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MG MGA - MGA gearbox

I'm just about to try to rebuild my MGA's gearbox. Any hints, tips or warnings will be gratefully received.

Thank you.
Geoff Love

Hi Geoff. Here are some notes I made during my last tranny overhaul. I hope they help. Glenn.

MGA Transmission Rebuild
1956 Transmission NO. NGN 2262

It is late November 2003. I have just finished rebuilding the transmission from my MGA 1500 roadster. Here are a few notes and pointers that will assist in the next transmission overhaul.

Degrease the outside of the transmission before disassembly. When removing the rear extension, re bolt the remote shifter assembly back in place, so you can use the shift lever to move the shift forks. Remove the speedometer drive unit, and the shifter interlock unit. The shift forks must then be moved and manouvered so that the rear extension can be removed from the main gearbox housing. This is best accomplished by putting the transmission in neutral. The three shift lever channels are then aligned, and the shift mechanism can be easily moved to one side, and the rear extension removed. The rear extension may be a tight fit and might have to be gently tapped off from the main case.

Once the rear extension is gone, the 3 steel shift rods need to be removed. To do this, remove the 3 setscrews holding the shift forks to the shift rods. Unbolt the steel block at the back of the main aluminum housing (2 bolts) using a socket wrench. The shift rods will have to be partially moved rearward to allow clearance for a socket wrench! After the shift rod block is free, then pull all 3 shift rods toward the rear, but DO NOT remove them from the steel block, or detent springs and ball bearings will fly out and get lost. Once all 3 shift rods are pulled back, the shift forks, inside the main housing can be removed. Remove the forks in the order they appear from the large opening in the main case. The reverse fork is removed first, then the other 2 forks are sequentially taken out.





Once the forks are out, the reverse gear can be removed, by loosening the single lockbolt (with lock tab) and removing the bolt. The reverse gear shaft taps out toward the rear.

When reverse gear is removed, the layshaft can be tapped out, and the laygear can be set in the bottom of the gearbox. The 3rd motion shaft is then removed by tapping it out from inside the gearbox. Use a long narrow brass punch or piece of hardwood. Be sure to check the laygear end play which should be around .002 to .006 inches. Oversize laygear thrust washers are available from Moss and elsewhere.

To remove the gearbox front cover, the clutch throwout arm must be removed first. When the front cover is off, carefully mark the front cover mating area in regard to location of the layshaft. The front of the layshaft is notched, and a semi circular section fits into a corresponding notch in the front cover. Failure to mark the orientation of the layshaft to the gearbox casing makes re fitting the front cover a guessing game. If marking IS forgotten it is JUST possible to put the front cover in place and then rotate the layshaft from the rear of the gearbox with small needle nose vice grips, until the layshaft is felt to engage the notch in the front cover. The layshaft can then be tapped forward where it will seat in the front cover.

When the mainshaft is out, the input shaft can be tapped out from inside the gearbox, using a long (brass) drift punch or piece of hardwood. Be careful not to pound too hard on the bearing, since it would be damaged.





Degrease the inside of the main and rear extension housings. Degrease all nuts bolts and washers. Wash out the case components with solvent. Replace front and rear main seals if required. Check the clutch lever pivot bolt and bushing for wear and replace as required.

In regard to disassembly of the mainshaft: Remove from the mainshaft, the first sliding hub that houses the 3rd and 4th synchro rings. DO NOT separate the parts of the sliding hub, as springs and balls will fly out. Reassembling the hub is difficult to impossible without special factory tools. Check 3rd and 4th gear synchro rings for wear, and replace as required. Then remove the next helical gear and bronze bushing that is fitted on the mainshaft.

This exposes a splined steel washer. Look for a spring loaded pin poking out from the mainshaft, that holds the splined washer in place. Using 2 jeweler’s screwdrivers, compress the pin, and rotate the washer in either direction. This lines up the splines on the washer, with the splines on the mainshaft, so the washer can then be removed. This washer can be difficult to remove.

Use one screwdriver to compress the pin, then wedge the other screwdriver between the pin and the splined washer. You should then be able to rotate the splined washer, and remove it. Once the splined washer is off, you can remove the next helical gear, and replace the second gear synchro ring if required. More disassembly of the mainshaft is not normally required unless wear further down in the mainshaft components is suspected. Reassemble everything in reverse order.

When putting the bronze bushing back on the mainshaft, be sure that the oil hole in the mainshaft lines up with the oil hole in the bronze bushing. This is important for lubrication of moving parts!


Replace the layshaft if it is worn. If the layshaft is worn, the layshaft bearings should likely be replaced at the same time unless in excellent condition with minimal slop.

When installing the laygear, use a “dummy” layshaft. I made one from a piece of standard 5/8” OD PVC plumbing water supply pipe cut to 6 and 13/16 inches long. This is long enough to hold both the layshaft thrust washers in place when the laygear is sitting in the bottom of the gearbox.

To re assemble the gearbox. Fit the dummy layshaft inside of the laygear. Grease the ends of the laygear, and put the thrust washers in place. The small washer goes on small end of laygear, the big washer on big end of laygear.

Install the input shaft. Tap the input shaft into the case, and make sure it bottoms. Heavily grease the inside of the input shaft, and install the 18 needle bearings.

Blow air through the mainshaft oil restrictor to confirm it is clear, then install the mainshaft through the rear of the gearbox, making sure the mainshaft spigot goes into the input shaft straight, and does not dislodge any of the 18 needle bearings.

Do not drive the mainshaft home, but rather, use the rear extension housing gasket to align the rear bearing housing pin in the proper orientation with the case. Then tap the mainshaft all the way home.

Fit the reverse gear, tapping the shaft in from the rear. Make sure the reverse shaft locking bolt lines up with the hole in the reverse shaft, and use a new locking tab.




With reverse gear installed, bring the laygear with thrust washers into mesh with the mainshaft gears. Slide the layshaft into the gearbox from the rear. The layshaft will push the previously installed dummy layshaft out the front of the case.

Install the 3 shift forks and shift rods in the reverse order they were removed. Before the 3 shaft are all the way into the gearbox, bolt the metal shaft block with 2 alignment pins back onto the rear of the gearbox, using a socket wrench. An ordinary box end wrench will not work! Slide all 3 shafts into the box, making sure that the shift levers for each rod are placed on their respective gears in the proper position. The shift rods are then pushed through their respective forks, and all three forks are locked to their shafts using the lockbolts. Make sure that the lockbolts are aligned with the holes in their respective shafts.

Place a new gasket on the rear of the main gearbox housing, and install the tailshaft housing. Make sure that the transmission is in neutral. Then lower the tailshaft housing onto the main case, amnouvering the linkage, so that the main shift lever engages the three shift shaft components, which should be in line, if the transmission is in neutral. Manouver the shifter parts so that the tailshaft housing seats on the main gearbox housing. Make sure that the single stud in the main case receives a locknot and washer before the tailshaft casting is seated on the main case. Bolt the rear housing to the gearbox.

Install the front cover, and clutch lever, with boot.
Making sure that reverse gear is engaged, install the Interlock unit under the small stamped steel cover, and bolt the cover into place using a new gasket. Screw in the speedometer drive sending unit, and bolt the main cover in place using a new gasket.


Install the shifter linkage on the tailshaft casting, (If not already installed) and check that the gearbox engages all gears properly. This can be done by manually spinning the input shaft and watching that the output shaft turns freely. Don’t forget to check reverse gear, and note that the output shaft should turn in the opposite direction to the turning of the input shaft.

Be sure to put oil back in the gearbox BEFORE starting the engine. 30 wt motor oil is specified in the manual, but 20 w 50 synthetic motor oil, or redline synthetic transmission oil is a better choice.

Be sure to check and install a new clutch release bearing, with the carbon portion facing forward, away from the transmission. The carbon block rides on the back of the clutch pressure plate.

Note that the remote shift linkage needs to be removed before the transmission can be reinstalled in the vehicle. Use 2 new gaskets when installing the remote shift linkage.
Glenn

A few added suggestions

1. When you take the gearbox out of the car, there will still be oil in it even if you have drained it - the extension doesn't drain well. If you have a 1500 slide in propshaft, the oil in the extension will be dumped on the floor, so put something down to catch it or soak it up. If you have a 1600 box with a rear flange, stand the gbx on the bell housing for 15 minutes, then drain the oil from the normal drain hole.

2. Use loctite threadlock instead of the various lock tabs. Some of the lock tabs appear to be unobtainable, and are anyway tricky to install. Threadlock is a perfectly good and easily used alternative.

3. Make sure that there is some free play around the front cover mounting holes to allow the front cover oil seal to centre on the input shaft. Otherwise it will leak. Make sure that the notch on the laygear is correctly oriented to allow the front cover to sit properly.

4. A piston ring compressor is a handy tool for reassembling the various springs in the fourth/third gear hub. This assembly can be put on the mainshaft incorrectly. If the fit up to the third gear is not tight, it's the wrong way round. Easiest to check out of the box. I always mount this on the shaft after I have put the mainshaft inte the rear of the box.

5. I use the new layshaft to hold the thrust washer at the front end of the box, and the old layshaft to hold the washer at the rear. Then you dont need a dummy layshaft. This is the trickiest bit of all in getting things back together (although laygear bearings are a pig too). Greasing the washers heavily to stick them in place is essential.

6. Rear gear layshaft has the slot to the rear of the box

7. You don't need to touch the lockwire on the selector rods. When withdrawing them from the block, cover the block with an old towel to stop things flying past your ear. For reassembly, a second pair of hands makes things much easier.

8. Gently file off any burrs from mating surfaces to make sure everything is flat on reassembly. Then it wont leak. A small dab of sealant at every gasket face, and especially at the sidecover bolts is essential.

9. The bronze collet at the gear shif extension has a nasty habit of dropping off into the housing as you reinstall the shifter assembly. It normally ends up needing retrieval a few times before it finds its natural place.

10. Grease the ball joint and slider shaft of the shifter assembly before installation. Makes a big difference to the smoothness of shifting.

11. Make sure the metal cover on the extension is the right way round to allow the interlock assembly to operate. In my Moss catalogue, it's the wrong way up in the illustration. The square dome should be at the front end of the plate. The only way to correct it once in the car is to take the box out again......

12. The rear seal numbering in many of the catalogues is also switched. Make clear when ordering which type of box you have, don't give the part number! (been there...)

If you install the shifter assempy bolts and slave cylinder bolts, the box sits on the bench with more stability to give access through the side cover. I put the tail on a wooden block when it is fitted.

It's all very simple, just looks complicated. Apart from the 3/4 gear hub, every piece only fits one way round. I can do a tear down and rebuild in about 3 hours once I have all the bits and the box on the bench.
dominic clancy

Thank you both for your extremely comprehensive and helpful replies.
Geoff Love

Geoff,
There are several discussions about modifications to the layshaft bearings in the archives. I chose to do this to my trannys during overhaul as I was concerned about the new caged needle bearings available from the Brit parts houses having only half as many needles as origional. I brought this up during the tech session at GT in Oregon and the opinion was that it was not a problem. It is not expensive and I believe it is worth it. Hope this helps and have a good day!

John
John Progess

Many thanks for the additional info.
Geoff Love

Geoff -

Barney has an extensive article about this at mgaguru.com. I will be using that and the above info as I do my rebuild.

I have the tranny out and cleaned. I was getting a growl and some "tick back" through the shift knob while in first gear so I thought a tooth might be missing but upon inspection, all gear teeth are there. Will dig in deeper!
Steven B

Best advice aside form all of the preceding excellent material is to convert to the later 4 bearing laygear if you can find one. Much stronger and better supported, and you'll never have a problem with it.

Favourite trick to play on a friend that has just buttoned up his gearbox - drop a couple of laygear bearing rollers on the bench and watch him go crazy thinking he left a couple out and will now have to tear it down again. :-)
Bill Spohn

Bill, that's just wrong. I'll have to remember that one.
Del Rawlins

This thread was discussed between 14/04/2007 and 16/04/2007

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