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Location:
Melbourne
Registered:
November 2004
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Location:
Gold Coast
Registered:
October 2002
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Re: 18RG block with unknown Pistons
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Fri, 12 August 2005 11:30
 
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they look like std 9.7:1
Daniel
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Location:
Adelaide
Registered:
September 2003
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Re: 18RG block with unknown Pistons
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Fri, 12 August 2005 11:43
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| munki wrote on Fri, 12 August 2005 21:00 |
they look like std 9.7:1
Daniel
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I reckon they are higher than that. From memory (it is pretty hazy I admit, but the 9.7:1's had a much flatter top section.
I would guess that they are more like 11.5 or 12:1
What is the history of the engine ?
Clean the top of the piston and if possible measure the height from the deck.
Cheers
Michael B
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Location:
Melbourne
Registered:
November 2004
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Re: 18RG block with unknown Pistons
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Fri, 12 August 2005 11:58
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i cant actually source the history coz i got it as a spare
with the working 18RG that i have
the guy i got it off reckons its from his mate and dont remeber what happened
it looks like the piston hit the valve somehow and broke it
it did came as is with the head sitting on top with a broken valve and 2 bent valves
no head bolts
no chains
no manifolds
but yeah the top of the pistons is not flat at all
i'll try to get some measurements and clearer pics
what am i looking for the measurements to know which one it is?
so its either 11.1 or 12.1 ????
thanx
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Location:
Perth
Registered:
May 2003
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Re: 18RG block with unknown Pistons
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Fri, 12 August 2005 12:33
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Unless someone can identify the brand of piston I think it will be a stab in the dark at the exact compression ratio.
If you really want to know then work it out the hard way...
You can always take a mould of the top of the piston then work out how much water it takes to fill the mould (use a syringe). At the same time determine the volume of the combustion chamber in the head (anyone know this already?) using the water method too. Head combustion chamber volume - piston top volume = actual combustion volume. Next work out the volume of the cylinder (pi x radius_of_cylinder^2 x (stroke + head_gasket_thickness)). Then divide the cylinder volume plus the actual combustion volume by the actual cylinder volume and you have the static compression ratio.
Or just get all your numbers and use one of these things:
http://www.turbofast.com.au/Tfcomp.html
http://www.rbracing-rsr.com/compstaticcalc.html
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Toymods Social Secretary
Location:
Sydney
Registered:
July 2002
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Re: 18RG block with unknown Pistons
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Sat, 13 August 2005 05:13
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combust chamber volume = 72mL IIRC
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Location:
Brisbane
Registered:
October 2002
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Re: 18RG block with unknown Pistons
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Sat, 13 August 2005 07:06
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they look like my HKS 12.5 to one, but BIGGER
contact Steve M, he has a set of Arias 12.5:1's too
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Location:
Adelaide
Registered:
June 2003
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Re: 18RG block with unknown Pistons
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Sun, 14 August 2005 01:09
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Are they forged? Only one way to tell for sure, show us a photo from the bottom of the piston.
If you can get a direct side on shot of the domes that might help.
If you could post the dome height and bore size that could help too.
The amount of flat between the valve cut-outs is slightly skinnier than the Arias 12.5s.
The sides of the dome are steeper and more square in shape than the Arias ones.
All I can guess is that they are definately not of lower compression than the Arias 12.5:1s unless their bore is smaller than 92mm.
The depth of the valve cut outs doesn't look like the reason that valves were damaged. They appear plenty deep enough.
Most likely the valves were hitting each other, the valve cutouts were on the wrong angle or there was a timing issue.
What are the cams that came with the engine, got any pics?
If Toyota cams, is there a number stamped into the cam on the flat that the cam timing cog goes onto? If so, what is the number?
Also what are the valve sizes?
If they are TRD ones, I may be able to source some new ones for you through an upcoming Toysport Order.
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Location:
Melbourne
Registered:
November 2004
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Location:
Brisbane
Registered:
October 2002
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Re: 18RG block with unknown Pistons
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Mon, 22 August 2005 11:35
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the block is ok to use, these are very high compression race pistons, not a road going item
i'm more worried about the shitload of metal filings everywhere
what u doing with the pistons
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Location:
Melbourne
Registered:
November 2004
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Re: 18RG block with unknown Pistons(with close up pics)
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Mon, 22 August 2005 11:48
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yeah its just a bit of dust and shit
i havent done anything with the pistons
but its notmaly covered up
im planning to use the whole thing and put my EFI on it
just wondering if i can do that due to high compression piston
everything is good but i might need to pull everything apart but havent got the time and motivation
but yeah hook me up with those gasket of yours please MR DOHC
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Location:
Brisbane
Registered:
October 2002
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Re: 18RG block with unknown Pistons(with close up pics)
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Mon, 22 August 2005 12:01
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bolting up the efi will be no worries, but running naything but a stock 18RG and the stock computer shits itself
so a fully programable comp will be needed
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Location:
Melbourne
Registered:
November 2004
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Re: 18RG block with unknown Pistons(with close up pics)
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Mon, 22 August 2005 12:09
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i have a link computer fuel only
what will happen if u just use the one i have now?????
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Forum Sponsor
Location:
Shepparton (Vic)
Registered:
May 2002
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Re: 18RG block with unknown Pistons(with close up pics)
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Mon, 22 August 2005 22:42
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I suspect that they are the TRD 2 ring Forged pistons. I had a set of these many years ago. Most of the after market pistons for these engines had all the machined faces flat. These have rounded sides.
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Location:
Melbourne
Registered:
November 2004
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Re: 18RG block with unknown Pistons(with close up pics)
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Thu, 25 August 2005 07:51
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| andrewvibert wrote on Tue, 23 August 2005 08:42 |
I suspect that they are the TRD 2 ring Forged pistons. I had a set of these many years ago. Most of the after market pistons for these engines had all the machined faces flat. These have rounded sides.
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tell me more
did you get to put it on
how did it go
yeah
i have a link computer from new zealand that controls fuel only
how will it go if i use this computer with this pistons???
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Location:
On your mum!
Registered:
May 2002
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Re: 18RG block with unknown Pistons(with close up pics)
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Thu, 25 August 2005 10:11
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Running your EFI cams will almost certainly not be a good idea. High comp requires cams with more overlap and greater duration.
If you try to run these bigger comp pistons with std cams your cylinder pressures will most likely become too great.
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Location:
Melbourne
Registered:
November 2004
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Re: 18RG block with unknown Pistons(with close up pics)
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Thu, 25 August 2005 20:28
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i have 253 heads and 250
can i use thier cam instead of 270?????
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Location:
Adelaide
Registered:
June 2003
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Re: 18RG block with unknown Pistons(with close up pics)
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Fri, 26 August 2005 08:22
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88250 stamped cams would definately be more suited to these pistons. 88231 or earlier would also be better than 88251 or later.
With the compresion that you have, the issue is fuel type. Using 98ron fuel, you'd probably have to have 310 degree or more cams, just to lose the compression.
The 12.5:1 compression pistons I've got were used with methanol and mechanical injection in a sprint car.
The only TRD pistons I've seen had "dome" shaped tops on them, as do the Arias 12.5s. I've never seen 18RG pistons that have such square sides as yours, they must get quite close to the sides of teh conbustion chamber. I've only seen forged ones that have more of a tennis ball shape dome.
JP engineering in SA make some that are squarer, I think, I've not seen them in a while, but they are cast.
If you are able to measure the volume (accounting for valve pit volume below block height) we should ba able to help you out with a rough compression ratio.
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