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MG Midget and Sprite Technical - Engine Rebuild Fail

Well, After setting myself the challenge of rebuilding the engine of my 1500 in a week, and swapping the gearbox... I have to report that I failed. At 11pm last night, I decided to go home leaving the radiator to be put back in and the carbs fiddling with... plus all the checks I needed to do (best done when awake). Engine looks good though! (see photo)

Question about the gearbox though, my gearbox had a different coloured speedo pinion... one green one orange.. one had 20 teeth one had 21... Has anyone swapped these over to change the speedo reading when enlarging tyres? - or does it required changing the driving wheel as well?

Christian


C L Carter

Christian,
You can use either speedo drive gear without changing the pinion on the gearbox output shaft. Both will mesh properly. Clearly one tooth difference 9in 20) will make a 5% change to the speedo reading. Depending on which tyre size you have used, this could be used to correct, or partially correct the speedo reading in use. Given that the speedometers are not very accurate at the best of times a bit of experimenting may well improve things. Use a GPS satnav or smartphone App to check the results.
Guy

coincidence, I read something about this last night whilst looking for some thing else

GAN6-223510 (Jun 78)
Gearbox speedometer drive gear changed from 22 to 20 teeth(RHD)

which is curious as from GAN6-200001 (Aug 77) the diff ratio was changed from 3.9 to 3.7 and different speedo

info from Terry Horler's book

MGOC Spares shows 20 teeth from June 1978 on

the 3.7 diff makes more sense for the 1500 engine
Nigel Atkins

I wouldnt call your situation a failure.... Heck edison didnt give up on the light bulb after his 1 st attempt

Failure happens when you start the engine rebuild process snd aboanden it after a few set backs

Trust me im an expert at the concept of engine building failure... More so then anyone else here on the bbs ... So if i sAy you havent failed yet... Accept it as gods truh... You havent failed... It a minor set back / inconvinance is all your experiancing...its not a failure

One week to build install and debugg is ... Very ambitious

Prop
and and the Blackhole Midget

If that's a failure - I need to shoot myself now! I've had my carbs sitting in a box in my kitchen for so long that I've forgotten how to put them back together!!
Ally

..failure would be when you turn the key and the engine blows :-)

you definitely made the right call, there's always a bigger risk to miss something crucial when rushing, which not only apply to working on cars, allot of babies were made that way for instance.
Alexander Sorby Wigstrom

Hey alex

You definitely made the right call, there's always a bigger risk to miss something crucial when rushing, which not only apply to working on cars, allot of babies were made that way for instance.

I suffer that problem also when i get tired and stressed on time... Do i push or pull...hahahaha

prop
and and the Blackhole Midget

Well, I went back at it today and. SHE LIVES ONCE MORE.

Started beautifully first time on the button and is running smoothly as ever - lapping the valves was the best thing I did (advised by Josh)... now she sounds beautiful and has so much more power in low gear.
a couple of teething issues...

1. need to rebleed the new clutch as it feel very light on the foot (although it is disengaging)
2. Need to invest in a new speedo cable (mine is somewhat buggered at both ends now) - So i need to find out what sort to get and what length I need. Only then will I be able to test the gearing.

Interestingly, the tappet clearances had increased to 12 thou (from 10) - After grinding the valve seats I would have expected it to go the other way... weird?

Very happy now. Especially as the change over gearbox (£30 on ebay) is sweet as a nut... no more rattling in first - huzzah!

C L Carter

Hi Christian. I doubt you ground that much of the seats/valves. But anyway you had the head off, and put it back with a new HG. So it's likely that the 2 thou difference could be the head sitting that much higher above the block face. Often when you retorque when hot, you then have to loosen of the rockers as the gap closes up.
Lawrence Slater

DON"T GIVE UP!!!

Carter, Very nice looking assembly you've got there.

Recap of what we just learned, as to clarify:

20 pinion green speedo drive = 3.7 ratio cars

22 pinion orange speed drive = 3.9 ratio 1500's?

correct?

~ Ron.
Ron Koenig

Ron,

not quite as clear cut as that what about the 21 teeth, I don't know the answer

also note previous post and this refers to RHD cars

also from Terry Horler's book - from GAN6-200001 (Aug 77) the Spitfire speedo was fitted
Nigel Atkins

Ron Koenig-
22 tooth is red
21 tooth is green
20 tooth is orange

Nigel quotes from Terry Horler's "Original Sprite and midget":

GAN6-200001 (Aug 77)
diff ratio was changed from 3.9 to 3.7 and different speedometer

GAN6-223510 (Jun 78)
Gearbox speedometer drive gear changed from 22 to 20 teeth(RHD)

and he also notes MGOC Spares shows 20 teeth from June 1978 on

I also note that an old Victoria British catalog says the 20 tooth was used on gearbox #101-5331

Still makes one wonder when the 21 tooth green was used! My car has the 21 tooth green and its GAN6-216232 made in Sept '78! Gearbox # 241697.

I notice the Victoria British catalog shows other variables for the 1500 gearbox, but what's curious to me are the gearbox #'s. There seems to be some distinctly different number sequences for the 1500 gearbox as VB's catalog notes for some other parts:

to gearbox #59162 and from #60001-65224
from gearbox #59163-60000

to gearbox #241363
from gearbox #241364-on

from gearbox #101-5331
from gearbox #5332-on


Brian McConnell

One tooth difference represents only a 5% change. As does the change from 3.9 to 3.7 diff. So the change from 22 to 20 teeth (10%) is unlikely to relate solely to the change in the diff update(5%) I suspect it also coincides with rationalisation of parts, including the speedometer head, between the Spitfire and the Midget. Production costs management. Together with this, speedometer gearing was never that exact a science on older cars! It was a compromise of getting close rather than spot-on with the speedometer reading, and more often having a speedometer that read slightly optimistically because it looked better for sales!

But I wonder if it is related to the tyres originally supplied from the factory? Did the export cars get radial tyres (bias belted?) whilst the UK cars were still supplied with the cheaper cross ply? The impression is that size for size, cross ply "stood taller" - I think the side walls are less flexible. Whether the rolling circumference was actually different I am not so sure.

Incidentally, this use of cross ply tyres with stiffer side walls may be the reason why, as Nigel noted recently, the cars in period photos and brochures always seem to sit higher than most of our current cars.
Guy

If this table is to be believed then 5.20 x 13 crossply = 135/80 (145/70)
http://www.carbibles.com/vintagetyresizes.html

which is about 5% difference

in Terry Horler’s book, page 83, because of the different rolling radius German cars with radial tyres had recalibrated speedos because the TUV(?) regulations only allowed 5% speedo errors as opposed the 10% for home market

Guy,
I think the reason the cars stood taller was the suspension sitting higher perhaps being new and different back then, though the crossply may look higher sidewall because of the tread pattern layout

from table above the crossply have a smaller rolling circumference and slightly lower sidewall height but may appear to be same size or bigger

radials appear to be an optional extra on UK cars until ‘72(?)
Nigel Atkins

This thread was discussed between 19/03/2012 and 02/04/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.