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MG MGB GT V8 Factory Originals Technical - Gearbox Propshaft Tunnel Modification

Okay, I've built the piece of metal that provides the upward extenion - its about 7-1/4" wide x 14" long x 1-3/4", exactly where on the top of the transmission tunnel does it go? Right after the fresh air box? Right at the front turned lip of the transmission tunnel?
anthony barnhill

Oh, this is the piece of metal that is on pages 36 & 72 of "How to Give Your MGB V-8 Power".
anthony barnhill

10 minutes work with a BFH at the tripple seam about 10" back from the front of the tunnel & you do not need the extension. That is what most people have done. I used what is properly called a drill hammer from the days of star drills. 2 lbs of head on a short handle. Not much mechanical advantantage as you would get from a longer handle, but easy to control. Just bang away until you think your arms are falling off, then rest a bit & give it one more session. If you trial fit your engine at this point, you will see if any more work is required.

Anyone fit an extension?
Jim Stuart

I've done one. Turned out pretty good. If you guys want some pictures, let me know. The pictures show it better than I could describe where it goes.

The guy doing my body work likes to cut things with his plasma cutter so this made him really happy.

Simon Austin

This question seems to be a function of the transmission. If you're using a T-5, as Jim states, a little metal massage at the triple seam is sufficient (about 3/8" should do it). If you have a taller Rover box, the metal extension maybe necessary.
Graham Creswick

I'm using the TR-8 (aka Rover) 5-speed & have already built the metal, I'll use it...

....if I could get the photos, I'd appreciate it...think its placed towards the front of the tunnel right behind the heater outlets...gonna have to cut bottom off radio support, I, please...
anthony barnhill

Anthony - only if you have a pre 74 shell - otherwise you are creating interior work for yourself too.

Roger...I have a '79 shell....are you saying the TR8 tranny will slide in without any modifications? The book says they all have to be modified to fit a Rover tranny!

I've had my engine in & out several times for fitting but this will be the first time with the tranny attached...I need to test fit it so I can move my plumbing up inside the tranny tunnel so it won't be near my dual exhaust system
anthony barnhill


I got your message off-site and will send you the photos tomorrow (Thursday).

Roger- not sure about only the pre-74 shell. When I had the gear-box (SD1) in the car (it's out right now as body work is finished), there wasn't a lot of room at the top even with the extension. Probably as much as a stock MGB 'box has. As far as I understand the tunnel modifications, it was a width issue, not height, with the early cars so don't know why you wouldn't need the extension on early cars as well.

The TR8 'box is the same as the SD1. T5 'boxes aren't as tall so don't need the extension.

Page 72 of the "MGB V8 Power" book shows the height difference between the SD1 and a stock gear-box.
Simon Austin

In 74 the MGB design was modified to take both engines with the same tapered gearbox (different ratios). The gearbox casing was good for 137BHP but not 200 so, on this side of the pond we tend to use the SDI and TR8's LT77S which as pointed out earlier is taller and rubs on the seam about 10" down the tunnel - and which as Simon correctly pointed out can be dressed back with BF and BI.

This pre-supposes use of the RV8 cross member plus rubber mountings on the gearbox and the left (port) hand narrow engine mount which will lower the front end enough to get the standard plenum chamber under a RV8 bonnet. It also assumes OE engine mount brackets on the engine and use of the standard brackets welded onto the rails. I understand that some of the US repro engine brackets sit the engine a little lower still - and that will reduse the amount of dressing needed.

and while we are on the subject - there are small variations on the length of the bellhousing - a TR8 is I believe a fraction longer than others and will mean that you will need to move the gearshift hole 3/4" further aft.

and another thing - try to use the later LT77S box -the earlier ones were not as long lasting - or - the German Gentrag box fitted to the last of the RV8s.

Simon - it is the early cars (not all) that need the mod. Yes it was also a width issue unless you had a car with a 4 syncro + o'drive.

Persevere - the V8 is a smashing car to drive - RW


We've already modified my shift lever so it pops up through the original lever TR-8 is the later one & is narrower than my MGB tranny (good thing as I plan to run plumbing down passenger side of tranny tunnel)....I've built the tranny modification piece so I'm gonna use it even though we've slotted the motor mount holes to lower the engine even further than original
...Simon: thanks for the photos...look forward to getting them...think I understand the location but want to be certain before I crank up the grinding wheel (Roger needs to provide better guidance in next edition of book)
anthony barnhill


Pictures are on their way.

Roger- thanks for the info. I just re-read my post and it didn't make sense. So much for writing late at night.

I guess what I was trying to say was I don't see why you wouldn't need the extension on the later cars due to the fact the tunnel height wasn't any different between early and late cars. Maybe I'm missing something here.

Tony- what gearbox cross-member are you using? I'm using an original MGB one with the brackets from Clive Wheatley welded on. It bolts on as if it came from the factory. I don't know if it's similar to the RV8 one or not. As for the shifter, I didn't modify the arm but had to make a small....ahh...."adjustment" to the hole in the top of the tunnel at about the 4 o'clock position to be able to get it into 5th...and this was with the 'box lined up straight with the rear diff hub.
Simon Austin

Oh, tranny cross-member...I'm using a rear cross member that's been modified to bolt to the tranny & also to the original MGB location.....I modified my shifter....actually, when Glen Towry rebuilt my tranny he dod the rear crossmember & shifter modifications...
anthony barnhill

Tony, you have seen & ridden in my 72 roadster with the BIG T56 6sp. & my 74 chrome bumper G.T. with the rover 5sp. & there is NO scab plate welded to the top of the trans. hump! The B.F.hammer does the job real good. I look at the scab plate years ago & said to myself, you know I got better things to do than make this plate up, weld it in, (2 hrs. later) were a hammer will do the job real good.
Glenn Towery're absolutely correct...after I cut a big chunk out of my tranny tunnel & had the metal piece formed ($25 at my local machine shop), I realized all I needed was a BFH at the low spot right behind the radio console & I'd be finished....

...oh, gotta talk w/you about the tranny crossmember...
anthony barnhill

This thread was discussed between 11/11/2003 and 16/11/2003

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