Stuck syncro?

Helped my friend try to figure out why an old military vehicle won't go into 2-3 and 4-5 gears. It's an old WW2 tracked landing craft that you can drive a jeep into the back of and is powered by a 7 cylinder Continental radial engine. It hasn't been driven for 15 years and the previous owner didn't mention any tranny troubles but????

Granny and reverse are in a separate case behind the transmission case and they do shift and work. 2-3&4-5 are inside the tranny case and we can't get them to shift into gear. The gear shift lever just won't move.In this tranny the top shaft is the input and bottom is output. There are two syncronizers on the bottom shaft, according to the parts book. One for for 2-3 and the other one is for 4-5. The shift lever is on the outside of the tranny and the three shifting rods come out of the end of the tranny about 5" and that is where the shift lever engages them. The rods for those two gear selections move freely just a bit and then they act like they hit something hard. I even tapped on the end of one with a hammer while trying to shift it in with the shift lever and it feels like it hits something rock hard. We have the drive shaft off between the engine and tranny and can turn the tranny with the nut inside the input yoke with a socket. If we try to move the shift lever into a gear the input shaft starts turning hard so evidently the syncros are moving just enough to make a little friction, but they won't engage beyond that. Like I said, when moving the shift lever it feels like the shift fork hits something rock hard. I don't know much about syncros, but can they sieze up and prevent the collar from engaging the gear? I have no idea what the make and model is. It's old, it's military and it's not a common gearbox, that's all I know about it.

The engine is not running but he has a spare engine that will be going in. Jim

Syncros can lock to the cone. They can also split and prevent engagement. The sliding shift ring can also lock to the splines they ride on.
I would drain the oil out of it and take a look to analyze the issue before taking it apart. My cousin had one and used it for several years as a toy. Wear ear protection when running it they are so way loud and hot inside that it is a short time on task vehicle. Jim

Thank you for the information. Like I said, I know very little about syncros. The old stuff I work with doesn't have them. When I sat down there in the drivers seat of that beast I could just hear the noise and feel the heat without it even running. That airplane engine is right behind the driver's right shoulder with a thin tin wall in between, the drive shaft from the engine to the transaxle is right beside the driver's right shoulder and the transaxle is by his right arm. The transmission part of the transaxle is aluminum and has cooling fins-right beside the driver. WHEW!

He wants to tow it a mile to a neighbor who plans to replace the engine. The book says to take off the drive shaft from the engine and put the tranny in fifth gear for towing. That keeps the tranny lube pump turning while towing. It also warns to go no faster than 7 MPH. It won't go into fifth and that was what he was worried about and justifiably because the lube pump doesn't turn if towed in neutral. But after I looked at the parts book awhile I determined the pump will spin with the tranny in granny, because it spins the top shaft which turns the gear on the bottom shaft that spins the pump, but it'll be spinning might fast. I told him he'd probably be OK if he did't go faster than 1 MPH, maybe 2 MPH. I don't know how fast granny is on this rig when it's running. The engine is governed at 2100 RPM's.

To check to see if the lube pump worked I rigged up a hydraulic motor with a 1" square output shaft, and I put a 2 1/8"- 1" drive socket on it. Then I put the whole works on the nut inside the input yoke on the tranny and plugged the hyd motor into a tractor. It spun the tranny over just fast enough to eventually get oil pumping so he does have a lube pump that works, it's just going to be spinning pretty darned fast when he tows this thing with the tranny in granny.

He said the previous owner used this vehicle to pull stumps. I got a chuckle out of that, thinking about a vehicle with an airplane engine pulling stumps. Seems kind of out of character! Jim

Using a cordless drill or other battery operated motor, the trans could be spun modestly to assure lube while towing in neutral. Much easier than risking overspeeding, or going so slow it seems rediculous. Jim

I mentioned that to him but he didn't seem very wild about the idea. The reason for the hyd motor in the first place is the input turned too hard for a drill. Ater we ran it for awhile with the hyd motor it did loosen up enough to be turned with a drill. Jim