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Old 05-11-2019, 04:57 PM   #1
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Default Rear diff rebuild process question

I've read all the Tom Endy procedures but I have a question as to why you have to make sure the carrier locks up as the first step. Why can't you just start with the gaskets installed and check for the torque on the carrier. If you set it up and can get the correct drag on the carrier bearings by shifting gaskets, wouldn't that be sufficient?

Curious to the purpose of having to lock it up first.

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Old 05-11-2019, 06:22 PM   #2
Joe K
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Default Re: Rear diff rebuild process question

Its been a while since I read Tom's narrative, but I suspect he wants to get to a condition of uniform start so you can tell if you're actually making any progress.

IIRC, I did mine using a string wrapped around the carrier and a fish scale. NO axles involved. This method was described in "How To Restore Your Model A" Volume 6 Page 20. The string comes out through the pinion hole. He warns to not allow the string to "touch" any parts of the housing lest the friction upset the reading/force. He also "locks up" first.

His descriptive:

The purpose of this exercise is to make sure that the carrier is very difficult or impossible to turn by hand when no gaskets are placed between the banjo and axle housings. If it is possible to turn the carrier with finger pressure through the pinion race hole, shims will have to be placed either under the carrier bearings or under the races in the axle housings. These shims can be cut from brass stock by hand if not available at automotive shops. I would suggest adding at least 0.008" under each carrier bearing if it turns freely, but test again to make sure the carrier is not easily turned (i.e. the uniform start) once the shims are added.
Then - by adding gaskets equally to each side, for used bearings he looks for 3 to 10 lbs on the scale with a target of 4.5 to 5.5 lb continuous once the carrier is moving. There is a distinct difference in the force necessary to "start" the carrier and the force necessary to keep it moving.

For new bearings he looks for 7.5 lbs. All this according to his calculation which embodies Timken pre-load recommendation.

Shifting gaskets from one side to the other will not change the pre-load. Adding or substituting gaskets will change the pre-load. Shifting gaskets from one side to the other WILL change the contact point of the pinion on the ring gear. And this is the NEXT step once you have established proper carrier pre-load.

Joe K
Shudda kept the horse.

Last edited by Joe K; 05-11-2019 at 06:28 PM.
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Old 05-12-2019, 11:33 AM   #3
Jim Brierley
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Default Re: Rear diff rebuild process question

Be very careful doing that 1st step, the axle housings can easily be bent and will leak oil if they are. My method is to install the right housing with no gaskets, using 4 bolts. then with the left side up, vertically, slip the left housing on with feeler gauges supporting it, adjust clearance until proper drag is felt. Feeler gauge thickness needed is the total number of gaskets needed. Re-assemble with gaskets on both sides and adjust them for proper backlash.
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Old 05-12-2019, 06:25 PM   #4
Tom Endy
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Default Re: Rear diff rebuild process question

In the differential rebuild tutorial I wrote I indicated that the first attempt at determining carrier bearing pre-load was to see if the axle housings could be bolted up to the banjo without gaskets such that the rotation of the carrier could be locked up.

The reason for this is a starting point for a first time rebuilder who may not fully understand what it is he is trying to achieve. If you can lock the carrier without banjo gaskets installed you are assured as you add banjo gaskets that it will unlock the carrier and you probably will be able to establish a pre-load.

If you are unable to lock the carrier without gaskets you will be unable to achieve a pre-load setting unless you add shims under the carrier bearings. You can also add the shims under the races in the axle housings instead if that is your preference. However, I find it much easier to remove a bearing than a race and the carrier bearing shims are readily available from Bratton's.

Jim Brierley is correct about not aggressively over tightening the axle housing bolts when seeking the carrier lock up. The method should be to torque the bolts on the right axle housing down to 35 ft. lbs. On the left side start tightening the bolts and at the same time spin the carrier with your fingers reached in though the open banjo flange. You will achieve lock-up long before you can do damage to anything. If you end up with all the left side bolts torqued down and you can still turn the carrier, you are going to need shims.

I have to admit I do not follow this procedure as I have done enough rebuilds to understand what I am looking for. I start out with two .010 banjo gaskets because that is the least amount of gaskets I want to end up with. If I find that the carrier turns too freely with the two gaskets, I am going to need more bearing shims. If the carrier is too tight with the two gaskets, I am going to have to add more banjo gaskets.

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