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Old 05-28-2014, 09:31 PM   #1
122ti
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Default measuring squish clearance

Hi, everybody. I'm in the middle of resealing a 93 B230ft and want to do the tight squish thing. I'm not planning on a rebuild, just gaskets and stuff. So, I started measuring the pistons poke using a deck bridge and dial indicator and they all seem about 4 to 6 thou proud of the block until I rock them. If I rock the pistons at TDC, I can get them to stick up as much as 11 thou on the sides. A little less front to back.
Should I just call it 6 thou and stop being anal? Should I be worried about that much piston rock as far as general engine condition goes? I tried to center them the best I could, but this is where I stop trusting my measurement interpretations.
Would like to hear what you guys have done.
Thanks,
Dave

P.S.
The point of all this is that I want to get a Cometic head gasket and need to know what size will get me about 35thou piston to head clearance. I can do the arithmetic, just want to see what you guys think about the piston rock. This motor probably wont see anything over 6500 rpm.

Last edited by 122ti; 05-28-2014 at 10:32 PM.. Reason: clarification
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Old 05-29-2014, 10:08 AM   #2
culberro
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I use the clay/putty approach and put 4 balls of it on the piston around the edges, and then one under each valve. Rotate engine over and then remove head and measure clay thickness. It's more work but you get a really good idea of what's going on.
As far as the way you are doing it, measure the high and the low and average them. The piston shouldn't rock due to somewhat equal pressure across the face, but they probably will when the engine is running.
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Old 05-29-2014, 06:07 PM   #3
122ti
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Thanks for the response. I'll go with my average measurement for now, I guess. I will definitely try the clay when i get my head freshened up and pick out a cam. I already torqued this engine up once already but took the head off for my daily driver. I guess I'll be putting it together and taking it apart a few more times!
Thanks again,
Dave
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Old 05-29-2014, 11:26 PM   #4
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Based on all I"ve read, at 35 and 6500 rl you're not going to hit anything anyway. My hope is to do this eventually, and I'm gonna shoot for 30-32. But the piston rock factor is certainly interesting, I had wondered how much to consider that.
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Old 05-30-2014, 09:59 PM   #5
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Yeah, when I measured the "rock", I really smooshed the pistons up and down on the sides as much as I could just to see the worst case scenario. If I average it out I get basically .006" proud of the deck and I'll probably go with that, but I would like to try the clay. Better to do it now with the engine on a stand than in the car. It just seems to me that I would have to torque it down with a cometic to use the clay method. I'd rather not squish a new gasket twice, but that probably would be the way to really make sure.
The reason I asked this question about the rocking is that when I first tried measuring, my pistons seemed to be a little lower than the .006" figure I keep hearing, so I started pushing on them at tdc and I could make the dial indicator move alot. I guess even at .011" out I would still be fairly safe. And I agree, they probably would have fairly even pressure across the piston, but I'm not an expert.
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Old 06-01-2014, 08:05 AM   #6
DET17
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My 94 squirter block (had 128K miles on it) measured out at .008" ~.010" above the deck; yes, those pistons also of course "rock" so I took my measures at the pin centerline and ignored the rock +/- . I had purchased the 036 Cometic, but after consultation with StealthFTi I decided to buy an 040 Cometic and used that, which has had no issues now with around 8K miles since the install.

Would I have been OK with a 036 Cometic? Perhaps, but I had planned to rev to 6500 and didn't want things getting too cozy.

FWIW, I was chatting with a local SBC rebuilder, who told me he had once disassembled a 350 engine which had previously been "tight quenched/squished". To his surprise, the numbers on the top of the pistons had transfered onto the head chamber surface in CARBON! Now I believe that's getting pretty close

BTW, if you need a Cometic 036, shoot me a PM, I've got a zero mile unit ready to go
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Old 06-01-2014, 11:03 AM   #7
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Hi, DET17. That's a funny story about the SBC. People seem to love to rev those things and they do sound good when you twist em up! I'm trying to decide how close I want to go. I'll think about that 036 you have but it would put me at about .030. I might be more comfortable with a .040 myself.
Have you noticed any of the ping resistance or gas mileage benefits of this mod? Did it make your setup any snappier? 230FT motors feel kinda "all turbo" in stock trim.
Thanks,
Dave
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Old 06-01-2014, 11:34 AM   #8
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Quote:
Originally Posted by DET17 View Post
To his surprise, the numbers on the top of the pistons had transfered onto the head chamber surface in CARBON!
If I was building a tight squish motor, that would be a hole in one in my eyes! Can't get any better than that lol
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Old 06-01-2014, 01:52 PM   #9
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Quote:
Originally Posted by 122ti View Post
Hi, DET17. That's a funny story about the SBC. People seem to love to rev those things and they do sound good when you twist em up! I'm trying to decide how close I want to go. I'll think about that 036 you have but it would put me at about .030. I might be more comfortable with a .040 myself.
Have you noticed any of the ping resistance or gas mileage benefits of this mod? Did it make your setup any snappier? 230FT motors feel kinda "all turbo" in stock trim.
Thanks,
Dave
I also am Very interested in the bold part. And I also am interested in the Cometic, but I also have a 94, so your number of 8-10 is concerning; I had only ever heard of 3-5 before.
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Old 06-01-2014, 02:46 PM   #10
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Well I can say that I've not noticed any pinging whatsoever with this engine. SteathFTi claims you can run low octane fuel with the tight squish arrangement.

I have no baseline comparison with this B230 as I did all the improvements at the same time, not incrementally. I wouldn't expect any performance increase unless you count knock resistance under boost ( I am at 17 PSI ) and as far as I know I have not experienced any of that. To be fair, I don't have any data logging proof as I am running LH2.4.
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Old 06-01-2014, 08:20 PM   #11
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I guess we would need to be able to adjust the ignition timing. I'll be running LH2.4 as well. There is that mod where you can ground pins on EZK for more timing, but now I'm getting ahead of myself. I'll be changing alot of things at the same time, too. This engine is going into a 92 240 that is my daily, so I'm sure it'll feel like a whole different car!
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