I have no reference to gearbox/engine/chassis numbers but I do know that
early MGA gearboxes used a selector that had a long tube cast as part of the
selector itself. Later 'boxes used a shorter selector with a tube on the
shaft. Probably it was less expensive to do it this way.
Lawrie
British Sportscar Center
-----Original Message-----
From: Barney Gaylord <barneymg@ntsource.com>
To: Baker, G. <gbaker@customcpu.com>
Cc: mgs@autox.team.net <mgs@autox.team.net>
Date: Sunday, October 25, 1998 10:16 PM
Subject: Re: Transmission rebuild
>At 11:10 AM 10/25/98 -0900, Baker, G. wrote:
>>.... My veh is a 1956 MGA, not chasis # but engine #BP15G-1444.
>1. I've got no spacer tube on my 1st/2nd shaft (Moss 3461-140).
>
>Probably never was one in that (early) model gearbox, but you could add one
>if you like. I think it goes on the 3rd/4th rod, not on the 1st/2nd rod.
>I believe this part was introduced at or shortly after the start of the
>1600 (type) gearbox that came in just before the end of the 1500 engines.
>And I think the tube spacer is intended to stop the motion of the 3rd/4th
>shift fork/rod as it is entering 3rd gear position, such that the ball
>detents in the 3rd gear hub and on the shift rod will fall directly into
>the detent notches and not allow an overshoot condition which would
>encourage it to rebound and pop out of 3rd gear. Other small changes were
>also made at this time to reduce the pop-out-of-3rd problem, see more below
>with item 6.
>
>2. I've got no shim (Moss #461-280) on the input shaft. Could these be a
>later or unnecessary modifications or has someone been there before me.
>
>Someone has likely been there before you. I do not recall ever seeing less
>than two shims (at 0.002" each in this location, and sometimes 3 or 4
>shims. The idea here is to minimize end float for the input bearing in the
>aluminum housing. Proper measurements and correct shimming can control end
>clearance for the bearing at 0.001" to 0.003". Making the clearance zero
>or less (interference) will defeat the front cover gasket and result in an
>oil leak. Leaving axial clearance of more than 0.003" results in end float
>of the input shaft on acceleration and deceleration in gears 1 through 3
>and reverse (helical gear mesh with laygear) which may cause undo wear to
>the bore of the aluminum housing. I know you've read my web page, so you
>know how to fit the shims. Buy a few, they're cheap, maybe 2 each of
>0.002" and 0.004".
>
>>3. How do you suggest one removes the circlips holding the needle
>bearings in the laygears and once they are out can they be replaced with
>newer encased bearings I've read about? My layshaft does show significant
>wear so the bearing should be renewed. Any experience in this would be
>appreciated.
>
>The encased bearings are interchangeable with the original loose needles,
>and lots easier to use. Buy the shaft, buy the needle bearings, get
>creative and get the circlips out somehow. Whatever you do, you will break
>or otherwise damage them beyond use, so have new ones ready to install.
>You should only have to remove two of the circlips, whichever two are
>easiest to reach, leave the third one in place. I seem to recall retaining
>one end of the clip with a small flat blade screwdriver to prevent rotation
>in the groove while poking a sharp pointed awl under the other end of the
>clip to lift it out of the groove, then putting the flat blade behind the
>lifted end of the clip ring and bending it inward with a twist of the
>blade. For sure the first clip you do is a knuckle torturing two-beer job,
>but once you figure it out the second one is out in a minute or two.
>
>>4. All my gears look pretty good.......to me..........but maybe I'm
>missing what to look for. The reverse gear does have some very small
>"lipping" on the gears but I think I'll just dress them. Other than the
>minor wear on reverse the only real wear is on the Baulk(Synchro) rings,
>the bushings and the layshaft.
>
>If there are no obvious broken chips in the ends of the teeth, use a
>Gibbonizer to gently touch off any sharp edges or burrs, bless the gears
>and put them back in. Check the bushing in the reverse gear, usually
>perfect but it doesn't hurt to check.
>
>>5. I am in a quandry as to the Forks because I don't know what to look
>for. I suspect they're worn but I don't know what warrants replacement.
>
>It's a little subjective, but the forks should not rock excessively if
>wiggled in the hub slot with the shift rod removed, and they should not
>bind in the rotating hub with the shift rods in place. They likely had
>0.002" to 0.005" of end float when new, but if wear has produced no more
>than 0.020" end float now, they should be perfectly functional. This is
>from no published source, but from some years of mechanical engineering
>experience.
>
>>6. Has anyone modified their 3rd/4th selector shaft by deepening the
>detent so as to prevent jumping out of 3rd gear.
>
>Not I, but .... the workshop manual section FF (exactly one page) describes
>the modification from the factory with the 1600 gearboxes. Shallower depth
>of drill hole for the 3rd gear rod detent spring can be achieved with a
>small shim button in the bottom of the bore before the spring is installed.
> Two detent notches in the shift rod were deepened by 0.018", which can be
>achieved by changing the shift rod to the later production part, or if you
>have an "in" with a machinist you may have the two cylindrical notches
>ground deeper (not the center one). Also check the length of all the
>detent springs under load, the book gives the details of load vs length.
>Personally I have no problem with the original design, as by now my car has
>trained me to keep my hand on the shifter when downshifting into 3rd gear
>at speed, so now I never notice it pop out of 3rd gear (like it used to).
>
>>7. I don't see a lot of wear on the thrust washers but plan on mikeing
>them. Is there anything to look for with these? I notice Moss sells three
>different sizes.
>
>One size for the front, four different sizes for the rear. These are
>hardened steel washers running in oil in the lowest part of the gearbox, so
>usually not much wear. If you don't change the laygear the original
>washers are the ones to use. Rear washers are available (or at least
>spec'd) in 0.003" increments. Can't recall where I saw it, but I think the
>end clearance (dry) is intended to be 0.004" to 0.008" (2 washers = 4 oil
>spaces). A little more clearance probably won't be noticeable, it just
>adds up to a little extra backlash in the gear train as the helical gears
>can float axially. If you're like me and always on the gas or on the
>brakes, you'll never notice. If you're in the habit of lugging the engine
>under load below 1000 rpm you may get a resonant rotational shaking of the
>engine on the rubber mounts which may take advantage of said backlash and
>shake your neck accordingly, reminding you not to do that.
>
>> I plan on renewing baulk rings, layshaft, springs, needle bearings(four
>sets), bushings(3), gaskets, roller bearings, seals, and possibly thrust
>washers. Any other thought?
>
>Thrust washers are probably okay. New gaskets, seals, lay shaft and needle
>bearings are a given. Most springs are likely okay, but check length
>against load per spec's in the book. If you buy new ones, check them also
>before installation.
>
>Bronze bushings and interlocking ring on the layshaft I have only replaced
>after I have damaged them during R&R. I have not known them to wear out in
>service. Either the gear is locked while engaged with no relative rotation
>on the bushing, or the gear is free-wheeling on the bushing with no load.
>
>For the big ball bearings, rotate them in your fingers with a small thrust
>load and a bit of oil inside. If you feel any vibration or scraping
>greater than perfectly smooth, or can hear any noises from within, replace
>the bearing. The input bearing is more prone to wear than the center one.
>I have very little experience with the tail bearing in the 1600 box, but
>the same test applies.
>
>One last big headache may be the big steel/bronze bushing carrying the
>sliding spline drive shaft front yoke in the rear of the 1500 gearbox. Two
>years of hunting says this part is made out of Unobtainium. For
>replacement I used a plain bronze bushing procured from the local bearing
>supply house. Original bushing is 1-3/8" ID x 1-1/2" OD x 2-3/4" long.
>New bronze bearings do not come that long, so buy two pieces each 2" long,
>cut a 3/4" long piece from one and install it along with the 2" long piece
>to make up the 2-3/4" length. I have one in service now for 30,000 miles,
>no problem. And these plain bronze bearings will not need to be honed
>after installation.
>
>When you place the drive shaft front yoke in this bearing with a little
>oil, there should be no perceptible side play. Any noticeable radial
>motion here will quickly destroy the rear seal, allowing the oil to escape
>from the gearbox, possibly causing catastrophic failure of this rear
>bearing, the drive shaft yoke, and maybe the housing as well (been there).
>First clue that this may be happening is a nasty vibration coming from the
>drive shaft, very similar to the effect of a bad u-joint.
>
>For a much detailed synopsis on what to look for in worn parts during the
>gearbox rebuild, see this web page:
> http://www.ntsource.com/~barneymg/mgtech/gearbox/gt101.htm
>
>Have fun with the tinker toys,
>
>Barney Gaylord
>1958 MGA with an attitude
>
|