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Gearbox Measurements


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The question pops up relatively frequently on here, about which gearboxes can be used to install a V8 into a Defender, and it seems theres no real satisfactory answer bar using a disco box or paying big money for a rebuilt box with the right bits on it.

I've read (cant remember when or where, think it was some yank as the tdi boxes are rare over there) that a V8 box can be fitted to a TDI, so i've had an inkling that the other way would work too. I finally pulled the LT77 out of the v8 disco last night, so i took some measurements:

The input shaft on the V8 LT77, protrudes 10mm from the bellhousing, so if we can get this dimension in a defender box with a V8 bellhousing we're looking good.

I also have a 300Series Disco V8 bellhousing and a Defender 300TDi R380 sitting around so measured those too:

The input shaft on the Defender box is 288mm long, from the bellhousing mating face.

The 300 series disco housing, is 278mm long.

Meaning when the 300 series housing is attached to the defender box, the input shaft ends in exactly the same place as a real V8 box.

The clutch splines on the V8 box are a little shorter (they start further away from the tip of the input shaft), but from the witness marks from the spigot bearing indicates this shouldnt be a problem, and theres plenty of length on the TDi spline to take the V8 clutch.

The only slight sticking point is the spigot bearing itself. The V8 has a 19.1mm spigot, and the TDi has a 22.2mm spigot. When i get the engine out, i will have a measure to see if its possible to simply have a different bush machined up for the end of the crank, and if there is, it would suggest that the combination of a 300tdi defender box, and a 300series disco v8 bellhousing should successfully mate up to a V8 engine.

Hope someone can find this useful, and i'll follow up with the engine measurements when i get them!

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Ok so some googling seems to suggest this spigot issue is quite common in series conversion, as the series uses the same 7/8" input shaft as the TDi.

There are two approaches, either grinding down the input shaft to the V8's 3/4", or boring out the spigot bearing on the V8 to take the larger 7/8" input shaft from the TDi.

The first one probably requires the gearbox to be disassembled, meaning your back to square one.

Leaving us with the boring out of the spigot bush.

It seems that its possible, a PM from jamesmorfee confirms that both his series and fridges original 109 used this method. The spigot bush does end up VERY thin, as its outside diameter is only around 24-25mm, but it does seem to work.

Now who whats to be the guinea pig? :lol:

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