Triumph engines and large wheels!

[Greybeard49]

Hi all. I've just bought a replica MG TA off ebay, based on a Herald chassis and engine with a GRP body, and I'm hoping someone will be able to advise on a suitable (preferably Triumph) engine.

It was apparently a runner about 20 years ago but has been stored since. My problem is the wheels - they are 19 inch wires with motorcycle tyres, and having owned a succession of Spitfires many years ago, I have no faith whatsoever in the ability of a 1300(?) Triumph lump to turn these big wheels with any enthusiasm. As it is a non-runner anyway I plan on replacing it - but I'm not sure with what.

I have a good Dolomite 1500 engine/box to hand and a more torquey alternative might be the good ol' straight six if there is space under the bonnet. Otherwise maybe a (heresy) MGB engine? I'd very much like to hear from anyone who has 'been there, done that...' with any advice or suggestions.

Greybeard49

[jonathan binnington]

stick the 1500 engine and gearbox in for a start and see how things go

wheels, triumph 13" are easy enough to come buy...

j

[Greybeard49]

Thanks for the reply but I think an MG TA replica would look a bit odd on 13" wheels!

[heraldcoupe]

Most Herald  based 30s type specials look so wrong with the small wheels. First of all, work out how the rolling surface of the wheels compares to a Herald on 145/80-13 tyres. A Herald or Spitfire/GT6/Vitesse diff can have a final drive ratio anywhere from 3.27:1 right down to 4.875:1. Most Herald diffs will be 4.11:1, so a combination of the 19" wheels and the lowest ratio diff might not be such a ridiculous combination. The difference between a 4.11 and 4.875:1 diff is around 18.5%, which I suspect is not too far from the difference in rolling circumference of the wheels - what size tyres are fitted?
For engines, the Triumph straight six is hard to beat. The 1600 is the least powerful, but unarguably the smoothest. The 2-litre and 2.5 variants are outstanding engines, and not significantly heavier than the bag of nails used in the MGB. Given all that, if a low ratio diff is fitted then a 1300 engine should pull it fine, so the choice is yours,
Cheers,
Bill.

[Greybeard49]

Hi Bill and thanks very much for the info which is very helpful. From what you say I could almost certainly get away with the 1500 Dolly engine I have available (the Herald lump in the car doesn't look too good) if I use the high ratio diff.

I used to own a Spitfire (many, many moons ago) with a 1600 straight six crammed in, and I have never since owned a car that was more fun to drive. Sadly I suspect the engine bay of the TA replica may be too small for one of these (although I also own a Moss Roadster that will probably get one fitted when I get around to that project!)

The wire wheels on the replica are fitted with 5 inch deep tyres giving an overall rolling surface of 46 inches.  I've just measured a Herald wheel I have lying around and this comes to 35 inches, so 46/35 x 100 = 129, i.e. roughly 30% greater rolling surface.  Hopefully the extra whoomph of the 1500 unit plus the relatively light weight of the replica will make up the difference.

I don't suppose you have any info about external identification of a 4.875:1 diff from ident numbers?

Cheers, Rex

[heraldcoupe]

The 4.875:1 diff will have a number prefixed with "G" on the central web on the bottom of the nose casting. "GA" or "GE" would make it 4.11:1, "Y" is 4.55:1. Lots of other diffs out there with different prefixes, let us know if you find something you don't recognise.
The only complication is that the early diffs (G, Y and most GA) will be weaker in their construction than the later types, the output shafts are smaller in diameter. It is possible to build the low ration crownwheel and pinion into a later diff casing to get the combination you want. The small shaft diffs were never used by Triumph for anything larger than the 1147cc engine, though a 1300 engine shouldn't break them. The 1500 engine is likely to break the shafts in the long term, so I think it's worth getting the right combination built.
You may find the following link useful for calculating wheel/tyre combinations:
http://www.mintylamb.co.uk/?page=tyre.htm
Cheers,
Bill.

[Greybeard49]

OK Bill, thanks for the comprehensive info. The torque loading on the output shafts is likely to be relatively high because of the wheel diameter, although I'm not planning on any drag racing. That said, the Dolomite engine easily spun the wheels of the rust-bucket I bought to get it - I'm sure it could quickly wreck a power train designed for 1147cc.
A hybrid diff using transplanted gears seems like the best engineering solution unless i can identify a heavy-duty diff with a 4.875: ratio (spitfire/vitesse/GT6?).
Thanks again for all the help.
Cheers, Rex.

[heraldcoupe]

If you need a 4.875 diff to donate the crownwheel and pinion, I should have one or two available.
Cheers,
Bill.

[Greybeard49]

Looks like the way to go. I notice your 'rarebits4classics' business is in Swindon, and I will be passing through there in a few weeks on my way up to Leicester. If you could give me an idea of cost this would obviousl;y be of interest (private email: mail(at)abacus-tp.co.uk). I'm also going to need a replacement radiator cowl/shell for an MG T series if you come across one at some time (scruffy / dinged would be fine - the replica is never going to be a 'concourse' job!)
Rex.