Pistons and rings for 9U D6 Pony Motor

Found , bought and shipped a ""RUNNING"" pony motor for my 9U D6. Installed the motor and it runs, but with so much blow-by it pushes the dipstick out of the tube it sets in. It only ran one time and since it runs for a few seconds and then dies. It is just not getting enough compression to keep running. Spark is 1/2 to 3/4 inch long and dark blue and the carburetor has been cleaned and is working properly. A compression check starts out at 25 psi and builds to approximately 100 psi. This is using the electric starter to turn the motor which takes the motor through several cycles of compression. I have not tried to turn the motor by hand and measure the compression for one cycle of compression. I suspect it will be about 25 psi. I plan on doing a leak down test by pressurizing the cylinder with compressed air and see how long it holds pressure or where the pressure goes. This would indicate if he valves weren't seating good. Measured number one cylinder bore with .003 out of round which is the limit according to the Servicemen's Reference Book. Everything indicates it is time to re-bore the motor and install oversize pistons and rings. Hastings has the rings in standard thru .060 oversize and Regal list the pistons but says call for availability. Is there another source for .020 or larger oversize pistons or is caterpillars price of $106 for semi finished pistons the best deal? Better than that is there anyone who has a set (2) .020 or larger oversize pistons for a 9U D6 Pony Motor? I plan on taking the motor back off the machine tomorrow and start tearing it down for reconditioning. I will have better information later as soon as I get the motor apart to do more extensive measuring.

Would be interested in hearing from someone with a couple of pistons for sale. Also interested in comments concerning the blow-by which indicates to me the motor needs some help. Call any time 573-433-9277. Thanks Brian Boaz

I just sent a 9U to the Netherlands and it would do the same thing. the oil stick would sit there and bounce up and down hissing. that starting engine would start first pull using the rope start. it ran really good, had good power for turning the main engine.

How long did the motor set? Rings might be stuck a bit. If you can keep it running I would put it through a few warm up and cool down cycles before I rebuilt it.

check the crankcase breather that is on top, under the air intake tube, it may be clogged causing more crankcase pressure, doubt it though, 0.003 out of round isn't bad, you could try to just give the bores a good hone to get some crosshatch back on the bottom side of the cylinder walls, before doing a complete overhaul.

I have 8 pistons with rings 2f3669 but they are standar

Im not real smart but I always understood that to do a compression test once it was proved the compression was down to find weather its the rings or the valves squirt some oil in the cylinder and try again if the compression comes up it means its the rings if it stays down its the valves have never heard of pressurizing the cylinder ????? the only time Ive heard of that is if you need to change valve springs or colletts with out pulling the head

Paul

With any testing method, the old expression "the devil is in the details" certainly applies.

With compression testing, how strong is the starter, the battery (or how long is the rope and how strong is the guy on the end of it), Is the choke open, is throttle open, is air filter restricted or clean, Do you have a good seal in the sparkplug hole etc The biggest one is how many "puffs" do you collect before you read your pressure. I've always used 3 which seems to be nearly the peak for most decent cylinders even if you were to let it turn over longer. I think the first puff is probably the most important, but any problems with seal in sparkplug hole etc and that number can be very skewed.

With compressed air/blow down testing, how big of an orifice is the air going through into the cylinder, what pressure is the air source regulated at etc.

The important part is that the cylinders of a multiple cylinder engine have similar results. There are just too many different things that can affect the resluts to say definitively that the compression is OK or too low etc until the results are way out.

I like to do the blowdown testing to diagnose leaky valves (which valve), which cylinder etc much more easily than when using compression testing. Doesn't tell me much about cylinder and ring wear because I don't have the calibrated orifice and pressure regulator etc that CAT specified, I just turn whatever shop air pressure I have available into the cylinder at TDC compression stroke and listen where its going.

On this pony motor, I would get the pistons out and see if they are reusable and price a set of rings before deciding whether to bore to an oversize or have it bored and sleeved back to standard to use the original pistons. Of course, boring to an ovesize will raise compression ratio, so the engine may have slightly more power after that treatment than if you went back to standard. Probably the least effective course would be to hone and re-ring it as is, but that would almost certainly help it.

I also have dealt with a D6 poiny that was puffing the dipstick out of the hole and I was certain that it was on its last legs and nearly to the point of being unusable, until I cleaned the carburetor completely. Made a "new" pony out of it! I sold the tractor to a friend last year and within a week, he was calling to tell me he couldn't get the pony started. So I went to see what the problem was and found that he (like me when I first started) had neglected the advice to ALWAYS stop the engine by turning off the gas supply valve. The pony had a liberal coating of gasoline diluted oil splashed all over the governor/waterpump belt and that side of the engine. Changed the pony lube oil and it fired right up and starts the main just fine. I don't know what the compression test would show on that engine, but it has to be quite low.