Gearbox crossmember modifications

Gearbox crossmember modifications

I’ve started working through the list of jobs I made for myself after trial fitting the engine. The biggest of these by far is the changes needed to the transmission tunnel to accept the taller R380 gearbox, and I’ve already made some headway on that by cutting out the necessary hole in the tunnel. I’ll write a full post on that task once I’ve finished fabricating and welding in the new taller piece to cover the hole, but in the meantime, now that the gearbox can sit correctly in the tunnel, I went ahead and made some modifications to my original MGB gearbox crossmember to make it work with the new gearbox.

I started by dropping the engine and gearbox back into the car, here is a view of how the gearbox now sits in its correct, final position, now that it is no longer pivoting against the top of the tunnel.

It’s now completely level and there is no issue at all in connecting the propshaft. In this position the mounting points are also not too far adrift from the original gearbox crossmember mounts, so as I mentioned in the previous post, I will try to adapt the crossmember itself rather than drill new holes in the chassis rails for it.

Whilst researching solutions for this issue I came across this excellent post on another MGB blog about a similar modification, so I very much used that for inspiration. I found I had to make slightly more extensive changes to the crossmember as the R380 is a slightly different shape to the LT77 referred to in that post, but the principle is very similar.

Starting with the standard MGB crossmember…

…the first job is to cut off the old mounting brackets for the MGB gearbox.

When I offered the crossmember back up to the car it would not go back on since the bottom of the gearbox sits quite low into the space where the crossmember is attached, so I had to cut a channel into one side. I kept the mount for the old engine brace bar since it will be a good support for the new gearbox bracket, but I cut it back into the new shape shown below.

Next part involves modifying a Rover SD1 gearbox mounting bracket (I found this on eBay relatively cheaply, they seem fairly easy to come by). They normally look like this:

In standard form, the SD1 bracket is too wide to fit into the MGB transmission tunnel however, so the extra mounting points on the side need to be cut off, like so:

At this point I crawled back under the car and bolted the SD1 bracket onto the gearbox with the correct rubber mounts, and bolted the MGB crossmember back into the original mounting holes. I bolted it on the opposite way round though, so that the modified engine brace mount is now to the rear, closer to the gearbox bracket. I carefully measured how far away the bracket was from the crossmember and then unbolted the whole lot again.

Away from the car I measured out a piece of 4mm steel plate and welded it onto the modified engine brace mount, and then welded the SD1 bracket onto that, so in the end I came up with this beautiful creation!

Back under the car I bolted the completed crossmember into place and it fitted perfectly. Not the prettiest piece of fabrication perhaps, but it was a fairly simple job, and definitely preferable to cutting up the floorpans for me.

I’ll have this whole piece properly blasted and powder coated before it goes back on for real, which should help to make it look a bit more presentable, but for now I’m glad to have sorted another one of the bigger challenges with this conversion.

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