|The MGA With An Attitude
REBUILDING THE MG GEARBOX - GT-203A
GT-203A - Test Drive Inspection - (you are here)
GT-203B - Open Inspection
GT-203C - Disassembly, External
GT-203D - Disassembly , Internal
GT-203E - Disassembly, Gear Clusters
GT-203F - Disassembly, Gears and Synchronizers
GT-203G - Reassembly
This dissertation was intended for an MGA, but most of it applies to the MGB as well, exclusive of any overdrive unit.
Test Drive Inspection
If the gearbox is still in the car and drivable, it can be helpful to do some testing before removal. Take the car for a short test drive to see if it shifts smoothly and easily. Stop the car (still running), shift into neutral, release clutch pedal once, depress the clutch fully to the floor and count to five before trying to engage a gear. If it grinds the gears in 1st or reverse (or any gear for that matter), or if it is very resistant to going into gear for 2nd, 3rd or 4th (or 1st for a late MGB 4-synchro unit), then the clutch is not fully releasing. You need to fix the clutch first (most likely just bleed the air out of the clutch hydraulic line), then try again.
If you get a loud squealing noise when you depress the clutch, that could be a worn spigot bushing in the tail end of the engine crankshaft, or a worn spline in the clutch disk. The same problems might cause a groaning noise just as the clutch is being engaged.
With a working clutch, engine idling, standing still, shift into neutral, depress the clutch, count to five, and listen. To muffle the engine noise, it helps to get another person to put their foot over the open end of the tail pipe and nearly stop it off, not enough to kill the engine mind you, but almost. The clutch and gearbox should be very quiet when the clutch disk and gearbox parts are stationary. Then release the clutch (still in neutral) and listen again. If it exhibits a noticeable hissing sound when the clutch is engaged, that's the ball bearing on the input shaft telling you it's tired. This is not immediately terminal and may continue to run okay for a long time. As the input bearing wears it will get much noisier before it causes catastrophic failure. If you get any other louder noises that start up when the clutch is engaged (in neutral), you might check the oil level in the gearbox. An empty gearbox can be a little noisy even if it is in otherwise good condition. Putting oil in it should quiet it down to normal. If it is still noisy with the proper oil level, then it likely has internal bearing or gear tooth problems.
Drive the car and check the shifting action while in motion. If the shifter moves easily into gear without noise when standing still, but it tends to grind the gears while shifting into 2nd, 3rd or 4th while moving, you may conclude that the synchronizer rings are worn (especially common in 2nd gear), and the gearbox will have to be removed for disassembly.
Shift into 1st gear, start up from a dead stop, and accelerate moderately briskly to 2500 rpm engine speed. Do the same for reverse gear. First and reverse gears in the MGA and early MGB have straight cut teeth and may whine lightly. That is normal, but you may not notice it with a good gearbox (and a little normal engine noise). MG Midget on the other hand is notorious for whining noticeably in 1st and reverse gears. But even there, loud whining may be bad. If you hear a loud clicking noise in 1st or reverse that increases tempo with engine speed, that would be a broken gear tooth on one of the straight cut gears, most often the smallest gear ring on the laygear. This requires disassembly to replace the broken part.
Start off in 1st, shift into 2nd gear at low speed, and accelerate moderately to 2500 rpm engine speed. Shift into 3rd and do the same. Then shift to 4th with light acceleration. If you hear a whining noise in 2nd and 3rd gears that goes away in 4th gear, that would be the layshaft telling you it is worn and needs to be replaced. The whining noise actually comes from slightly misaligned gear teeth as the shaft wears and allows the laygear to run a little off center.
If you hear a loud clicking noise in any gear, that would be a broken gear tooth, or two. A single broken tooth on 2nd or 3rd gear may not make noise, as the helical cut gears will have two or three teeth engaged at the same time. If it clicks it likely has two broken teeth. If it has three adjacent broken teeth on one gear, it may spin free without drive, or it may (more likely) lock up and refuse to allow shifting into gear when in motion. I have seen quite a few of these with a broken tooth or two, which usually does not cause any other damage. On the off chance that a broken tooth might get jammed between a mating pair of gears when running, it could pop out of gear (if you're lucky), or the interference could (rarely) do some very serious collateral damage, like bend a shaft or split the housing. As such, I can only advise you that it not a good idea to continue to drive the car if it has loud clicking or grinding noises in the gearbox. If you feel inclined to drive it home, try not to use the gear(s) that makes noise.
While you are driving about, notice the action of the manual shift lever. It should have a crisp feel with very little slop or free play. If the hand lever is particularly sloppy, moves around a lot, or seems to move too far (especially for reverse gear), that could be badly worn selector parts or sloppy support bushings for the selector shaft(s). That requires disassembly for repair.
Or it could be a broken or missing bushing between the front lever of the remote control shaft and the rear lever of the selector shaft (also called remote control shaft). This bushing can be replaced by removing the remote shift housing from the top of the gearbox, which can be done via the interior of the car without removing the gearbox. Find more information on this bushing in GT-120.
Park the car with one end on a tall curb, or over a ditch, or jack it up to get underneath (use jack stands please). Grasp the propshaft firmly just aft of the gearbox, and shake with vigor. If either universal joint in the propshaft wiggles, it will need to be replaced. If the sliding spline in the 1600 type propshaft wiggles it will need to be replaced. But that's not what we're after here. What you need to observe is any motion of the gearbox output shaft. For the 1500 type gearbox with sliding spline coupling, if the propshaft front yoke moves up/down noticeably in the tail of the gearbox, that indicates a worn tail bushing that needs to be replaced. This is generally accompanied by a badly leaking rear seal. There should be little or no perceptible radial movement of the propshaft yoke in the gearbox tail bearing. For the 1600 type gearbox with flanged output shaft, any radial motion of the flanged coupling indicates a failed ball bearing. If the gearbox tail bearing is okay, but you have a simple oil leak, the gearbox rear seal can be replaced after removing the propshaft (and the output flange on the 1600 type unit).
Also check for lateral play at the bottom end of the clutch release arm (at front end of the clutch slave cylinder pushrod). There should be only a little sideways motion of the release arm, less is better. If it has substantial lateral wobble the pivot bushing and/or pivot bolt (inside the bellhousing) are likely worn and in need of replacement (which requires removal of the engine). Also check the clevis pin and pushrod for play. Again there should be very little slop here, but it is quite common for both the clevis pin and the pushrod to be substantially worn and in need of replacement (along with the rubber boot).
If you feel confident that the car is in good enough condition to drive it with spirit at higher speed, there is one more test you can do, but only for MGA with one of the low starter 1500-type gearboxes. On an open unobstructed road, get the car up to road speed in 3rd gear. Hold the throttle full to the floor while it accelerates fully to 6000 RPM.
Road speed at 6000 RPM in 3rd gear will be as follows:
74 MPH for the 1500/1600 with 4.3:1 final drive ratio
78 MPH for the 1600-MK-II with 4.1:1 final drive ratio
81 MPH for the optional 3.91:1 final drive ratio
70 MPH with the optional 4:56:1 final drive ratio
Somewhat lower speed if it has an abnormally high final drive ratio.
Remove your hand from the shift lever. When you achieve 6000 RPM release the throttle fully and depress the clutch fully to the floor. Hold it like that for several seconds while the car will coast and the engine will slow down on its own with clutch disengaged. Observe to see if the gearbox will pop out of 3rd gear during this high speed overrun (automatic disengagement). If it does pop out of gear you may refer to the following documents related to this problem:
GT-202 - Why it Pops Out of 3rd Gear and What To Do About It
MG-241 - Jumping Out Of Gear (csm)
FT-038 - Incorrect Spring for Shift Detents
If you determine that the gearbox needs internal repairs, then you need to remove the engine to get the gearbox out of the car. See workshop manual for this procedure, but do not believe the bit about having to remove the floor (except for a few very early cars with no tunnel top cover). I find it easiest to remove the engine first and the gearbox separately, but that's for another discussion. For the 1500 type gearbox with low starter and sliding spline output, do remember to drain the oil from the gearbox. If you don't drain the oil it will run out the back end when the gearbox is separated from the propshaft.