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MG MGA - Time to Perform Layshaft Replacement

It's time. I will have the engine and tranny out in about a week to fix a couple leaks and to replace the MGA clutch with an MGB setup. While the tranny is out, I would like to replace the layshaft (and assorted bits) as well as the 2nd synchro. Add to that a fresh front cover/seal and a complete set of gaskets.

Assuming nothing goes terribly wrong, how much time should I budget for the tranny work. Pulling/replacing the engine will be no problem.

Tommy

Hi Tommy. I had my gearbox apart, layshaft and 2nd gear synchro replaced, and back together in about 7 hours, excluding transmission removal from the vehicle. I found it difficult to compress the small spring loaded peg, and to turn the splined retaining washer. You might want extra hands to help with this part of dismantling. Cheers, and good luck, Glenn
Glenn

I am in the process of replacing my second synchro and centre bearing (the parts arrived today)

About an hour to dismantle. The trick with the selector rods is NOT to remove them individually but to withdraw them indivisually so that the bronze gear slectors can be removed, then to undo the two screws and withdraw the three rods and their block as an assembly. It's much easier to remove the detente springs and balls without bits flying everywhere if you do it this way.

Getting the laygear bearings out is tricky, getting the new ones in almost as bad. After that it's plain sailiing, everything is relatively easy as long as you have access to a hydraulic press (to get the centre bearing off the shaft).

The needle bearings between the input and output shaft are best held in place with assembly lube or Vaseline to stop them falling out when putting things back together.

The lock tab for the big nut on the output shaft is NLA, so threadlock is called for. Ditto on the input shaft.

Make sure that the laygear is correctly oriented otherwise the front cover will leak (been there years ago.....). Make sure that lockwires are also used where original (and use threadlock too)

The assembly process will take about 1.5-2 hours if done methodically, everything is clean and all parts and gaskets are available. A second pair of hands is always helpful with holding things steady, but it's not hard to do alone. Lay everything out in correct order and orientation before you start, then it's easy.

dominic clancy

Hi Tommy. Here are some rough notes that I made last time I had my transmission apart. Perhaps they will help with your overhaul. Good luck, 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

Glenn,

Thank you for the detailed explination!

I will print them out and carry them with me.

Tom
Tommy

Hi Tommy
I have a couple of hints for you. 1. to remove the bearings in the cluster gear it works best to split the lock ring rather then try to remove them. 2. Rather than drive in the input shaft and main shaft, I found that a little heat around the hole ( careful not to heat the bearing) will let the shafts drop right in. 3. I was able to put in the trans with the shift tower on. I only removed the shift lever. 4. To reinstall the shift lever retaining spring back on it works best to use a tube that fits the top of the spring retainer (I used an alum. tube but PVC will work) cut it just shorter than the shift lever. Slide the tube on put on a washer on the top end and put on a nut to tighten down the tube and install the lock ring. 5. I found that if it slips out of gear it is usually the detent spring that is too weak. check this before reinstalling. 6. There is a new steel second gear and synchronizer that will end the 2nd gear problems. 7. I found that my front cover for the input shaft had a loose fit for the seal, it is worth checking. 8. I used dental picks to push in the lock pin on the main shaft, it worked very easy. 8. I cheated in that I did it on a pair of saw horses with a sheet of plywood work surface covered with cardboard. I drilled a large hole on one side to put the input shaft into( this made it easy to work on with the trans standing on the front end).

I hope this helps. It is not a real hard job, though it took a little time.
Bob J.

This thread was discussed between 17/01/2007 and 25/01/2007

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