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 Post subject: 75mm Pistons
PostPosted: Mon Dec 30, 2013 2:36 pm 
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I've done a lot of searching, and can't find anything quite definitive...

I've got a G10 block, and G10T rods, and a 3tech performance cam (higher lift?).

I'd like to find some 75mm pistons to use for somewhere between 7.0:1 and 8.5:1 with floating bearings just like the G10T pistons.

Does anyone know some inexpensive pistons will actually work in this scenario?

I've seen references to vitara pistons, but it's not clear to me whether the valves will work with these, as they seem to be 4-valve designs.

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 Post subject: Re: 75mm Pistons
PostPosted: Mon Dec 30, 2013 2:45 pm 
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You cannot make that motor interference, so don't worry about that. The Vitara piston will work, but compression is quite low. You plan on low boost, so you're probably good for 8.5-9.5 to 1. Stock pistons and rods will do the job.

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 Post subject: Re: 75mm Pistons
PostPosted: Mon Dec 30, 2013 4:19 pm 
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My bores are all oval. I'm going to bore it out, so I'm looking for oversize.

Are you saying that the stock G10 stuff is okay, for say, 15 psi, or are you talking about the stock G10T stuff, because I can't find oversize G10T pistons.

I should also say, I need this motor to run on 91 octane, as that's all that's available when doing rally, hence the lower compression.

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 Post subject: Re: 75mm Pistons
PostPosted: Mon Dec 30, 2013 5:59 pm 
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As long as you time and fuel it right, the stock G10 parts will work fine. Vitara pistons should work fine too, if you have the heavy rods. Thinking about it, the (Vitara) compression won't be that bad, as the G10 stroke is longer than the G13's that they're typically used in, so that will put them higher in the bore. The last thing you want is a 7.5-1 compression ratio G10 with a performance cam. Off boost power will be brutal.

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 Post subject: Re: 75mm Pistons
PostPosted: Mon Dec 30, 2013 6:10 pm 
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Cool, bought some G16 NPR pistons, rings and pins on ebay, the 8v kind... hopefully they put me about where I need to be. Thanks for the advice!

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 Post subject: Re: 75mm Pistons
PostPosted: Wed Jan 01, 2014 11:26 am 
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when i built my blue mk2 turbo3 engine i had to have the bore liners replaced and bored back to stock dimensions to fit the standard turbo3 pistons that suprf1y hooked me up with back in at spec. it's not a deal breaker.

when i built the black turbo3 engine i used vitara 16 valve pistons that jardamuth hooked me up with. those were 75mm pistons so the bores were taken to the max 40 thousandths over bore.

suprf1y is correct about the pistons being a bit higher. the crowns actually protrude slightly above the block deck and into the chamber. piston diameter didn't affect the head gasket fitment.

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 Post subject: Re: 75mm Pistons
PostPosted: Wed Jan 01, 2014 2:38 pm 
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viewtopic.php?f=32&t=55530&p=416390&hilit=compression#p416390


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 Post subject: Re: 75mm Pistons
PostPosted: Wed Jan 01, 2014 2:44 pm 
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viewtopic.php?f=32&t=54199&hilit=rebuild

Had to run lots of timing with this one.


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 Post subject: Re: 75mm Pistons
PostPosted: Wed Jan 01, 2014 3:22 pm 
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I'll try to get pictures, measurements and compression ratio estimates when I get the pistons in.

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 Post subject: Re: 75mm Pistons
PostPosted: Thu Jan 02, 2014 11:21 pm 
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Got the pistons in the mail today, and did some measuring.

Long story short:

According to my calculations, G10T rods + G16A 2 valve pistons = ~7.73:1 compression ratio
Using G10T rods + G10T pistons (74mm) = ~8.14:1 compression ratio

Here are my assumptions:
G10 has ~9.25:1 compression ratio stock (according to 3tech)
Head gasket is ~1.2mm (close to 0.050") compressed

Here's what I've measured:
Compression Chamber volume: 30cc (this is from a 3tech head)
G10 piston dish: 5cc
G10T piston dish: 5cc
G16A piston dish: 9cc

G10T rods are ~0.057" shorter than G10 rods between the closest two bearing surfaces (couldn't measure precisely due to not being able to press out the G10 pin)

All three pistons are 1.498" from the top of the piston to the bottom of the pin.

G10 uses pressed pin ~0.659" diameter (couldn't measure precisely)
G10T & G16A pins are floating, and 0.748" in diameter

G16A piston measured at 74.97 mm in diameter, and 235.00 grams in weight

Pictures of the G16A NPT pistons:

Image
Image
Image
Image

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Last edited by ofensus on Fri Jan 03, 2014 1:24 pm, edited 1 time in total.

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 Post subject: Re: 75mm Pistons
PostPosted: Fri Jan 03, 2014 9:53 am 
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i used the 75mm 16 valve vitara pistons on my last turbo3 build.
Image
Image

i forget whether mike and clark decked the head when they ported, polished, and built this one but piston clearance wasn't a problem.

when jardamuth sent me the vitara pistons he advised me that they would create an interference operation so i'll go with that.

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 Post subject: Re: 75mm Pistons
PostPosted: Fri Jan 03, 2014 11:37 am 
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The 8v pistons look like they have the same reliefs as the turbo ones and they will be sitting about .05 in lower in the block. I'm not convinced I'll have that problem yet. I'll make an overlay pic of the two pistons to demonstrate and make some more measurements on the dish topology.

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 Post subject: Re: 75mm Pistons
PostPosted: Fri Jan 03, 2014 12:35 pm 
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I've used same pistons in my build. Only problem I encountered was interference of the piston skirt at the radius and the crank. Not all will do that , my buddy bought different brand of pistons and had no problem. My engine has 100K hard miles on those pistons and his maybe 75K.
Thank you for all the calculations , I did some of them when I was getting ready to do my build and came up with similar numbers. Two valve pistons do not drop CR as dramatically as everyone was telling me they will. This confirms it. :D


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 Post subject: Re: 75mm Pistons
PostPosted: Fri Jan 03, 2014 1:21 pm 
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DTM GTi wrote:
I've used same pistons in my build. Only problem I encountered was interference of the piston skirt at the radius and the crank.


I'd read your thread on that, but thanks for reminding me. I'll check that when I get my block back, bored, from the machine shop. I did notice that the piston skirts looked like they'd been roughly shaved in that area already, so maybe I'll be lucky there.

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 Post subject: Re: 75mm Pistons
PostPosted: Fri Jan 03, 2014 1:44 pm 
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I used Rock brand pistons so if you didn't buy those you will probably be fine. Interestingly Rock pistons were 229 grams and if I remember correctly exact same weight as stock T3 pistons. Only one of them was 230g , good thing they come in sets of 4 :)


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 Post subject: Re: 75mm Pistons
PostPosted: Fri Jan 03, 2014 7:54 pm 
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Here's a very unscientific analysis of valve clearance, comparing G16A pistons vs G10T

The purple/black stuff is an overlay of the G10T piston, the silver, normal colored piston is the G16A piston.

The valve reliefs are larger and deeper on the G16 piston than the G10T piston. I think it'll work just fine. Also Note that the arrow on the G10T piston lines with the reflections on the pin fairly well

Image

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 Post subject: Re: 75mm Pistons
PostPosted: Sat Jan 04, 2014 8:59 pm 
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Are you comparing it to a mk1 piston? You are using a mk2 head right? Regardless, unless you're running a big dome, you cannot make a mk2 interference. I thought I'd already said that

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 Post subject: Re: 75mm Pistons
PostPosted: Sat Jan 04, 2014 11:58 pm 
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suprf1y wrote:
Are you comparing it to a mk1 piston? You are using a mk2 head right? Regardless, unless you're running a big dome, you cannot make a mk2 interference. I thought I'd already said that


You did say that, and this also confirms that it'll work :)

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 Post subject: Re: 75mm Pistons
PostPosted: Tue Jan 21, 2014 11:15 pm 
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Pistons were within 0.3 grams of one another.

I got one installed, and found no interference with the crankshaft (visually looks like at least 1/4" of clearance)

Also, incidentally, I made a solid 75mm ring compressor.

I was having a really hard time getting the pistons in with the compressor I bough at autozone, so I did some searching, and found out about solid tapered compressors.

Image

I turned my own on the lathe, and the piston just popped right in. I will never go back to the clamp style, it's going in the trash!

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