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Discussion Starter · #1 · (Edited)
How to check Deck Height

Today at Big Boyz Cycles we took a few pictures of how to check the deck height. Measuring the deck height helps in calculating your compression ratio. It can also adjust the quench.

This engine is being disassembled and the cases, cylinders and heads are being powder coated. If the engine wasn't being disassembled we would put clean rags in the spigot holes to prevent anything from falling into the cases. This particular engine is having the cylinders replaced for aesthetic reasons. This engine has had the cylinders milled and to keep the performance the same, we need to match the new cylinder deck height with the old. This also sets the quench.

This is an EVO engine but the same principles apply to a Twin Cam.

1) Remove old head gasket material from the cylinders.





2) Clean piston of any carbon build up. We use a razor to do this. Be careful not to drop any carbon or gasket material in the oil drain holes. If the engine wasn't coming apart, we would of block these hole for safety.





3) Using short head bolts and several wheel spacers, tighten the cylinder down. If the spacers have large center holes like ours, make sure they don't overhang the cylinder wall. Since this is an EVO, it has base gaskets. Leave the stock, crushed gasket in place when measuring. Twin Cams use o-rings and can ignore the gasket references.





4) Using a welded sprocket extension as a tool, we use channel locks to turn the engine over while finding TDC. Place a finger half over the cylinder and half over the piston and feel for the top of the travel while turning the engine back and forth.





5) With the piston at TDC, place a straight edge across the piston and cylinder. In this case the piston sticks above the cylinder therefore we measure the head gasket surface to the straight edge with a feeler gauge.
NOTE: The piston is .013" ABOVE the cylinder head gasket surface.





6) After our measurements, we need to change the piston and cylinder to the new piston and cylinder, then re-measure.





7) We are using Genuine HD parts for this conversion. Cast piston and New HD replacement cylinders.





8) Install new piston. Use rubber hose on the cylinder studs to protect the piston and studs from hurting each other.




9) Replace the cylinder using a new gasket. In our case, we had to torque a head down to crush the new base gasket to get a proper measurement. Then we removed the head and bolted down the cylinder as described above. Set TDC again and remeasure the deck height. In this case the cylinder is now taller than the piston therefor we are measuring between the piston and the straight edge.
NOTE: The piston is now .011" BELOW the cylinder head gasket surface.

To set the deck height of the new cylinders to match the old one, .024" needs to be milled (.011"+.013"). Using the stock .045" head gasket, this will give a .032" quench.

 

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FYJIGM
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Springer,
Do you have a similar post that shows how to file the rings during installation of new piston and rings?
 

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Discussion Starter · #3 ·
DNLEVESQUE said:
Springer,
Do you have a similar post that shows how to file the rings during installation of new piston and rings?
Use a ring end gapping tool. It's small hand operated grinding tool. Or if you only have to do it once, you can use a small fine file. Always remember to remove any burs left by which ever tool you choose. The operation is self explanatory but if you need a post on it, I'll make one. I am sure your service manual shows you how to measure the gap.
 

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FYJIGM
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That would be great Springer but it looks like The Snowman has posted a link with similar to what we're talking about. I'll read into what's there a little more.
 

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Thanks again Springer

You really know how to show these mechanical principles to us so we can understand these settings and measurements and help us understand the effect of them on our builds.
 

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Discussion Starter · #9 ·
Today the cylinders were turned to remove the .024" that was measured yesterday. Often times this is referred to as "Milling" but since it is actually turned in a lathe it is not milling. We often refer to "Milling" the heads but we turn those also.

NOTE: We are removing the material from the bottom of the cylinder.

 

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Yet another very excellent thread Springer.................keep them coming bro!


Ozzie
 

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Discussion Starter · #12 ·
While at Big Boyz today, I took a few more pictures.

1) Close up of the machined surface after turning.




2) Measuring cylinders after machining. NOTE: The pistons are now .013" ABOVE the cylinder again.

 

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How do you know how high to make the pistons above the cylinder? Was this one just a case of making it the same as it was? Also, what would happen if you had left the pistons lower?

Great info Springer!
 

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Discussion Starter · #14 ·
hdmarsh77 said:
How do you know how high to make the pistons above the cylinder? Was this one just a case of making it the same as it was? Also, what would happen if you had left the pistons lower?

Great info Springer!
To obtain .030"-.035" of squish using the stock .045" heads gasket, the piston needs to exceed the top of the cylinder by .010"+ The tightened squish area makes for a more efficient combustion chamber. Bean had previously adjusted the original cylinders to the proper deck height. With the new cylinders, the customer didn't want to lose any performance so we had to match the original deck height.

If left alone, the compression ratio would have been lower and the squish would have been larger resulting in less performance.

The same principle can be applied to the Twin Cam. In the Twin Cam you can bring the deck height to .000" and use a cometic .030" head gasket to obtain a desired quench. You could also set the deck height at +.010 and run a stock head gasket and obtain a .035" quench. In the EVO, turning the bottom of the cylinders not only adjusts the deck height but also provides a flatter surface for sealing the base gaskets.
 

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springer- said:
To obtain .030"-.035" of squish using the stock .045" heads gasket, the piston needs to exceed the top of the cylinder by .010"+ The tightened squish area makes for a more efficient combustion chamber.
The same principle can be applied to the Twin Cam. In the Twin Cam you can bring the deck height to .000" and use a cometic .030" head gasket to obtain a desired quench. You could also set the deck height at +.010 and run a stock head gasket and obtain a .035" quench.
QUOTE]

Springer sorry so late on this post, but on the TC you said the deck height of .000 to get a .035 quench. Would you mill the bottom like the EVO or can you turn it from the top of the cylinder sence there is no gasket at the bottom of the TC?

Get post by the way, as always :thumbsup:

Tim
 

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Discussion Starter · #18 ·
The same way, only on the sides where it is flat. The piston still has to clear the head, so the dome is only where the open combustion chamber is.
 
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