Piston to head clearance

[dieselman]

Well it's been over a year since I started this project, but I am back to it.  I checked the piston to head clearance. After taking out all the shims the clearance is .068.  I assume I will have to mill down the cylinder to give room for the shims to properly adjust the clearance to .050.  The gasket alone is about .055.  Has anyone run across this problem before? 

[SHIPCHIEF]

No,
But many of us have had leaky head gaskets because the cylinder liners stick up too high above the cylinder block deck.
Maybe your liner sticks up too high. If you machine the top of the liner flange down enough to limit protrusion to about .005", that would get you closer, and the fire ring will seal, yet water should not weep out.
If you mill down the top deck of the cylinder block, you will need to re-machine the recess for the liner flange, so if you go that route, measure the liner flange before cutting the recess, to set the liner protrusion. Then you won't need to machine the top of the liner.
If the top of the liner is fine with the top of the cylinder block, perhaps it would be easier to mill off the bottom of the cylinder block?
Also; the engine will run fine if the compression is a little low, but it may be harder to start. The engine is so easy to work on, and the head is particularly easy to remove, that you might consider giving it a try and see how it runs for you?
Scott E

[dieselman]

Then liner protrudes only about .010.  Not enough gain.  I have to take the head to a machine shop anyway because of a nice gouge in the surface of the head between the area from the cylinder and extending beyond the liner flange.   It came from Lovson like that.  Nice QC.  Thanks for your input.  Its a good thing there is no urgent need for this project.

[xyzer]

dieselman,
Is the engine in question the 14/1 in your sig? I would look into what the clearance is for that engine. 6/1’s and 12/2's are .050-.055 I have no idea what a 14/1 should be. Maybe it is the same I don't know. The problem you describe is a new one to me but I'm sure the "oid" people are capable.
Dave

[dieselman]

xyzer:  .050 is the clearance that I recall reading somewhere.  I'll have to double check that.  Any ideas where to find the info?

[xyzer]

Someone here should know. Mine always came with a manual that had all the basic specs listed.

[dieselman]

I'll check the manual again, Thx.

[cujet]

Rather than assume the bump clearance should be a particular number. I prefer measurements of the volume of the indirect combustion chamber and valves. You can easily calculate compression ratio. Set your bump clearance accordingly, without being too close (for obvious reasons)

[MacGyver]

Ahhh, but assuming that you are still below the target compression ratio, and you need to set the clearance to the minimum value... how close IS too close? What's the absolute minimum "safe" value?

[craig c]

Hi , with a Cast iron piston .020" piston to head clearance , just need to machine decompression lever so valve does not hit piston when starting engine , CraigC

[cujet]

As I mentioned before, too close is related to exhaust valve opening while using the compression release.

The exhaust valve is held open by some amount. The valve will hit the piston if bump clearance is too tight. Reducing the compression release lift makes the engine nearly impossible to hand crank. I suspect that anything closer than 0.045 inches will have either piston/valve interference or hard cranking (with no valve touching).

Chris

[MacGyver]

I'm running about .047" clearance, and my exhaust valve just barely touched the piston when cranking. It left a faint little circle in the carbon on top of the piston.

I whacked about .030" off of the decompression lever and it's fine now. It doesn't seem any more difficult to crank with the decopmpression lever shortened up a bit.