Piston or head gasket - Decision time

[73 Mike]

It's time to get started on my next engine. For those of you that don't remember, I had a minor mishap with my last one that resulted in me replacing it with a cheap, used crate engine that has less power than my old 72 Chevy Vega with a straight 4. Kept me on the road for the summer at least.



I plan on using the crank (3.75 stroke and already magnifluxed) and rods from the last build. I did replace the one that was slightly bent :bounce:




I also plan on using the iron head (72 cc chamber and 200 cc runner) and am looking for a static CR around 9.75:1. I'll likely switch from the solid, roller lifter set up that I used to a hydraulic roller, but that discusion is for another day.

I was able to pick up a nice Motown block that needed boring. Unfortunately, it came with installed o-rings that had to be decked off because the placement didn't allow for an overbore. The net result is that I now have a deck height that is 27 thousanths below 9.8 inches. I picked up a cheap piston from Summit and verified the deck is even accross all eight cylinders.

I think that I have three realistic options for the pistons:

1) Purchase a set of custom pistons with a lower compression height so that I have a zero deck. This is probably the most expensive but not by all that much. I'd also have to machine the intake, but it's only money :crap:

2) I found a set of Felpro head gaskets with a compressed thickness of .080 inches. The sample piston that I got has a chamfer on it so the "flat part" of the piston would be totally below the deck. I don't think that there would be any issues with the top ring or the ring land. Surprisingly, this isn't all that much cheaper than option 1 as the head gaskets are $110 each. People seem to be very uncomfortable with negative deck heights though.

3) Several of the pistons that I've looked at have pretty large top ring lands. I could get a set of these and machine off the top 27 thou. This is the least expensine option but makes me the most nervous. I'd also have to machine the intake.

Although I don't intend to race or autocross this motor, the truth is that I'm pretty hard on them. Although I'm getting very good at swapping motors, I'd like this one to last a little longer than the 200 miles or so that i got from the last one. I never really figurged out what went wrong so I'm trying to be a little more careful this time around.

 

[JeffP1167]

:amazed: that might not fall under small problem.

 

[Fuelie74]

I would probably consider a Cometic head gasket if going that thick. Sounds to me like the best option since you would keep the heads higher to lineup with the intake. Maybe I am missing a major downside to going that direction though.

 

[larrywalk]

So, what is the deck height? 9.8" is standard for a BBC deck height.

Standard SBC decks are 9.025"; if you are at 8.998, no problem - just use a .041" composition head gasket and you should have a quench height of .039".

 

[MYBAD79]

Deck height of 9.773" ... if you use -18cc KB pistons and a .041" gasket you'd be at 9.5:1 CR. The piston would however protrude .027" - probably not a good idea....

Machining the top land would make me nervous too.

On many pistons the pin bore is already in the oil ring groove, I doubt you can move the bore .027" higher. There's only so much space between the bore and the top of the piston and the rings have to go somewhere....

Below is a photo of the KB pistons at TDC with a deck height of 9.8005", the chamfer at the top is rather small...

 

[larrywalk]

Deck height of 9.773"...
Below is a photo of the KB pistons at TDC with a deck height of 9.8005", the chamfer at the top is rather small...

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No, no, no! That's a picture of a small block! Deck height of ~9.0" Even the special aftermarket raised-cam SBC blocks only have deck heights of ~9.3". Also, the stroke of 3.75" is small block (383, 400 or 406). If it were big block, the stroke would be 3.76 for the 396, 402 or 427, and 4.00 for the 454.

If the deck height is 9.8", it's a big block and that picture is not of a BBC.

So again, to the OP, 73 Mike, what is your finished deck height?

 

[MYBAD79]

yup, had a brainfart and thought he was talking deck height for a SB and I assumed he was building a 383 with 3.75 stroke..... deck height is 9.025 for a small block
funny that the numbers looked good when I used the CR calculator, I used zero deck with .041" gasket minus the .027"

 

[73 Mike]

Deck height of 9.773"...
Below is a photo of the KB pistons at TDC with a deck height of 9.8005", the chamfer at the top is rather small...

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So again, to the OP, 73 Mike, what is your finished deck height?

Oopsie. I was talking about a big block elsewhere and the number stuck in my head.

It is a small block and the machined deck height is 8.973 inches.

 

[MYBAD79]

then my numbers above are correct except the deck height, 9.5CR isn't bad and the KB pistons are not the most expensive ones out there.... now the question is: can the piston stick out past the deck and how close is the top ring to the deck surface ??

 

[73 Mike]

then my numbers above are correct except the deck height, 9.5CR isn't bad and the KB pistons are not the most expensive ones out there.... now the question is: can the piston stick out past the deck and how close is the top ring to the deck surface ??

That's exactly the question. Depending on the head gasket used, I think I can have a reasonably safe quench of about 40 to 50 thou. I just have little idea of what a reasonable or maximum "out of the hole" height is.