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Bombi Differential Questions

tango

New member
I’m in process of doing a bombi rebuild. I’m new to bombi and snow cats in general. This sight is awesome and answered a lot of questions just by looking at older posts. While turning the bombi I get the clunk noise. It sounds like drive sprocket is jumping cogs on track. I know that’s not the issue but trying to find root cause of the clunk. I have opened up differential housing last night and drained oil. Oil was very thick and heavy. I’m getting ready to pull brake bands and inspect them. I did notice on brake drums themselves have a little play like not tight on shaft or what ever there hooked to. Is this normal or should they be tight? I think I should pull axle tube as well to check bearings and straightness as well. I measured from drive sprocket to tub in several spots and measurements were same all the way around. If I do want to pull axle housing do I need to break track or will it loosen enough to pull axle out? Is there any thing else I should be looking for as well. Thanks for all the great help!!!!
 

mla2ofus

Well-known member
GOLD Site Supporter
If you only hear this when turning there is probably a broken tooth or two in the spider gears. When going straight the spiders gears aren't turning but when in a turn they rotate. To inspect them will require removing both axles so you can rotate the spider gears by hand to visually inspect them. Don't know which type of diff you have, whether it's the one with a removable cover over the carrier assy or the type with a removable carrier assy.
Mike
 

300 H and H

Bronze Member
GOLD Site Supporter
The "Chunk" is what you see in the diff case.

It is basically the same unit in all Bombardier machines, just comes in different sizes. There are no spider gears in the chunk, but there are 3 or 4 sandwiched planetary sets stacked side by side, with the brake drums on the outboard of the chunk. The gears inside are all timed with one another, and must be correctly timed if taken apart. If the ring and pinion look ok, I doubt the trouble is in the chunk, but maybe in the axle tubes and axles, so pull them for inspection of splines and bearings. Also examine the bottom of the case for debris that may lie in there, as there could be a clue as to what might be causing the racket.

Universal tractor oil, or HTB should be used when it goes back together. John Deere sells it as torq guard J20C. Caterpillar has a similar oil as well as many farm and home stores.

Let us know what you find..

Regards, Kirk
 

tango

New member
Thank you guys so much for the quick replies. I have drained the oil thick heavy green colored smells like 80-90. Took several hours to drain so thick. I have pulled brake bands out they are in very good shape. I would give them a 9 out 10 on a scale. Ring and pinion look great and the smaller planetary gears are all very good. Brake drums have no sign of wear. The brake drums have about .075 side play but no up and down. Are those mounted on-a spline slowing them to move or do you think I have something worn that I should look at. I ran a magnet in the bottom of pan expecting to get a few small shavings but magnet came out spotless. Not sure if I should button the differential up or wait for more replies from owners that may have seen this. I will pull the axles and tubes to inspect. Thanks again for the help on This unit.
 

GlacierSean

Member
SUPER Site Supporter
MNOUTDOORS will have the best answer. Here is my two cents, it could be track tension. My machine was clunking while turning on a previous trip. I put some grease in the offending track and the noise went away. Always try the cheap and easy fix first. The resistance from the snow or ground you are turning on can cause the machine to turn more difficulty or with more noise.

Listening to all of the crazy noises your cat makes and being able to determine witch are actually an issue is half the fun. My favorite game from grooming was (name that fluid leaking out of my cat).
 

tango

New member
Oops, I meant to say Are those mounted on-a spline allowing them to move instead of slowing them to move.
 

MNoutdoors RIP

Gone But Not Forgotten
GOLD Site Supporter
I’m in process of doing a bombi rebuild. I’m new to bombi and snow cats in general. This sight is awesome and answered a lot of questions just by looking at older posts. While turning the bombi I get the clunk noise. It sounds like drive sprocket is jumping cogs on track. I know that’s not the issue but trying to find root cause of the clunk. I have opened up differential housing last night and drained oil. Oil was very thick and heavy. I’m getting ready to pull brake bands and inspect them. I did notice on brake drums themselves have a little play like not tight on shaft or what ever there hooked to. Is this normal or should they be tight? I think I should pull axle tube as well to check bearings and straightness as well. I measured from drive sprocket to tub in several spots and measurements were same all the way around. If I do want to pull axle housing do I need to break track or will it loosen enough to pull axle out? Is there any thing else I should be looking for as well. Thanks for all the great help!!!!


Depending on which Bombi you have the front axles come in two variations.
the older one has a nut on the end,the newer one has a cap on the end with
Bolts around. The newer one is easy to remove the axle it’s a floating axle remove the cap and pull the axle. One area that is neglected on the newer Bombis is the sprocket hub. There is a grease zerk on the face next to the cap bolts that should be checked and lubed about every 50 hours of use.

As far as the .075 side movement in your drums that perfect. You are correct you want no up/down movement.

Bombi differentials are noisier than most. But with that said there have been many Bombi diffs that have had a issue with the woodruff key under the the planet gears that get some wear and or partially sheared and that throws the timing slightly off. And that will cause some noise.

Fluid that we prefer is the Amsoil power shift fluid in 50 weight if below -10 then 30 weight the Cat T04 in 50 weight is fine also and cheaper if your diff had 80-90 weight your bands may look new (1/4” thick) but maybe glazed, if glazed they will not grab like they should and therefore not turn as good.
 

snowcatt

Member
Hi
I’ve rebuilt quite a few Bombis .
And now when I get one I pull the diff change all the bushings and bearings woodruff keys clean up the brake drums and Time it that way it’s done good for years of driving .the bushings are cheap
I found the best place to split the track is just before the sprocket on top
I tack the bolt to the guides makes it easier to put back together
Good luck and have fun
 

tango

New member
Is there a way to tell if the timing is off. I really don’t want to pull differential if I don’t need to just because not sure how to reinstall with correct lashes on everything. Don’t want to make it worse if there is nothing wrong if you know what I mean. Is there something I should try or look at to check the timing on it? I think I will break tracks and pull axles to check bearings there. Thanks for all the good info and help on these things. Sure I will have plenty of questions before I done with this thing.
 

MNoutdoors RIP

Gone But Not Forgotten
GOLD Site Supporter
Is there a way to tell if the timing is off. I really don’t want to pull differential if I don’t need to just because not sure how to reinstall with correct lashes on everything. Don’t want to make it worse if there is nothing wrong if you know what I mean. Is there something I should try or look at to check the timing on it? I think I will break tracks and pull axles to check bearings there. Thanks for all the good info and help on these things. Sure I will have plenty of questions before I done with this thing.

Take a video a post with audio, before you tear it apart. many Bombi just make some noise.
 

snowcatt

Member
Did it make noise when you turn
If you hear a chatering noise I would pull it
No big deal to remove and reinstall.
I can tell you exactly what to do
Setting the lash is easy
Have fun
 

tango

New member
It did make some noise especially on sharp turn on hard pack. Maybe this is normal I have never been around one before. There is no locals around that have seen-or been around one before either. I do have differential cover off and drained right now so I can’t video it.
 
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