont know if the pistons will hit the head or not with them sticking out thirteen thou? Thats my main question.

terryo- When you bore out a block the compression ratio decreases not increases as you a putting the same ammount of air/fuel into a bigger space.[/b]

So u think a bigger area will take the same amount of air the standard bore size does <_<

The bigger the combustion chamber the more air it will draw in.


ur arguing with the wrong bloke....terryo is easily one if not the most knowledgable engine builders that uses this forum regularly[/b]

He only builds 4AGE's for a living :lol:

[4A-GE Power]

Shit sorry it sounded like i was telling him, i was asking :unsure:

I forgot i had an unfinished post

[Delazy]

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But i still dont know if the pistons will hit the head or not with them sticking out thirteen thou? Thats my main question.

terryo- When you bore out a block the compression ratio decreases not increases as you a putting the same ammount of air/fuel into a bigger space.[/b]

So u think a bigger area will take the same amount of air the standard bore size does <_<

The bigger the combustion chamber the more air it will draw in.


ur arguing with the wrong bloke....terryo is easily one if not the most knowledgable engine builders that uses this forum regularly[/b]

He only builds 4AGE's for a living :lol:
[/b]

hahaha...yea :2thumbs:

[terryo]

provided your pistons are not as tall as the head gasket is thick, then you dont have a static "hitting" problem. At high revs, the rod & crank moves up about 0.004" however, so dont try to get the result down to the last few thou.
the other problem you have is that the taller you have the pistons, the more likely the valves will hit if you jump a cam belt tooth or 2.
when you bore the engine out, or stroke it, then there is more volume in the bore that you eventually cram into the combustion chamber (head chamber section + head gasket + any dish in the pistons). that is why comp ratio increases. it is for this reason I keep cautioning people not to just assume you can fit a thin head gasket on an old engine, where the head chamber section has got progressively smaller from sucessive machining.
always happy to help with info,
Terry

[4A-GE Power]

Thanks man thats what i was after. sorry if before it sounded like i was having a go, i just typed wrong and left half the post out as i forgot about it.

[rthy]

9.4 is now a meaningless number. it increases each time the block is bored or decked or head skimmed. when all 3 are done, it rapidly increases. this is why using thin gaskets without having accurate data, is a dumb idea.[/b]

I tottaly agree terry, I however wanted to know for something related to my own engine and research in regard to it. I understand this guy is going to have to do some very carefull measurments to find out what is the current specs and the original static compression ratio is pretty irrelevant to him. You know looking at this engine I cant help but think it might of been someones poor attempt at raising the compression.

[4A-GE Power]

As i said i don't know what has been previously done to the motor in Japan. I have done a couple of calculations and i reckon i can use a 1.0mm metal headgasket. This means the pistons won't hit the head as there should always be minimum of .026" clearance between piston and head. The only other factor is valves hitting pistons but i think this should be ok

Terryo- Do you know what the valve to piston cut-out clearance is on a standard engine?

[rthy]

I think it would pay to cc the head, it could be they have done more than one trick to acheive whatever the hell they were trying to do.