I wish I would have measured the negative deck height prior to disassembly, but I didn't. Here's the deal...

I had the block milled .100. The new pistons measure .030 shorter than the stock pistons

Given an estimated original negative deck height of say .145 I should have a negative deck height of .045 with the original pistons and .075 with the new pistons, right?

Well, my admittedly crude measuring device shows negative deck height of 9/64" which equates to .160

Am I adding things wrong or measuring wrong or something? I want a true 10:1 motor when I'm done. Head cc'd at 44 cc.

I sure am glad I measured instead of just bolting things together. At least I can make up for the difference by milling the head more. That wasn't my plan, but after paying $200 to have the block milled along with other machine shop costs I didn't realize I can't bring myself to go back to that machine shop for awhile. Other than to take back the incorrect main bearings and get my head locating dowels they didn't replace.

Rob

Firstly Rob 9/64 = 0.140" not 0.160" so you've saved 0.020" there. According to my calculations, since you took a fair amount off the head to create the 44cc chamber, with a 0.020" head gasket, you're very close to 10. Have I missed something?

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Cheers!
Al T

Given that the block or head can be milled for compression is there any advantage to a smaller combustion chamber volume over the stock or larger chamber volume?

Are you measuring the whole piston or the from the pin which is what matters.

Will

I used my cheapie Harbor Freight calipers to measure from the top of the wrist pin to the top of the piston. That gave me the difference of .030 between the stock piston and the new one.

I spoke to Doug Dutra yesterday and he said that he has seen blocks with up to .190 negative deck height.

My head porter is going to take .080 off of the head which will result in a 43.7 cc chamber. His calculations come up with 9.75:1 compression ratio with a .020 head gasket (if I can find one). If I use the composite gasket it should be 9.5:1.

Once I re-re-re-measure everything and have it all worked out I'll post something. Right now it looks like it should work out to about 230 HP and about 270 ft lbs torque.

I sure am learning more about this tuff. This is a great teaching tool to show the kids the importance of measuring things and double checking.

Rob

Great stuff, Rob.

Tell the kids they should go and get science/engineering degrees later on. There are fewer and fewer science PhD students who even know how to use a socket wrench, much less a caliper... Technology innovation cannot be done only on a computer screen!

Lou

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Home of Slant6-powered fun machines since 1988

I wish I had had someone take me by the hand when I was 13-16 years old and show me this stuff. I've loved cars all my life, but have always held back on actually tearing deep into things for fear that I would mess something up. Well, I am messing things up, but it's okay. Hopefully the kids are getting something out of watching my mistakes

I borrowed a depth gauge from the machine shop last night (I had to leave my first born son) and here's the negative deck height measurements. I measured each one a dozen times at various locations and am confident that I was at TDC. I was surprised at the variance.

1 - .1390
2 - .1435
3 - .1395
4 - .1455
5 - .1430
6 - .1465

That's an overall variance of .0075

I had the crank turned .010
New .010 over crank and rod bearings that I verified tolerance with plastigauge
Deck milled .100 from whatever it was originally(next time I'm measuring first)
New cast pistons that are .030 shorter than stock pistons

So, the head was milled .080 that should equate to a 43.7 cc chamber. He is going to cc it again now that it is milled

I'll post more later.

Rob

Looks like you are on the right trail there, Rob. You should be right around 10:1.

The piston can rock in the bore some, and that will account for most or all of your variance there.

Lou

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Home of Slant6-powered fun machines since 1988

I just got off the phone with Gerald, my head porter. I had my numbers messed up. It was 47.3, not 43.7. He just cc'ed the head and it is actually a 49cc chamber after milling .080.
That should calculate to 9.544:1 CR with a .020 head gasket or
9.203:1 with the Fel Pro gasket I have.

It's not what I was originally shooting for, but I am willing to give up a couple horsepower. Hopefully that little bit of reduction will help it live a little longer for these teenagers. I'm hoping if I set it up right it will last the three years I'm gone. I'll find a good rev limiter and/or make the shifter only be able to go down as far as drive so it will shift itself rather than letting them do it.

Rob