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MG MGA - Help! Gearbox reassembly

I dismantled the mainshaft to replace the synchro rings. I mounted it vertically in a vice, taking parts off in order and placing them in an orderly fashion, right way up, for reassembly. Before disassembly, there did not appear to be any appreciable end float on the 2nd and 3rd gears, although I did not attempt to measure it.
After reassembly, I find that I can lift 3rd gear up by about 20 thou, and the same if I lft 2nd and 3rd together.I only dismantled far enough to replace the 2nd gear synchro ring. I took it apart again to measure any wear on the bronze thrust washer, per Barney's advice but could see none - .156 thick and no difference between the wear surface and the inner ears. So I am baffled - is this too much endfloat and if so how do I correct it?
Art Pearse

I see there is a rear thrust washer, listed, behind the 2nd synchro. I don't recall seeing that or taking it off and dropping it. How thick is that one? May have to diss again and look.

Art Pearse

Art -
The rear washer is about the same as the front locking one. The front washer was available in three sizes once - good luck with that one! But the total range of adjustment was only .005". Wear at the axial abutments of either steel washer to splines, the washers themselves, or any of the ends of the bronze bushes and washer are where play comes from. Nearly impossible to correct without replacing all worn parts, and it probably is "all parts worn". Be sure you align oil holes in bushes and shaft (that's a common reason for wear on these parts!).
I had trouble getting these parts when there were still dealers and the same gearbox was being supplied on MGB!

FRM
Fletcher R Millmore

Looks like I can buy .004 thicker front thrust washer. Does anyone know the thickness of the centre "interlocking ring" P/N 461-540 ? No point my buying one if it is no thicker than mine. Also the rear washer is listed N/A. I think it is unlikely the gear faces have worn. Another thought - maybe the main bearing has shifted backwards on the shaft during removal of the shaft (I have not touched the bearing, it feels good). Could this allow more end play?
Art Pearse

Art -
Mainshaft position is not an issue for this clearance.
The front & rear thrust washers abut the splines, and that determines position. Sometimes I had shafts that actually had a small burr formed where the washers abut. All else is cumulative wear on the spline ends and the various washer surfaces that mate, as well as the washer surfaces that the gears run on. In particular, the bronze bushes and the interlocking TW wear on the surfaces that mate axially, quite separate to the wear the gears cause. There are altogether four rotating wear surface interfaces, and another four non-rotating abutment interfaces.
I recall when you could get everything new, that you measured and set the total bronze assembly endfloat by selecting steel TW and shortening the bronze bushes. Which bush you shortened was a matter of endfloat on 2nd and 3rd gears respectively.
Within reason, the gearbox will work just fine with a good bit more clearance than spec.
The various books related to this box are murky. Magnette book says nothing about actual clearances, my MGA books are missing. MGB book says "see general data", but the only clearance is "Mainshaft endfloat .004-.006". Since the mainshaft is fixed by the center ball race, it has no endfloat really, so this figure must apply to the 2/3 bushes in some manner. Long ago I had exact info, possibly straight from BMC factory tech instructor, but that's gone now. The MGB book says you heat the bushes in hot oil in order to replace them; in practice I only had a couple that were that tight out of hundreds of boxes.

FRM
Fletcher R Millmore

Thanks Fletcher. There is room on the shaft for a front TW up to .025 thicker. Pity they don't sell them that thick.
Art Pearse

Art -
Measure the end float on 2 & 3 on their respective bushes, using the interlock washer and appropriate steel washer as it is fitted (use a bolt and flat washers to hold the assembly together). If the endfloat is too little or nil/neg, then the problem is in the wear on the bushes and/or interlock. You can try to get somebody, like Quantum, to give you measurements of new available parts. If you are good with a torch, you could build up the abutment faces at the interlock with silver solder and finish to desired length. Hard to make internally splined hardened TW!

FRM
Fletcher R Millmore

Fletcher, I measured the end float on the bushes only, using the washers as limits, as you said. 3rd gear .004, 2nd gear .003. All together, I can lift the 2nd gear(and 3rd on top of it) up .022 before the front TW hits the spline stop. If I insert a 16 thou feeler on the face of the interlock washer, it all feels good. So I am considering your suggestion of building up the interlock washer by .016 or so. Meanwhile, I have asked Moss the thickness of a new interlock washer. Is this solder repair a recognized way of doing things? And what grade of solder - 96 silver / 4 tin?
I can't move the 2nd gear bush or the rear TW - they are tight on the shaft.
BTW I am looking after a friend's South Bend lathe in his absence.
Art Pearse

Art -
Good on the measurements, now have you figured out where the wear actually is? I recommend examination under magnification - worn areas should be very obvious.

I also recommend asking somebody like John at Quantum Mechanics about available parts/sizes. I don't know him but he is widely recommended as actually knowing what he is doing AND having parts and sources for parts.

You are deficient in your old fart education: "silver soldering" is actually a "silver brazing" process; temps are in the 1200-1600F range. 96Ag/4Pb is soft solder (400-600F range), too soft for this application, though it might be good for a pressure lubed bearing. It is difficult to get recommendations for arcane and forgotten processes, but you want to get close to the characteristics of the phosphor bronze bearing alloy. I'd use Sil-Can 15 (15Ag/80Cu/5P) or 35 (35Ag/26Cu/21Zn/18Cd).

See: http://www.solders.com/brazing_alloys.htm

The practice of using a torch for overlays is or was common when people made/fixed stuff rather than buying parts; it comes back when the supermarket system fails, as it always will. I've saved a lot of pieces using these techniques. This can also be done with TIG, sterling (92.5Ag/7.5Cu) is a good candidate in this case.

Since you want to builds up a surface, an alloy with a wide range between liquidus and solidus is desireable.
You need to build it up enough that when you flatten it the corners are good, since the axial location is on the inner ID face area of the interlock washer.

FRM
Fletcher R Millmore

I'd say the wear is on the interlock washer, just by appearance. There is some scoring there. But the interlocking ears that don't get the wear are no thicker than the bearing part. Do you know which faces receive the thrust from the helical gears?
Art Pearse

Art -
Get a magnifying glass, and a good light, and LOOK at the parts.
Then get a micrometer and/or dial caliper , and MEASURE the parts.

I've been trying to point out that there are many places for wear and you have to look at each one.
I make it 12 non rotating and 8 rotating wear surfaces. One thou per and you have your clearance.

The helical gears give axial thrust, hence wear, in both directions, depending on whether the car is accelerating or decelerating; that's why there are thrust surfaces in both directions. Cars driven wimpish without downshifts have less wear in the overrun direction, but just taking your foot off the gas or driving on a bumpy road reverse thrust.

The constant reversing load hammers "the interlocking ears that don't get the wear" and the mating surface of the bushes, causing a distinctive wear pattern. It also hammers the steel washers against the splines, and the gears against the washers. Poor lubrication makes it all much worse. All of this is visible.

FRM
Fletcher R Millmore

OK, here are the gory details. First of all, I don't see how the bushes affect the end float at all, as they do not restrain movement of the gears laterally, at least not in my case. Basically I have measured wear of .0035 on washers and probably .0012 on gears for .0047 total of all axially mating surfaces of 3 washers and 2 gears. I have .021 of float. Replacing washers can get me to .0135 float. This float is combined 3+2 gears, as one will move tthe other, no way to separate the individual float.
To me it looks like it had about .017 float originally.
So - is .013-.014 float OK?
If not I will think about a thicker centre lock washer, probably have to get it made.

Inspection details:
Assembly on vertical mainshaft: Rear TW, 2nd gear, centre washer, 3rd gear, front TW.
Front TW turned to lock position.
Assembly can be lifted .021" before front TW hits the spline stop. (Feeler guages between rear TW and gear)
Same result with or without bushes installed.

Front washer:
Negl. face wear- still original machining marks over 50% of face.
Slight spline impact visible on front face. Possibly .0005
Thickness 0.1565 (probably was 0.157 new)
Available new up to 0.161

3rd gear:
Front face negl. wear (original m/c marks).
Rear face – polished, slight score. No variation in face to face measurements F-F = 2.1500

Centre washer:
Thickness 0.1545 – 0.1557. scored both faces.
New washer is 0.157 (John Esposito, Quantumechanics).
Wear = .0015 -.0025
With bushes and washer assembled, washer can move .003

2nd gear:
Unworn areas still visible. Measured across wear faces and unworn
1.3684 – 1.3678 = .0006 wear total.

Rear washer:
.001 wear, measured by diff. From unworn dogs.
.1895 across wear faces.
Impact marks from splines at rear about .0005

Front spline stop to rear shoulder stop: 3.040

Possible remediation:
New front washer, .161 .0045
New centre washer .0020
New rear washer .0010
Total .0075
End float .0135
Art Pearse

I'm thinking of turning a new centre thrust washer, .170 thick, giving me .006 end float total. Is 360 brass OK, that's what I see available?
Art Pearse

Art -
How I wish we were in the same place!
"First of all, I don't see how the bushes affect the end float at all, as they do not restrain movement of the gears laterally, at least not in my case."

There are two entirely different batches of float here.

First is the float of each gear on its bush. You would/did measure that by putting the gear on the bush and clamping the bush and the appropriate TW and measuring movement of the gear on the bush between the TW. This should be the ".004-.006". Given your measured clearances from above, this gives wear (shortening) of the bushes of .001-.005 total, not allowing for any wear on the gears or the TW faces the gear contacts. Either/both of these would mean the bushes have yet more (shortening) wear.

The second float group is the total TW/bush assembly on the shaft. The shortening wear on the bushes and interlock washer will add to whatever the original clearance was, plus any wear on spline ends or steel TW contact areas. The assembly of washers and bushes on the shaft should also have .004-.006 endfloat, with no gears in place, and putting the gear(s) on should not affect this clearance in any way.

The actual measured total float of either gear will be the bush assembly float on the shaft plus the float of that gear on its own bush, again irrespective of whether or not the other gear is in place. This would be .008-.012". (this is where the ambiguity in the factory spec comes in: if .004-.006 is total endfloat measured at the gear, then the gear on bush is .002-.003, same for bushes on shaft. Which makes your measured numbers even worse)

With both gears in place, pressing hard on either gear should not reduce the end float clearance of the other gear on its bush at all. This is the most critical point: The gear you are NOT in turns freely on its bush, but the gear you ARE in produces end thrust that will clamp the free gear in place if the float is not sufficient. This cooks things.

From your "inspection details": "Same result with or without bushes installed." indicates that the bush stack is worn to equal or less than the effective gear thickness, which means that gears can bind under load. It also means that there is not room for a reliable oil film, which both lubricates and acts as a cushion for shock loads. This may be why you have scoring on the interlock washer.

"Centre washer:
...
With bushes and washer assembled, washer can move .003"

My recollection is that when new the interlock washer was clamped between the two bush end faces, and had zero movement. This was a very nice tight fit, both on diameter and leg/notch width; it always impressed me. I do not know how much wear it takes before the bush legs contact each other and allow movement of the washer, but it is significant. At this point the washer and bush ends show a characteristic wear pattern.

Every time the drive reverses, the gear you are in hammers against the interlock washer and transmits shock loads through the bush stack to the far end of the shaft. Rover, on the 2000 gearbox, tried to make this bush/washer configuration as a one piece deal, and it was a constant and expensive gearbox failure point. The bush would break at the juncture of "washer" and "sleeve", then disintegrate into little pieces. The Rover bush cost successively more with each "upgrade", up to like $75, when the MG parts cost about $8-10 total for all three - the Rover bits still broke and the MG ones never did. I got a nice '68 TC for $600 in '71 with such a bust gearbox, really hard to drive like that, since it shifts randomly, sometimes by itself!

You "should" replace the bushes and interlock washer, but if driven reasonably gently it should work as is. I would not try to make one, since the fit of the "legs" is important, and 360 is likely not a good material choice. You can get bearing bronze from a bearing supply place, but it ain't free! I would rather build up the bush ends and/or washer as previous if I couldn't find parts.

FRM
Fletcher R Millmore

OK, I am beginning to see the light. So the "total float" which is what I have been measuring, is allowed to be in the range .012-.018 ? I am .003 over. I did not realize that the 4-6 mil spec applied to each of 3 additive floats. I think a new centre-lock washer will slightly spread the "within bush" clearances, b/c the washer tabs are slightly impacted (resulting in the .003 wobble). I can re-measure across the bushes themselves assuming a flat-all-over washer. The "bushes + gears" end float is presently .014, so I have 8 to go. I could get 4 from a thicker front TW and maybe 3 from the centre, so getting there! But it still looks like originally, my box was built at the very end of the allowances, as the measured face wear is small.
I managed to have a lump of aluminum bronze bar stock donated, so a slightly thicker washer may still be what I do.
Art Pearse

Just confirmed, I can get .005 clearance on each of the gear/bush assembles if I have a truly flat centre washer - using a straight edge across the ends of the bush that will abut the washer. So, a .161 new centre washer will give me .005 assembly float. Or a standard
Question - my rear bush is a tight fit to the shaft. I had to use the first gear hub to knock it out. Should it be an easy sliding fit?
Art Pearse

This thread was discussed between 02/04/2010 and 12/04/2010

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