Originally Posted by Bishop
My gear diffs and repair parts showed up today, the U-Pro really is a marvel in how tiny and light it is, 'very' notchy out of the packet though, how many runs do they usually take to smooth out if you just fit and go?.
I may just use mine as is for a few runs, I'm curious to see how they work out of the pack, before I go putting the 10k and 100k I also bought to tune mine with, likely if they still feel rough I'll toothpaste them also.
When i took my uPro out of the package and built it per instructions, i wasnt happy with how it felt. I mic'd out both housing halves and wondered why they would shim it they way they did in the instructions. Ill offer up my findings for anyone willing to try them.
My wording will be per the instructions included for the rebuild of the gear diff to try and eliminate confusion. This is also for new internals and housings... used parts will have different tolerances and may cause premature failure.
Pinion Gear Hard Shim - 8x2.5 - Qty2 (one on each side)
Bearing/Housing Shim - 8x6.5x0.2 - Qty2 (one between each bearing and housing half and NOT included in the package)
Main Gear Hard Shim - 9x5x0.2 - Qty2 (one between each main gear and internal o-ring. One 0.1 on each side may work best dependent on o-ring tolerance)
This equaled the measurements on both sides, and spaced the Main Gear Hard Shim against the inner o-ring just right. It was pressed against it... but not squeezing it. Each outdrive turned easily, but with a little drag.
In my opinion, a lot of you guys probably experience leakage here because the instructions want you to put thinner shims of smaller outer diameter between the Main Gear Hard Shim and o-ring. O-rings dont seal well against multiple surfaces like that.
Once i had it shimmed as above, i did a test assembly with some teflon impregnated silicone lube (Manufactured by SynCo) i use on all o-ring assemblies and metal to plastic surfaces in gear diffs. Each o-ring was coated before insertion, and a dab of the lube between each mating surface. This is about as 'dry' of a test fit you will get to check for gear lash before adding diff fluid.
To ease description, i had the same internal lash as a properly set 64 pitch gear set... pulling the outdrives away from each other as i turn them to simulate load. This lash also accounts room for the diff fluid to flow between the gears during use.
Once i added my 100k diff fluid, the diff was absolute butter, there was no notchiness, and no leaks.
In my opinion, from looking at various used uPro diffs, they likely leak between the two case halves because they are simply overtightened. The screws hold the housings together, the large o-ring does the sealing.
Tightening the housing screws more than that can distort the housing enough to promote leakage. I can think of 3 uPro diffs i have looked at that have indentations where the screws were simply tightened too much... both on the housing AND the pulley. All were coated in diff fluid.