5 Speed box for a Roadster?

[DAVEBORRIE]

Hi
I'm considering fitting a 5 speed box this winter. Can you recommend a type to hunt down? I've seen that the Sierra box will fit an MGB but that doesn't mean it will fit my marlin with its MGB engine!
Just sailed through my MOT with no advisories, which is great news!

[stevejgreen]

Re: 5 Speed box for a Roadster?

One approach, perhaps more economic and simpler to introduce is to swap the back axle.

Assuming you have the standard 3.63 Ratio Marina back axle from a 1.8, the option is to swap the existing differential centre with one from from a Dolomite 1850 Auto / 4 Speed TR7 Auto / 2.0 Ital Auto with a 3.27 Ratio. The cost of a rebuilt (new gears and bearings) diff centre will be in the order of £450, but installation will be less than half a day for a reasonable mechanic. They do appear on the likes of eBay but are rare and specialist.
That will drop your revs by around 10% in every gear, through the range.

If you plan increased power in the future, the complete rear axle from a Dolomite Sprint, with a 3.45 Ratio will give you a 5% reduction in revs. Installation is pretty straight forward but the axle needs some suspension bracket cutting and welding work and perhaps adapting the rear of the transmission tunnel to clear the larger diff nose. You will also need a new handbrake arrangement and a shorter Propshaft.

Neither of these options give you the flexibility of a fifth gear. but do make motorway cruising a little more relaxed.

The Frontline kit for fitting a Type9 to an MGB includes a special Bell housing. So it requires that the backplate of the standard Marina engine is changed to MGB. Unfortunately it does not stop there as a new starter motor and complete MGB flywheel and clutch assembly need to be fitted. None of these parts common with a Marina.

I have a Type9 bolted to the back of my Marina Bell housing. Retaining the Marina clutch but using a more expensive V6 version of the Type9. There are several Marlins and Marinas running with this mod.

Finally you can try to locate a Dolomite overdrive gearbox. I have never installed one, but there are several articles in Roadster Tech Tips that explain the issues in some detail.

Some people would say it is almost as economic to remove the B Series engine and gearbox completely, and drop in a new powerplant and drive train. It all rather depends on your egineering skills.

Search the Roadster section for Overdrive or PM me if you would like a chat to discuss the options.


Some quick approx numbers

Marina 3.63 diff @ 70mph 3686rpm
Sprint 3.45 diff @ 70mph 3498rpm
Auto 3.27 diff @ 70mph 3315rpm
T9 OD with 3.63 diff @70mph 3036rpm
T9 OD with 3.45 diff @70mph 2886rpm (a bit extreme)

[lil_red_roadster]

Re: 5 Speed box for a Roadster?

Steve covers the main options well.
The Type 9 is a good choice as it fits the standard tunnel (if a little tightly around the speedo drive)...but how you actually approach it depends on the depth of your wallet, your DIY ability and an eye to any future power upgrades you might have in mind.
A standard MGB/Type 9 conversion kit, plus the additional MG-specific parts & a reconditioned gearbox is going to cost something well north of a grand unless you get really lucky on fleabay. A used T9 unit and the Marina bellhousing conversion considerably less. 4-pot type 9's cost less than V6 versions and whilst the V6 is more suitable for higher-power applications the 4-pot will comfortably cope with MGB levels of power.

Whichever route you choose its well worth it...!

[stevejgreen]

Re: 5 Speed box for a Roadster?

A small addendum.
Any change in final drive ratio, either in tyre size, differential ratio or Overdrive will effect speedo accuracy.
It is possible to interchange speedo drive gears inside a Marina Gearbox to get a approximate result within the availability of parts.
Ultimately you may end up with recalibrating your existing speedo or installing a modern, programmable, alternative, whether a simple bike computer attached to a hub or a SatNav device.

[j.fernie]

Re: 5 Speed box for a Roadster?

Take account of the need to remodel the tunnel if you go for the overdrive option.
I ended up fitting a cross member to support the back of the box as the overdrive unit adds a bit of weight.

[stevejgreen]

Re: 5 Speed box for a Roadster?

When I did my http://5speedmarina.com/ conversion I managed to fit the box without modifying the tunnel though it is very close in places. I used, and modified an Escort crossmember, fitted in almost the same way as the standard Marlin.
A dolomite box is a little larger, Tech Tips has some very useful info.
One drawback with the dolomite box is the length of the input shaft splines, shorter than the Marina, so less spline takes the same load. That is unless you rebuild the Marina gearset into a dolomite case (for the overdrive) but that needs time and pretty good egineering skills.

[da_jones]

Re: 5 Speed box for a Roadster?

Hi
the box fitted to my Roadster is from a Dolomite Sprint which has an overdrive. The gear change has been extended to bring the lever to a more handy position. The box is quite tight in the tunnel and access to the overdrive solonoid becomes very dificult so make sure that it is tight with a good gasket fitted before insrallation.