JS Piston Deck Height

Dan1950

Dan1950

1974 MK II Roadster
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With a .021" copper shim under the cylinders the JS pistons are .125" above the cylinder deck.

JS Piston Deck Height


The quench pad in the combustion chamber is .123" recess. With .035" fire ring gasket compressed height. there is only .033" clearance between the piston crown and head deck. Too close for my comfort.
 
Last edited:
Dan1950 said:
With a .021" copper shim under the cylinders the JS pistons are .125" above the cylinder deck.

JS Piston Deck Height


The quench pad in the combustion chamber is .123" recess. With .035" fire ring gasket compressed height. there is only .033" clearance between the piston crown and head deck. Too close for my comfort.
Click to expand...
Seems to me that the dial indicator should be vertical for a correct reading.
 
If you had it at 45 degrees when you read .125, it would have been about .080. side a squared + side b squared = hypotenuse squared. But you weren't reading .125 in the initial post. You were reading .026. If you were at 45 degrees before and you are now perfectly vertical, you should read .018 so you weren't at 45 degrees before or you are not perfectly vertical now. All assumes that the indicator is zeroed on the top of the cylinder.

Forgive me if I've screwed up the math - it's been a long time!

Edit: I think you went all the way around and then some on the indicator based how high the piston is in the picture and ended where you did so I'm partly wrong above. .126 was your original reading and your vertical reading is .114 - that is correct, but originally you were not at exactly 45 degrees.

At any rate, you are at .114 if perfectly vertical.
 
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marshg246 said:
If you had it at 45 degrees when you read .125, it would have been about .080. side a squared + side b squared = hypotenuse squared. But you weren't reading .125 in the initial post. You were reading .026. If you were at 45 degrees before and you are now perfectly vertical, you should read .018 so you weren't at 45 degrees before or you are not perfectly vertical now. All assumes that the indicator is zeroed on the top of the cylinder.

Forgive me if I've screwed up the math - it's been a long time!
Click to expand...
You are failing to take into account that the dial made a complete revolution (.100") that needs to be accounted for so yes the readings were . 125" and .114".
 
The good news is that sounds like you are OK. To be sure, you need to check after tightening the cylinder base nuts. Even if the copper base is lying perfectly flat you'll read a few thou more.

BTW, I hope the cylinder base came annealed or you annealed it!
 
marshg246 said:
Ya, corrected that just before you posted.
Click to expand...
As far as your math. I doubt that I had the angle at exactly 45°.

But, FYI 45° runs and offsets can be calculated with the square root of 2 (1.414)

Since the hypotenuse are equal the square root of 2 is the common factor.

Something from my pipe fitting days.
 
marshg246 said:
The good news is that sounds like you are OK. To be sure, you need to check after tightening the cylinder base nuts. Even if the copper base is lying perfectly flat you'll read a few thou more.

BTW, I hope the cylinder base came annealed or you annealed it!
Click to expand...
I tightened the 4 through bolts and I plan to use Loctite 518 on the shim.
 
Dan1950 said:
As far as your math. I doubt that I had the angle at exactly 45°.

But, FYI 45° runs and offsets can be calculated with the square root of 2 (1.414)

Since the hypotenuse are equal the square root of 2 is the common factor.

Something from my pipe fitting days.
Click to expand...
For easy math, I used 45 degrees. That means that side a and side b with a 90 between them are the same length. So, as you say the square root of 2 can be used. In any case, a squared = b squared + c squared for a right triangle whichever way you do the math.

It's fun once expectation and reality meet :)
 
maaseyracer said:
Which pistons do you have? The medium or the high? Is this a 750 or 850?
Click to expand...
jsmotorsport.com

Complete set of two forged Norton lightweight pistons 750 to 1007cc for stock Norton rods (most sizes are in stock). Contact us for 500 to 650 Dominator sizes.

Norton pistons for stock rods
jsmotorsport.com

jsmotorsport.com

Graphite Piston skirt coating for two pistons. More durable than Molly and other coatings. Recommended for all uncoated pistons for tighter installation clearance, reduced friction and longer life. Note that many of our flat top pistons come precoate

This aftermarket graphite coating is superior, longer lasting and extends the piston life. Our domed, extra high compression flat top, Domni and special order pistons are not coated.
jsmotorsport.com

Graphite coated skirt high compression 850.
 
Dan1950 said:
jsmotorsport.com

Complete set of two forged Norton lightweight pistons 750 to 1007cc for stock Norton rods (most sizes are in stock). Contact us for 500 to 650 Dominator sizes.

Norton pistons for stock rods
jsmotorsport.com

jsmotorsport.com

Graphite Piston skirt coating for two pistons. More durable than Molly and other coatings. Recommended for all uncoated pistons for tighter installation clearance, reduced friction and longer life. Note that many of our flat top pistons come precoate

This aftermarket graphite coating is superior, longer lasting and extends the piston life. Our domed, extra high compression flat top, Domni and special order pistons are not coated.
jsmotorsport.com

Graphite coated skirt high compression 850.
Click to expand...

I Just built two 850 MK3 engines one with the high compression pistons and one with the low compression pistons. Jim sent me an Excel doc that allowed me to calculator compression with his pistons fairly accurately. I would reach out to him and ask him for that.

I am running the same base gasket and a 0.020" head gasket. Should get me just above of 10.5:1. My measurements are not that far from yours.
 
Thanks for making me aware of these pistons and I'm really surprised the head and cylinder line up well enough to stick the piston out the top like that. On other motors even with alignment dowels you have to do some checking and maybe adjust a little but thats with an angle edge on piston and combustion chamber. On my 2 850 heads the bolt hole pattern is at least .025" off to the side compared to the squish pocket so I guess bolt the head to cylinder and reach down the bore with a hook tool to see that the squish pocket is outside the bore and call it good? Either way i'll be buying these pistons next time. Thanks for making them
 
edgefinder said:
Thanks for making me aware of these pistons and I'm really surprised the head and cylinder line up well enough to stick the piston out the top like that. On other motors even with alignment dowels you have to do some checking and maybe adjust a little but thats with an angle edge on piston and combustion chamber. On my 2 850 heads the bolt hole pattern is at least .025" off to the side compared to the squish pocket so I guess bolt the head to cylinder and reach down the bore with a hook tool to see that the squish pocket is outside the bore and call it good? Either way i'll be buying these pistons next time. Thanks for making them
Click to expand...
Funny you should mention that.

 
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