Spitfire Spitfire 1500 differential whining

[Greyherren]

The differential of my 1500 1979 Spitfire starts whining past 50 MPH...I do have a spare one from a 1975 Spitfire, but not sure if it's an OK one. Don't want to replace the faulty one unless I am sure the replacement unit is fine. Does anyone know a good repair shop/mechanic that could test the differential and rebuild it if and as necessary? I am near Sacramento, CA

 

[bobhustead]

This response is generic, as I do not have experience with your model car. The noise you describe could be from several sources, but it seems likely to be ring and pinion. These appear from the Moss site to be third member style differentials, so testing and rebuild on the bench (instead of under the car) would be available. The thing is assembled in a detachable front part of the housing. The things to look for in the potential replacement unit are wear pattern where the teeth run on each other, how serious the wear is, and how bad the bearings are. You can find online pictures of suitable wear patterns on the gears for comparison. I suspect that you have a factory set pinion depth (distance from back of pinion gear to center of axles). This measurement is set by shims under the rear bearing on the pinion gear. Wear on the pinion bearings can affect pinion depth and the proper parallel alignment of pinion teeth with ring gear teeth. Bearing slop here can cause the whine you mention. If these bearings are bad, they can be replaced by a machine shop with a press, using the same shims in the same positions. The other dimension is called lash, or the distance between a pinion tooth and the tooth it is about to engage on the ring gear. This is measured with a dial indicator against an accessible tooth on the exposed part of the ring gear, with the pinion held fast, while rocking the ring gear to measure how far it turns when a tooth engaged with the pinion moves thru the space between two pinion teeth. This is adjusted with shims under the two carrier bearings, which control the side to side position of the ring gear (by controlling the side to side position of the carrier). These shims are typically in the same thicknesses and in the same positions for a replacement ring gear as they were on the original. These are the things to think about in assessing suitability of your old potential replacement third member- Are the bearings tight (not sloppy) and is the lash within limits.

Problems arise from loose bearings (pinion or carrier) and from excessive wear on the ring/pinion mating surfaces. You should be able to get a good idea about the conformity of this stuff from manipulating the system on the bench and any decent mechanic shop can check up on you. The finicky bit on replacement of ring and pinion (must be replaced in sets) is pinion depth. Each new ring and pinion is run together at the factory and the optimum pinion depth is determined by a machine that finds the least noisy depth. The pinion is then engraved by a handheld tool with the recommended depth for the set. You then use a specialized dial indicator the mount of which is adjusted to be in the axle center and the probe of which is adjusted to touch the flat rear foot of the installed pinion. You then compare the measurement you get to the inscribed depth to figure out how many shims of what thickness you need to add or subtract. Is this facet of the job a pain in the patoot? You betcha. A work around without using the weird dial indicator is to use some arithmetic on the inscribed depth specs of your old pinion and your new pinion and add or subtract shim thickness to accommodate. I don't suggest you do this yourself, but explain only to give you an idea of what is involved.
An eyeball test for gear wear is that the tapered edges of the pinion teeth that contact the ring gear will gradually get more knife-like as the faces of the teeth get worn down. If you see this, the gearset needs to go.

Bob

 

[Greyherren]

This response is generic, as I do not have experience with your model car. The noise you describe could be from several sources, but it seems likely to be ring and pinion. These appear from the Moss site to be third member style differentials, so testing and rebuild on the bench (instead of under the car) would be available. The thing is assembled in a detachable front part of the housing. The things to look for in the potential replacement unit are wear pattern where the teeth run on each other, how serious the wear is, and how bad the bearings are. You can find online pictures of suitable wear patterns on the gears for comparison. I suspect that you have a factory set pinion depth (distance from back of pinion gear to center of axles). This measurement is set by shims under the rear bearing on the pinion gear. Wear on the pinion bearings can affect pinion depth and the proper parallel alignment of pinion teeth with ring gear teeth. Bearing slop here can cause the whine you mention. If these bearings are bad, they can be replaced by a machine shop with a press, using the same shims in the same positions. The other dimension is called lash, or the distance between a pinion tooth and the tooth it is about to engage on the ring gear. This is measured with a dial indicator against an accessible tooth on the exposed part of the ring gear, with the pinion held fast, while rocking the ring gear to measure how far it turns when a tooth engaged with the pinion moves thru the space between two pinion teeth. This is adjusted with shims under the two carrier bearings, which control the side to side position of the ring gear (by controlling the side to side position of the carrier). These shims are typically in the same thicknesses and in the same positions for a replacement ring gear as they were on the original. These are the things to think about is assessing suitability or your old potential replacement third member- Are the bearings tight (not sloppy) and is the lash within limits.

Problems arise from loose bearings (pinion or carrier) and from excessive wear on the ring/pinion mating surfaces. You should be able to get a good idea about the conformity of this stuff from manipulating the system on the bench and any decent mechanic shop can check up on you. The finicky bit on replacement of ring and pinion (must be replaced in sets) is pinion depth. Each new ring and pinion is run together at the factory and the optimum pinion depth is determined by a machine that finds the least noisy depth. The pinion is then engraved by a handheld tool with the recommended depth for the set. You then use a specialized dial indicator the mount of which is adjusted to be in the axle center and the probe of which is adjusted to touch the flat rear foot of the installed pinion. You then compare the measurement you get to the inscribed depth to figure out how many shims of what thickness you need to add or subtract. Is this facet of the job a pain in the patoot? You betcha. A work around without using the weird dial indicator is to use some arithmetic on the inscribed depth specs of your old pinion and your new pinion and add or subtract shim thickness to accommodate. I don't suggest you do this yourself, but explain only to give you an idea of what is involved.
An eyeball test for gear wear is that the tapered edges of the pinion teeth that contact the ring gear will gradually get get more knife-like as the faces of the teeth get worn down. If you see this, the gearset needs to go.

Bob

Thanks so much Bob, will start with looking at wear patterns as well as check the bearings. I guess that anything beyond that will be beyond my pay grade......and seek help from a qualified mechanic. also, I may not have all the tools needed for a full repair. I will also take some pictures to post for any support/advice. again, thanks a lot, Hans

PS, if anyone has info on good mechanics in the Sacramento area, would be grateful to get details.

 

[bobhustead]

Yeah, you absolutely need a press for adjusting shims (gotta remove bearings every time).
Bob

 

[Greyherren]

Yeah, you absolutely need a press for adjusting shims (gotta remove bearings every time).
Bob

Ok, did check the gear ratio of a spare diff from a savaged Spit, it’s 3.9, which is right for my Spit with overdrive. But I can’t really look inside and check/repair if needed so looking for a reputable shop/ mechanic in the Sacramento area who could handle that job. Thanks in advance for any hints

 

[CJD]

Rear ends that are wearing out tend to start whining between 25-35mph, very loudly between 55-65mph, and again at 90mph if you take it that high. As the wear progresses, it will eventually whine at all speeds. To diagnose the gears:

1) Gears will stop whining or change tone when you go from drive to coast.
2) New gears that are set up wrong will whine. If caught soon, you can fix that with a proper set-up.
3) If the diff has not been rebuilt recently and starts to whine, then the gears are shot and cannot be set-up properly, no matter what you do. Thicker oil may extend the replacement for a few thousand miles, but they are toast at this point.
4) If the whine is at ALL speeds and does not change tone between drive and coast, then you have a bearing problem...not gears. It is not uncommon to have both, though.

 

[Greyherren]

Thanks to all for the detailed advices, but I am still procrastinating....it's such a pain to take out the diff, that I wonder if some teflon additive in the gear oil may help. Would it be ok to try, or would that create new issues?