Big Boyz Head Porting  


How to check Deck Height

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.

The cylinders were turned to remove the .024" that was measured above. 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.

Close up of the machined surface after turning.

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

As always feel free to ask question !!!

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