Originally Posted by
gerryndennis
That's interesting that OS didn't mark the piston, I've never seen an OS scneürle piston without the arrow so assumed they all had it.
The .60FSR I got at the same time as this engine has no marking on the piston either... Both engines are late 70s vintage - the .60 looks like it ran about a dozen tanks, crashed, and got disassembled and put in a bag. The guy I got it from said both engines had been disassembled and bagged for the last 20+ years...
I'm sure you're fully aware of the requirement for the con rod big end chamfer to face the crank shaft, and that baffled pistons, (pinned) ringed pistons, and pistons with holes and cut outs that match the transfer ports to be fitted the correct way. It's also advisable to keep lapped and ABX Pistons the same way round as they were run in for best compression seal.
Once the the engine is as worn as it sounds like yours is, I guess it doesn't matter as much though. You obviousely have found that Diesels need better comp seal to start than what a glow can get away with.
The conrod was removed from the piston - the entire engine was disassembled completely - bearings removed, needles removed from carb, rod out of piston, etc. I know which way the rod needs to face - that was marked with an "R" for rear anyway. Piston was marked with just an arrow. Initially I assumed the arrow would face the rear since the rod was marked as such (I always look for the chamfered bushing on the rod to confirm its orientation), but after confirming with the previous owner, the arrow was supposed to point forward instead. I did run a tank or two through it with the piston flipped around (the piston had no other distinguishing markings or cutouts to signify which way it's supposed to face either), but later "put it right". It ran absolutely no differently. I do understand the importance of piston and rod orientation in regard to running in and keeping that the same way - with these engines - that wasn't easy to do.
Davis deliberately makes the comp screw short enough that beginners won't be able to accidently have the piston hit the contra (very hard on the con rod). I replaced the screw on my head to get the comp high enough to run. I found that the contra was protruding well into the chamber. The piston was dark coloured under the bowl and squish band part of the head, but had a ring of clean metal under the contra. The contra must have been very close to the piston.
In theory the contra should be close to flush with the chamber. I replaced the head gasket with a thinner one. The contra is now in a much better position, and the original comp screw now works. I was surprised how much difference a small change in head gasket thickness made.
This wasn't an issue with most original Diesel engines because the contra is full diameter and ran in the bore, they were either dead flat with no bowl shape, or conical shaped to match a conical piston. You could set the contra too close to the piston but the engine wouldn't run that way (unless the compression seal was really low). The conversion heads mostly have the contra run in the head insert so it's possible for the contra to hit the piston, but the engine still be under compressed, if squish clearance is too high. A worn P/L makes this worse.
Dave H
I've replaced the comp screws on a couple of heads because they were too short - I'll do the same with this one. I installed the Davis head out of the package with the factory OS head shim as the other Dave has suggested. The Davis head came out of the bag 1/4 turn out from bottomed out, yet the instructions say the starting point should be one turn out from seated. I set the head to that setting (1 turn out) and it wouldn't start that way. I had to prime it heavy and increase compression pretty much to that 1/4 turn out before it would pop and eventually fire. I have some other engines that I'm gonna see if the bores are the same. If they are, I'll see if there are any Pistons that fit the liner better than the stock one. Can't hurt to try. Piston seal is paramount to a good low idle and hand starting. That is partly why I don't care for ringed diesels. They're not that easy to hand start either it seems.