Which shim to use?

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ArthurB
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Which shim to use?

Post by ArthurB » Mon Feb 17, 2020 8:40 pm

I am adjusting the pinion gear lash. I'm trying to choose between a thrust plate shim and ring gear shim. It seems to me the ring gear shim will only change the pinion gear lash. What makes my head hurt is trying to figure out what the thrust plate shim will change. Will it change the axle to axle clearance? The required thickness of the right side bronze thrust washer? The amount of left axle sticking out if the housing? The amount of right axle sticking out if the housing?

My guess is "no" on the axle to axle clearance, and "yes" on all the others. Am I thinking straight?

If it matters, I have used gears and I'm trying to reduce the lash from about 0.020" to 0.010".


Norman Kling
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Re: Which shim to use?

Post by Norman Kling » Mon Feb 17, 2020 8:53 pm

The axle to axle clearance is set by the washer between the axle ends within the differential. The length the axle protrudes out each side is set by the thrust washers at the inside bearings. The distance of the parking drum from the backing plate will fit with original parts unless there is some wear within the hubs or the axle ends. However, if you have auxiliary breaks with separate hubs, you might need the shims to move the internal parking break backing drum because it is bolted inside the auxiliary drum.

The shims for ring and pinion clearance should be determined according to the gear mesh. The pinion shim will move the pinion back farther which could cause it to be too far toward the rear and not mesh completely with the ring gear. So you need to adjust it to be centered front to back with the ring gear. Lastly a shim behind the ring gear will move it toward the pinion gear So a combination of all these things will determine the best mesh for the gears. you will also need to face the sleeve inside the front of the driveshaft tube to keep the pinion from moving back or forward in the tube. Get the rear axle book and it will have pictured step by step instructions on how to rebuild your rear axle assembly. Also if you use aftermarket pinion bearings, things will be even different. In that case follow the instructions furnished by the manufacturer
Norm

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ArthurB
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Re: Which shim to use?

Post by ArthurB » Mon Feb 17, 2020 9:54 pm

Thanks Norman, I've been following the Chaffin book and the Fun Projects instructions. I've reached the point where Chaffin says I can use a shim under the ring gear or the "more primitive" solution of a thrust plate shim. He mentions the thrust plate shim will shift the entire assembly to the right, but I'm trying to figure out what exactly that means in terms of setup. Seems like a ring gear shim is the easiest solution, but I want to make sure I fully understand what changes what if I use a thrust plate shim. Thanks for the heads up on the brake drum. I hadn't considered that one.


Scott_Conger
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Re: Which shim to use?

Post by Scott_Conger » Mon Feb 17, 2020 11:24 pm

I understand that the rear axle book mentions the use of a ring gear shim, and most suppliers sell them. That does not mean their use is either ideal or superior to other remedies. Anytime you place a shim between a gear and the assembly that it drives, you are introducing another, very large surface area which the bolts are trying to clamp against and maintain rotational stability. This increases the possibility of the gear shifting and placing great strain on the bolts. A more mechanically sound approach is to shift the entire assembly "over" by selectively increasing/decreasing the bronze thrust washers and the use of thrust washer shims. Such a shift should be essentially invisible to the mechanism as to how it relates to the housing(s). If there exists some sort of alarming excess clearance, the cause should be fully understood before remedial action takes place, and the remediation should solve the Root Cause of the clearance problem, not simply mask it's existance. Point being, if there is a clearance issue, it is nearly certain that it is NOT caused by a fitment problem between the differential housing and the ring gear.

All of the above assumes that the pinion is absolutely correct in its relation to the ring gear and that at least a cursory examination for correct contact pattern has been established between both gears, all prior to correcting for depth.
Scott Conger

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Re: Which shim to use?

Post by AZTerry » Mon Feb 17, 2020 11:35 pm

Hello Arthur,

I will try to address your individual questions.

1) It seems to me the ring gear shim will only change the pinion gear lash. You are partially correct, it does this by moving the gear teeth deeper into the mating gear.

2) What makes my head hurt is trying to figure out what the thrust plate shim will change. The (driver side) thrust plate shim changes two things at the same time. It changes the centering of the differential carrier in the axle housings. It also changes the depth of engagement of the ring and pinion gears which is what changes the backlash.

3) Will it (the bronze thrust washer shim) change the axle to axle clearance? No, the thrust washer between the two axles in the differential carrier changes the axle clearance. Will it move the position of the axles at the backing plates, yes.

4) The required thickness of the right side bronze thrust washer? Yes, if you add a shim to the driver side thrust washer you will reduce the size of the required passenger side thrust washer.

5) The amount of left axle sticking out if the housing? The amount of right axle sticking out if the housing? Yes, if you add a shim to the driver side bronze thrust washer it will reduce the driver side stick out by that amount and increase the passenger side sick out by that amount.

6) If it matters, I have used gears and I'm trying to reduce the lash from about 0.020" to 0.010". Yes it matters. If you have 0.005 worn off the face of both gears, you may be at the correct pinion depth now.

I have not read the manual in many years, but my suggestion at this point of the project is forget about the backlash for now. Go to the next step of setting up the differential carrier end play. This will tell you what thickness of thrust washer you need on the passenger side. If you are using new bronze thrust washers and you need to use a shim under the thrust washer then consider shimming the driver side thrust washer.

I hope this helps
Terry


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Re: Which shim to use?

Post by Allan » Tue Feb 18, 2020 5:30 am

Arthur, have you had the assembly fully built and the two halves bolted together, to see if you have room for a shim anywhere? If the combined stack of the four steel discs and the two bronze thrust washers is too thick to allow the two halves to be bolted together, then you can start to fiddle. With the two halves lightly bolted together, you can measure any distance between the two diff halves to give you a rough idea how much you need to reduce the washer and bronze thrust stack. You can grab a few thou, if you have a selection of steel washers from which to choose. Any more needs to come off the bronze thrusts. I like to make these .010" different in thickness. If you have to take out .030", take .020" off one and .010" of the other. That way, you can juggle your thrusts and any varying steel washers from either side of the diff carrier when setting your crown wheel and pinion mesh.

As previously indicated, a shim behind the crown wheel is not good practise. It should not be necessary if you can juggle the thrust stack.

Hope this helps,
Allan from down under.

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ArthurB
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Re: Which shim to use?

Post by ArthurB » Tue Feb 18, 2020 11:56 pm

Thanks guys, very helpful information. Since I first posted I used Prussian Blue to check the gear engagement. It looks spot on, so I am hesitant to change the gear lash. Lash is at .020 +/- . A mechanic friend looked at it and said "don't touch a thing."

My gears are used, axles and drive shaft are new, FYI. I think I'm now properly educated to make a call about the washer stacks. I feel like I'm entering the home stretch.


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Re: Which shim to use?

Post by Scott_Conger » Wed Feb 19, 2020 10:51 am

Arthur

very glad you're comfortable with your progress. If you please, it's always nice for readers to follow a project to the end...with that in mind I'll ask that you report your findings once on the road, along with your thoughts on the process. This would make for an interesting read and a good source for future questions for future readers regarding a rebuild. Thanks.
Scott Conger

Full Flow Float Valves - deliver fuel like Henry intended!


Allan
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Re: Which shim to use?

Post by Allan » Wed Feb 19, 2020 6:02 pm

Arthur, there are some who would say .020" backlash is too much. If the pinion is engaged for the full length of the teeth, and your prussian blue indicates full engagement on the teeth, you have it right. That is preferable to having less backlash but only part engagement at the gear faces. Well done.

Allan from down under.

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