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TR2/3/3A TR3 Transmission Differences

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Hey Randall, If I wanted to swap my TR3A eng and gearbox for a TR4A eng and od gearbox, what am I getting myself into? Its a later TR3a 79,000 or so. I've got the TR4a engine but am looking at a gearbox to buy. ( I know you've answered this before but I can't find all the info). Does it all bolt in without modification? Do I use the TR4a exhaust and carbs? Thx. Kevin
 
Brinkerhoff said:
Hey Randall, If I wanted to swap my TR3A eng and gearbox for a TR4A eng and od gearbox, what am I getting myself into? Its a later TR3a 79,000 or so. I've got the TR4a engine but am looking at a gearbox to buy. ( I know you've answered this before but I can't find all the info). Does it all bolt in without modification? Do I use the TR4a exhaust and carbs? Thx. Kevin
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The front plate has to be changed (TR4/A motor mounts are farther apart) but the 3A plate will bolt to the 4A engine.

I'm not sure about the TR4A carbs & exhaust, but I think you'll have trouble there. Seems like someone told me that the TR4A dual exhaust hits the body on a 3A. The throttle and choke linkage is also different. And I'm guessing you may have interference problems with the longer intake manifold causing the carbs to hit the inner wing (even though the carbs themselves are somewhat shorter). Might work though, depending on what air filters you use.

However, the TR3A manifolds, carbs & exhaust will bolt right up to the TR4A motor. You'll also want to transfer over the road draft tube.

If you want a stock fan, I suspect you'll need to swap over the TR3A components there, too. The 4A fan extension is longer (I think) and probably won't fit behind the TR3 radiator.
 
thx, ok , I'll got all the extra misc. to do it then. What will I gain in HP by doing this? Or will the TR3 carbs and exhaust work against me? Yes I want to keep the stock steering and fan but would like the extra power and the sychro gearbox with the od. Does the tunnel and mounting in the rear need work too? or the driveshaft?
 
Hard to say how you'll make out on power, depends a lot on the condition of each motor and what parts it has inside. There are a lot of TR3 motors around with 87mm liners in them, as for a long time the 87mm liners were cheaper and easier to find than the stock 83s. The 4A changes supposedly improved mid-range power (which after all is most important), but the peak power rating didn't improve by much (100 bhp to 104).

For your case (late TR3A), no changes are required to the tunnel or driveshaft. The slip joint in the driveshaft has enough travel to accommodate the slightly longer gearbox, and the tunnel will already clear the starter bulge.

Of course you'll need to add the wiring for the OD, but that is easy enough. I'll make one suggestion though : add a smallish fuse in the power to the solenoid. That way, if the solenoid somehow hangs up and doesn't open the contacts inside, the fuse will blow before the solenoid burns up. I use an 8 amp "slo-blo" just because I've got a box of them, but I believe even a 5 amp regular fuse will work fine.
 
Brinkerhoff said:
Thanks. So if I keep the TR3 engine , the TR4 gearbox will work with the TR3s clutch? I know the TR4a has a diff. clutch.
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Yup, should be fine. Just use the throwout bearing and carrier to match your clutch.

The front cover on the gearbox (where the TOB carrier rides) will be slightly shorter (about 3/8"), but that won't hurt anything.
 
Something to consider if you are looking to seriously increase horsepower:

Kastner had gearboxes fail on the first TR4 cars as they developed cracks where the bellhousing meets the case. The factory added a large puck-shaped steel part onto the tailshaft to alter the damaging harmonics, but he found that he had to weld thick (1/2'') strips of aluminum along the sides of the case and onto the bellhousing to stop the failures. The factory eventually changed the design of the casting to include similar webs on each side of the case, but he doesn't say when that change occurred. Anybody?
 
During the TR6 run, I believe. The TR5 was beefed up some, but it got even beefier later on.

Which, BTW, is exactly the reason I'm running a later TR6 box. The TR4 housing with the flange broken off is still in the scrap pile somewhere.

The internals got beefed up as well, several times, including changes in gear tooth angles and steel bushings rather than brass.

DSCF0023.jpg
 
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