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GEARBOX LUBRICATION - GT-201

Subject: Oil flow in the gearbox, and towing. -- This dissertation was intended for an MGA, but much of it applies to the MGB as well, exclusive of any overdrive unit, especially the models with the 3-syncro gearbox.

At 02:58 PM 12/15/98 -0600, Robert Allen wrote:
>.... I'd be curious to hear from those who have successfully rebuilt the manual transmission of a 'B' or Midget and who took the time to figure out the lubrication mechanism of said transmission, then describe for us slow-witted doomsayers how it would be perfectly okay to _not_ disconnect the driveshaft when towing.

Okay, I accept the challenge (but maybe not the conclusion). I have yet to do a late 'B' 4-syncro gearbox, but I have done a Midget 1275 gearbox, and MGA gearboxes (several). The MGA units are very similar to the early MGB units, and all of the above share a similar system for lubrication. And especially for the "slow-witted", I will be using lots of small words, so bear with me.

Before going any farther I want to state soundly that the 2nd and 3rd gear bronze bushings (and 1st gear in the later 4-syncro boxes) are not just splash lubricated, but are in fact pressure fed (low pressure) with a flow of oil through a drilling in the mainshaft. The source of the forced oil flow is a helical screw oil pump on the mainshaft, just behind the rear main bearing and in front of the speedo drive gear. This part is a little illusive, because the catalogs usually just call it "spacer" or "distance piece". Please remember that this part is mounted on the output shaft, so it will turn whenever the propshaft rotates, whether the engine is running or not. So here's how it works.

In the front gearbox housing, just inside of the side cover, cast into the inside top of the case, is a trough that will catch oil being splashed up by the gears on the mainshaft. Oil runs to the back of this trough, through a hole the back of the front case, and into a passage cast in the front face of the rear case, where it runs down (by gravity) to a point just below the rear main bearing. From here there is a passage cast into the rear housing leading back and up to a point in the bottom of the close fitting bore just under the helical screw "spacer" (which I will hereafter refer to as the "pump"). Since the origin of this oil flow is at the top of the case, well above the mainshaft, the resulting small pressure head causes the oil to flow readily upward into the close fitting chamber around the pump.

1500 mainshaft, center main bearing, oil pump spool, speedo drive gear, lock tab and retaining nut. When the mainshaft turns in a clockwise direction (vehicle traveling forward), the helical groove in the pump draws oil forward. (Click for larger picture). At the front end of this helical groove is a radial hole that conducts the oil flow inward to the inside of the pump, which has a hollowed out cavity around the mainshaft. From here the oil flows into a radial hole in the mainshaft to get into an axial drilled hole in the center of the shaft, where it then flows forward towards the front of the mainshaft.

In the front end of the mainshaft there is a restrictor orifice. This allows a little oil to flow out the front to lubricate the needle roller bearing between the input shaft and the output shaft. The function of said restrictor is also to retain a little oil pressure inside of the mainshaft to induce the oil to flow to other places. Also in the mainshaft there are more radial holes leading to the bronze bearings inside of the 2nd and 3rd gears (and 1st gear in the 4-syncro MGB boxes). Assuming the unit is assembled correctly, there are also aligning holes in the bronze bushings that allow this oil to flow out to the small space between the bushing and the steel gear to create an oil film bearing, similar to the journal bearings on the engine crankshaft.

The bronze bearings inside of the 2nd and 3rd gears (and sometimes 1st) are normally idling, and the only load on these bearings is the weight of the gear itself. This is true when the engine is running in any mode other than 2nd or 3rd gear (or 1st for the later units), and also when the car is being towed without the engine running. When you actually engage one of these gears while driving, that gear is locked to the mainshaft to provide the output rotation, and as such there is no relative rotation between the gear and the shaft while it is transmitting the driving load. As such, if there is any oil at all in the gearbox, these bronze bearings seem to last indefinitely. The only one I have ever had to replace was one 3rd gear bushing that was damaged on one end when the third gear thrust washer came loose from the mainshaft.

Now there are other bearings in these units. The oil level, when properly adjusted, lies about at the top of the laygear hub, so the layshaft and its needle roller bearings and most of the laygear and reverse gears are submerged in oil. Whenever the input shaft is turning lots of oil is transferred from the laygear to the gears on the mainshaft, and oil is being splashed all over inside of the housing. The large ball bearings on the input shaft and on the main shaft are just dipping into the oil so the ball races are liberally bathed in oil while turning.

Besides being submerged in oil, the layshaft also has a form of forced oil flow to the bearings. There are holes drilled in the front and rear of the main case, and channels cast into the front cover plate and rear housing, such that oil can get to both ends of the layshaft. The layshaft has axial holes drilled in from both ends, and also a few radial holes to allow oil to flow into the bearing cavity inside of the laygear. Rotational motion of the laygear causes some centrifugal force (oops, sorry for the big word) in the oil between the laygear and the ends of the main case (at the thrust washers). This force moves oil away from the layshaft at the ends, effectively dragging oil through the bearing cavity inside the laygear, and creating oil flow around those needle roller bearings. Without this flow metal particles could accumulate inside of the laygear and accelerate the wear on the layshaft. With this flow any particles there will be flushed out and (hopefully) settle out in the bottom of the gear case.

And while we're on the subject, in light of this last statement, how many of you know that you should change the oil in the gearbox and rear axle about every second oil change? More importantly, how many of you actually do it? Better check the maintenance schedule in the shop manual. For most people this may nearly match the interval for flushing the brake fluid, another sorely neglected point.

Now there's one more bearing in the gearbox, the one at the far rear of the mainshaft where the driveshaft connects. MGA 1500s have a bronze bushing here with a sliding spline connector for the driveshaft yoke. Very late MGA 1500s and all later MGA and all MGB gearboxes have a ball bearing here, and a flanged coupling for the driveshaft. In the case of quick acceleration a generous splash of oil may find its way back there. At steady cruising this will not happen, but this action is not left to chance. Along the inside of the rear case, about half way up on the right side, is cast another oil catch trough. Whenever the output shaft is turning oil is splashed up here by the speedo drive gears. This oil runs to the back of the rear case where it can enter a cross drilled hole and end up just in front of the rear bearing. There is a large flange in the casting here surrounding the mainshaft, so as to encourage the oil to flow farther back and to flow through the rear bearing. Behind the rear bearing there is a groove inside the bottom of the case to allow oil to flow forward below the rear bearing and through a hole back into the forward part of the rear case. As such, oil will flow here constantly and bathe the rear bearing whenever the mainshaft is turning.

As to the question of whether you can tow one of these things with the driveshaft connected without damage, the answer is a resounding "it depends", and what it depends on is the type of gearbox and having the proper oil level. The real parts in question here are the bronze bushings inside of 2nd and 3rd gears (and sometimes 1st), and also the needle roller bearing at the front of the mainshaft. The oil pump being driven on the output shaft will supply forced oil to the mainshaft while being towed. This is because the oil supply to the pump is from splash in the top of the main gear case. This splash oil comes from the large 1st/reverse bull gear (in the 3-synchro unit) that will be dipping in to the oil while being towed.

In the case of the 4-syncro box, the large reverse gear on the mainshaft will always be turning with the mainshaft, and being unshielded will throw plenty of oil up into the catch trough on top. So if the oil is at the correct level the entire gearbox would be properly lubricated by the pump while towing. So as a challenge at this time, I would like to know if anyone has ever towed an MGA or MGB with the driveshaft connected and incurred any internal damage to the gearbox? I think this would not be a problem, and these can be towed with the driveshaft connected without worry.

I myself have towed an MGA more than 150 miles on the rear wheels without disconnecting the driveshaft, with no apparent problem, and no noticeable wear on the 2nd or 3rd gear bushings when disassembled many years and many hard miles later. Once there is a proper oil film in the mainshaft bushings it will remain there for quite a long time, as the gears in question are simply idling stationary with their own weight on the bushings as the mainshaft turns.

Barney Gaylord

    PS
In recent years I have been getting more confident that the MG can be towed for long distance without damage to the gearbox. My own MGA has been towed up to 150 miles on a few occasions with no problem. I have heard of a few others that were towed up to 2000 miles with no problem (some more than once). The only horror stories I have heard (very few indeed) were cases where it was not noted if there was any oil in the gearbox, and this was not verified even after inquiry.

For sure the success of keeping the gearbox lubricated during towing depends on having the correct amount of oil in the gearbox. If oil level is extremely low, first gear on the mainshaft may not dip into the oil, so there would be no splash oil supplied to the oil pump. Before towing your MG you should always check the oil level in the gearbox.
    BG

Addenmdum:
At 06:39 AM 11/11/2009 -0700, Chris Logue with a ZB Magnette in England wrote:
I have raced various cars using the 1500 gearbox, and estimate that I A-framed one of them 5,000 miles and the other two about half of that. No other component gave any trouble. None of the boxes were rebuilt units, and had already done around 60,000 miles before being raced.

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