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 a an EVO engine but the same principals 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.

 

Very informative post Springer, Thank you.

 

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.

 

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.

 

Thanks for the illustrated instructions on checking deck height.

Now that I know HOW to do it, I have a question as to WHEN to do it! I have a factory stock TC88 with 35k miles on it and plan to have the cylinders bored and minor head work done. I will be using the factory HD SE cast pistons (22851-99A). I would like to check the deck height and have the cylinder base or heads machined if necessary to end up with the correct clearance. My question is can I remove the heads and measure the deck height with the stock bore and pistons or do I need to send the cylinders out to have them bored, install the new pistons, and then check deck height? If the manufacturing tolerances on the pistons is very tight and the wrist pin center to top of piston is the same for the old and new pistons then I should be able to measure deck height with the original piston before I send the cylinders out. On the other hand if the piston dimension can vary a few thou from one to the next then I better wait until I get the cylinders back and check with the new pistons. Measuring with the new pistons would be best but if turned out the deck height was too high I would end up having to send the cylinders out a second time to have the base machined.

Thanks in advance for your advice!

Andy

 

Good Post Springer

I like to use a degree wheel to find tdc , and I check both front and rear. I also like a 30 thou head gasket so I would have to remove less from the cylinder. That keeps me closer in spec with the distant between the heads for intake alignment and not causing any issue's with exhaust alignment either. Great pictures as well. Keep up the good work

 

Any advice from the experts? Can I measure deck height with my old pistons or do I need to bore cylinders and install new pistons first?

Thanks

 

Pardon the archaeology

Could you please tell me how you fixed the cylinder in the lathe?
I see some kind of ring (torque plate?) and can't figure out how it attaches to the cylinder.

I'm afraid of damaging the fresh hone, when trying to get the correct deck height on lathe.

 

You do realize this thread is 17 years old?

 

At least I tried.
And the methods of fixing the cylinders are unlikely to have changed since then.