Forums General Counties Cars Discussion Gearbox rebuild

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    I purchased my A40 pickup earlier this year. Inevitably, after about 500 miles the cylinder head gasket failed. On dismantling the top end of the engine it became apparent that a full engine rebuild was required. The top of the engine block was distorted and hollow on the joint face ( the head was fine).  I’m not familiar with these vehicles but I also suspected that I had excessive play in the column gearchange which was causing vagueness in gear selection. As I also had oil leaks  I decided to pull the gearbox out at the same time to address these issues. Here is a description of the faults  I found and how I tackled them.

    Lost motion: On dismantling the gearbox I discovered that the bronze selector forks were worn in the slots where the gear engagement lever locates.  I was able to build up the worn areas with silver solder. Then on my little model makers milling machine and with a file, I was able to re-profile the slots.

    Next I saw that the ends of selector arm were badly mushroomed over and slack where they push against the gear engagement lever. I first tried building them up with my tig welder using plain mild steel welding rods. When I came to re-profile them back to shape with a file I felt that the material was too soft and might not last well in service so I removed all this metal and reworked the ends using stainless steel rod which I hope will be a bit more durable.

    Oil leaks: I think that these are the 2 main areas where play can build up in the gearbox. There is a third area which probably doesn’t cause much lost motion but it doesn’t hurt to reduce the wear that may occur in it. This is the cross shaft which runs directly in the  gearbox casting, and after 65 odd years, wears where the the shaft exits on the right hand side. My gearbox had about .25mm movement here which will probably cause oil leaks in this area even if the seal is replaced. There are a couple of ways to rectify this fault . The simple way is to give the stripped down box to a machine shop and get them to bore out and fit a bush to the worn bore that the cross shaft operates in. I tackled it in a different and much too complicated manner. This involved fitting a hardened and ground sleeve to the cross shaft, then boring out the gearbox case to accept a sealed needle roller bearing, followed by a second oil seal.

    Next, I dealt with the oil leak from the back end of the gearbox where the prop shaft enters. I was disappointed to find that original oil seals are no longer available. I removed the steel oil seal assembly from the gearbox and was able to make, on my lathe, a new end cap from alloy using readily available seals from an online bearing supplier. I chose a narrow type which meant that I was able to squeeze in 2 seals.

    Before, when the car was being driven there was a clicking from the clutch when it was depressed and bad graunching noises sometimes emanated when selecting gears. I found that one of the retainer springs that hold the release lever plate in place was missing which I think might have caused the clutch plate to lift unevenly. Also the clutch operating shaft and its bearings were worn resulting in lost motion. I was able to turn down and sleeve the shaft back to size and also, make up new bronze bearings to fit in the sides of the bell housing. The cross shaft lever was also worn on both ends. This was repaired with weld. The end where the trunion fits was then drilled and reamed back to size and the other end filed to fit the cross shaft. A complete new clutch actuating rod, clevis and trunion assembly was then made.

    All this took a lot of time and i hope I dont have to work on the box again. Ive now had the car back on the road for nearly 4 weeks and have covered just over 600 miles. The gearbox is much nicer to use. Gears snick into mesh much easier and quieter with less waggling of the lever.

    I have an appointment with a rolling road this Tuesday.  The plan is for them to set my new electronic ignition to the optimum position. Hopefully I can then make some timing marks so that I can keep an eye on the ignition timing with a strobe. Ive also fitted proper inlet valve stem seals from an A-series engine and a proper, more efficient  paper air filter which I hope will protect my new engine. I suspect these mods may alter the carburation slightly so I’m taking some spare, different size  jets with me in case they want to swap them.
























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