JS Piston Deck Height

Dan1950 said:

First thing I did was mark the perimeter of the quench pocket with black permanent marker.

Then I bolted the head to the cylinder and reached up into the bore and scribed a line around the bore perimeter..

Bear in mind that this line represents the amount of intrusion which is a bit more than it appears.


After that I placed the head gasket over the studs and scribed another line around the ID of the gasket.

I will use my Dremel with a sanding drum to relieve the pocket out to the line that represents the head gasket leaving the line.

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Dan,

What does it look like with the head and head gasket both mounted on the jugs? I was courious if to know if the gasket extended into the bore, or possibly formed a "shelf" or lip where the squish area was over the gasket opening? Hard to explain but I think you get my meaning.
Pete

Deets55 said:

Dan,

What does it look like with the head and head gasket both mounted on the jugs? I was courious if to know if the gasket extended into the bore, or possibly formed a "shelf" or lip where the squish area was over the gasket opening? Hard to explain but I think you get my meaning.
Pete

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The gasket does not impinge onto the .020" overbore. There is enough ID to allow . 040" overbore, but it would be close.

Did you run a scribe line on the head in the right side bore for grins? Will the two squish pockets end up with the same volume?

Speaking of head gasket shelves. I had a problem with the JS copper head gasket and the two cNw 5/16" reduced shank studs at the front of the barrels. Gasket slid forward a little on the reduced shanks when I dropped the gasket down on the Pliobond and I got a thin shelf (impinging) into the right hand bore. The gasket didn't appear like it was forward enough to make a shelf when assembling. I could have been in a hurry though. cNw doesn't sell the reduced shank 5/16" studs anymore. The reduced shank 5/16" studs don't show as part of the bolt kit on the cNw website. Could be others ran into the same kind of issue.

These cNw studs look cool, but are not a good idea when dropping a head gasket down on Pliobond. The gasket is stuck and won't move with that stuff. Standard shank studs or the JS reduced shank studs would ensure proper gasket alignment.

Schwany said:

Did you run a scribe line on the head in the right side bore for grins? Will the two squish pockets end up with the same volume?

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Yes there was no impingement into the combustion chamber.
Since I am buying seat cutting tools to finish some pitted valve4 seats I will measure valve head clearance at overlap and fit 1.56" intake valve if possible. Either way I will be altering the valve seats . I am ordering a chamber cc measuring kit from Jegs and will adjust volume so that both chamber match in volume.

Schwany said:

Speaking of head gasket shelves. I had a problem with the JS copper head gasket and the two cNw 5/16" reduced shank studs at the front of the barrels. Gasket slid forward a little on the reduced shanks when I dropped the gasket down on the Pliobond and I got a thin shelf (impinging) into the right hand bore. The gasket didn't appear like it was forward enough to make a shelf when assembling. I could have been in a hurry though. cNw doesn't sell the reduced shank 5/16" studs anymore. The reduced shank 5/16" studs don't show as part of the bolt kit on the cNw website. Could be others ran into the same kind of issue.

These cNw studs look cool, but are not a good idea when dropping a head gasket down on Pliobond. The gasket is stuck and won't move with that stuff. Standard shank studs or the JS reduced shank studs would ensure proper gasket alignment.

Click to expand...

I'll set the head gasket over the head studs. They do a good job of holding the gasket in place. I will be using my tried and true Copper Coat spray for the head gasket. I've been using it for over 40 years and see no reason to change.

Before taking the dremel to the head, maybe borrow another cylinder head / barrel to see it resolves the problem?
I've found head gaskets to be off in the past. And heard of cylinder overbore jobs which are not straight either. (perpendicular to the flange)

Remember that the pistons did not protrude on the 850, so it’s likely the factory placed no importance on this tolerance for manufacturing purposes.

elefantrider said:

Before taking the dremel to the head, maybe borrow another cylinder head / barrel to see it resolves the problem?
I've found head gaskets to be off in the past. And heard of cylinder overbore jobs which are not straight either. (perpendicular to the flange)

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And while I'm at it, I'll pick up a pound of unicorn dung. Never mind having wasted $$$ on the machine work and the cost of a questionable use cylinder if that doesn't resolve the issue.

Dan1950 said:

And while I'm at it, I'll pick up a pound of unicorn dung. Never mind having wasted $$$ on the machine work and the cost of a questionable use cylinder if that doesn't resolve the issue.

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I would get some unicorn race dung to reduce detonation with the high CR pistons.

Looks like the JE high compression pistons for stock rods are not a drop in kit for "all" 850 heads. They are very close though. Typical 3rd party performance parts fun. You'll get there with the Dremel.

Which type of unicorn race dung do you recommend. Could I use full synthetic?

Mart UK said:

Which type of unicorn race dung do you recommend. Could I use full synthetic?

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All unicorn race dung is synthetic. It is synthesized from unobtainium. Use the types with either corn or peanut additives.