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MG MG Y Type - Fan Belt length

Hi all,

I have a Dunlop B36 fan belt but it is very tight. it won't fit over the pulleys unless you actually undo the dynamo pivot bolt as well as removing the one on the adjusting link. But then I can't fit the tacho reducing gear as the dynamo is too close to the distributor when reassembled.
Can someone tell me the dynamo pulley diameter so I can see if the problem lies there.
Also the dynamo when moved about three quarters of the travel in the adjusting slot , hits the lip on the body. Thus either the dynamo is too big, or the motor is positioned a bit far to the left or the wing has been pushed out of shape a bit. Or maybe it was always like this.
I built a spreadsheet to calculate the belt length given the pulley sizes, and can work out if a B37 or whatever is going to work. But if the pulley on the dynamo is wrong, maybe I should change that.

The dynamo is the correct one according to LTBY's , but of the type fitted to early cars.

regards KGM
K G Mills

I have detailed records from the previous owner of my YB and reading through Ive found this -

Fan Belt R106

5/8th wide & 7/16th thick

39 1/8 " outside dia & 36 3/8th inside dia

generator Lucas model C39, pv2 Type L/O
Part number 2258B

Maybe something there that can help you.

Thanks
Kevin
K Weeds

I also use the B36 in BX version, that is a toothed belt. And my generator pulley is 80 mm.

regards
Walter
Walter Prechsl

Likewise, I also use a BX36. Here is the belt currently in use. Regards, Ewan

E.J. Ward

Hello all, thanks for the responses.
The info on the YB confirms what is in LTBY's. And my pulley size is also 80mm (or maybe 81). Which is a relief because it means the tacho gearing should be OK. The earlier YT dynamos were replaced by a smaller one evidently, which makes sense.

I have ordered another belt, a B37 (!) but it is 925mm which is actually 36.4. Probably the same as the 36 and 3/8thh length that Kevin reports. here's hoping. So maybe not all B36's are truly 36 inches . I rather wish I had gone with the BX36. Its extra flexibility should mean it is easier to handle than I found with my old rather stiff Dunlop B36
My calcs show the B36 (if it is exactly 36) is about 2/10ths of an inch longer than the fit over the dynamo pulley and very nearly the same over the water pump pulley. So any stiffness in the belt would make it very difficult to fit. With the bolt removed from the adjusting link, it is a little better but the dynamo body hits the head of the bolt holding the side plate cover. So that might have to be removed too. And the Dunlop belt measures a little less than 36 inches as best I can determine.
Now back to the tacho drive problem. YT's ( (early ones have this bigger generator) but all have a vertical entry distributor cap. (from LTBY's) Mine has a side entry, like the YA and YB. So now I have to swap my distributor cap. Only then can I fit the tacho. Otherwise its cable and the HT lead from the coil will interfere with each other. This is also evident from the picture in LTBY's , which I hope doesn't infringe copyright. It also shows how close the dynamo is to the body, and the tacho drive to the distributor.
Joy.
(Know anyone who wants to swap distributor caps?)
Sometimes I think this car just delights in presenting these little difficulties for me.
regards KGM


K G Mills

Fitting a top-entry cap is a 'correct' way to solve the clearance problem.

But if you don't have such a cap like to tinker, you could also change the orientation of the distributor in steps of 90 degrees. On my Y the ignition leads point to the (more or less) 'north', but I've also seen east, south and west pointing leads.
Changing the orientation involves repositioning of the ignition leads in the correct firing order, but that's all there is to it.
Willem van der Veer

Just adding what I learned from experience on my YT, once needing a new belt in Dubbo NSW in the middle of a long road-trip...

It's important that it's a B-series belt, not an A-series or other. The B profile is 21/32" x 7/16". An A-series belt, that some mechanics may have, sits too low in the dynamo pully, which effectively reduces its diameter. It'll work, but the dynamo is spinning faster than design in relation to the crankshaft pulley, which means the tacho is over-reading. Having the right profile belt gives a noticeably different tacho reading.

There's a wider range of industrial drive belts available than Repco's range of automotive belts, or than most mechanics carry. I found a Carlisle Blue Label B37BL 15M/045 was about perfect in profile and length. Very durable too.

As to length, 15" (940mm) is perfect. Any shorter you run out of clearance between the tacho drive and the distributor. Any longer and the dynamo risks encountering the offside bodywork.

I hope this is of use.
regards,
Robert





R Ades

Hi all,
Robert, your pics are very informative, thank you for posting them. Some of the numbers I can see on your dynamo are the same as mine. Though the service number seems different (228334 vs 228664 maybe I misread it.) And you are quite right about the B section question. I note that your dynamo seems to be set at the end of its adjusting link. If I move mine to towards that , it hits the body. So perhaps my body work has been "customized" shall we say. I do know from other evidence that the car has had a fairly serious whack on the front/left side sometime. the chassis was bent, and inside the scuttle panel on that side there is some repair work. Whatever is the explanation, I think your belt length of 940mm (the 15" doesn't seem right, it translates to 37 inches) might be a tad beyond the upper length that I could use by my calcs, which could be a bit off of course.
Willem, taking Robert's pics as a guide, it looks like a north pointing configuration could very well work. But his dynamo is right at the end of its adjusting link's range, which doesn't accord with my setup. But it still could be OK.
One of our club members here has offered a top connecting distributor cap, so that's great for me.
Now I just have to wait on the belt I have ordered to see if the length is OK (ie fits over the pulleys but doesn't let the dynamo over travel and impact the body work.)

Thanks again all for the guidance, I don't think I could progress this restoration without this bulletin board, both in response to my probs directly and through topics that have been covered by others.

KGM
K G Mills

In the interests of full disclosure, my distributor is a small bodied Bosch unit, not the original Lucas. This gives me a bit more clearance at that end.
There's still a bit more adjustment in my dynamo link, it's not at it's limit yet, just close. I found the next size belt down left the tacho drive too close to the dizzy, and reduced the contact arch on the dynamo pulley.

FYI, the belt in my picture has been running for over 4 years.
R Ades

Thanks Robert,
the belt I have bought has arrived.
"Made in Romania. Optibelt VB B965 Ld/17x925 Li B37"

They did leave a bit of belt left without any inscription. But "Romania"?
Even at a "B37" it still needs you to crank the engine to get in on the pulleys. It is 925mm as against your 940mm, both still nominally B37. The 940 (true size) you have would be an even better fit.

It is on now but I still have to fit the tacho drive.

regards KG
K G Mills


Generator sizes

For comparisons sake: please let me have the diameters of the earlier "bigger" size and the "later" smaller size generators.

Regards
Anton
Anton Piller

Hi Anton,
the dimensions of my generator. ref Image 1

The distance A to B is 160mm, A to C 206mm

The diameter of the body is 114~115 mm

The distance from the centreline of the generator to the lower pivot bolts centreline is 80mm but it is difficult to measure accurately.

The second image shows where the adjustment link sits with the belt in place.

The vehicle is YT/ERX 3740, delivered here in Adelaide South Australia 23 of Feb 1950. The engine number is still 13392
The generator is stamped
LUCAS
C45YV GC24
228334

LTBY's lists the genie for this vintage YT as
C45YV-3 C24

but with the same service number.

A PO evidently chromed the cover band, and it is a bit of history I might retain.

regards KG






K G Mills


Thank you KG, for the in-depth information. I have to compare with my MG: YT 4220, late 1949 model, engine No. XPAG/TR/14077

Anton
Anton Piller

K G

Here is what v-belt I use. It is of the toothed BX type, whithout saying so.

Does it sit too low in the dynamo pully, which effectively reduces its diameter?

I seem to remember that the belt was bought from Moss many years back, but only was fitted two years ago...

Anton





Anton Piller

Hi Anton,
your belt certainly looks like it sits lower in the pulleys. However the depth of the belt may be smaller than the one I have, so the effective diameter of the pulley could still be similar.

And as an academic exercise-----
If the belt sits low, I imagine it would do so on all three pulleys. The generator pulley is 80mm, while the crankshaft one is 95 mm. reducing each of these by say 4mm , would only alter the generator speed by a factor of 1.008, . That seems a pretty small sort of increase. The water pump would actually slow up slightly.
But if I could, can I check a question on the windlace fixing. So I have resurrected an old thread you contributed to. Can you look there please?

KG
K G Mills


KG
Sorry about my ignorance, but I do not understand the last sentence of your post. "But if I could, can I check a question on the windlace fixing. So I have resurrected an old thread you contributed to. Can you look there please?"

I don't even know what a windlace fixing is :o(
Anton
Anton Piller

Hi KG,
I'm not sure it's correct that a narrow belt will sink down equally on all three pulleys.
My memory is that the V shape and depth in the three pulleys is not consistent. The V in the dynamo pulley is deeper than the other two. So the wrong belt profile will sink lower on the dynamo pulley than the others, spinning the dynamo faster.
Apart from the visual differences, when I installed the correct belt (over a narrower one) the tacho measurement altered by ~200rpm at 60mph. Significant enough to be noticeable on the gauge.
Regards,
Robert
R Ades

Hi all,
Anton ..I met the term on the net and had to look it up then. But from our help when all else fails, (aka Google)

What is a Windlace?
Image result for windlace
"Windlace is a piece of fabric folded over a soft foam core giving it its round shape and gets placed around the opening of doors and gives the upholstery a finished look. Replacing windlace should be done during the upholstery process. The windlace offers a second seal to help protect against the outside elements.10 Sept 2020"
The bit I am concerned about how this is best fixed across the door sill. Your images in an old thread ("door seal" 2018 ) showed you must have overcome this problem.



Robert, good point about the potential differing pulleys. Standard design ought to mean their tapers will be same, so the change in effective diameters ought to be the same. (Vee belts Classical sections and pulleys were standardized well before the YT's were made.) The operative word(s) being ought. And there is the question of wear too. And it appears Anton's belt seems well down on the dynamo pulley as you suggest. While paradoxically his belt is actually specified as a little bit wider than a standard B section. So so much for standardization. While I am surprised at the size of the effect on your tacho, nothing beats an actual observation, so I stand chastened.

regards KG
K G Mills

Hi KG,
No chastening, I was equally surprised when I made the observation. I had to dust off my engineering studies to understand why.

It's the effective circumference of the pulley compared to the length of the belt that defines the final gearing. Any difference in diameter is therefore multiplied by Pi (3.1415926). Please check my maths, but I calculated that that a belt sitting just 2mm too low in an 80mm diameter pulley will give ~4.76% difference in gearing. At around 4,000rpm on the tacho, that's close to 200rpm error.

For full disclosure, I now run a 4.22 CWP set in my diff, so 60mph on the recalibrated speedo gives 3800 rpm on the tacho, with the correct fan belt.

Regards,
Robert
R Ades

Hi Robert,
At the risk of criticism for needlessly cluttering up the bulletin board, some-one once told me that retired clergymen and policeman (there were no "literate" women in days of yore) were used to calculate tide tables. But who and how calculated Pi? π. I know it is "known" to at least 100,000 figures.
Anyway, my favorite people are engineers. I spent half my life helping them learn. (I don't think you 'teach" people.)
Back to tachos, I don't think π is involved in the relative effect. The belt speed is what it is, and that means the tangential velocity at any linked pulley is the same. So since that equals r * ω , r being the radius and omega the angular velocity. the product r * ω is unaltered by changing r the radius. Translating angular velocity to rpm does involve π, and a few other constants. But that doesn't alter the % change from just changing r.
Sorry, I love this stuff.
Dropping the belt in 2mm, on only the dynamo pulley, is 2 in 40 or 5% (of radius not diameter) so 5% of 3800 is 190rpm, which is indisputably what it is. My earlier calcs dropped the belt in on all pulleys. Which is what might be expected.

I want to know more about the diff change. What is involved?

Cheers KG
K G Mills

KG, I prefer your maths. Either way, 2mm drop of the belt into the V means ~190rpm tacho error. QED.

As for my diff, some years ago I commissioned MG Workshops to modify the Y-type housing to take a variety of crown wheels. After trying several BMC standards, I finally settled with a Midget/Sprite 4.22 CWP set using the original Y-Type spider gears. This gearing is perfect with the original (restored) gearbox. The original XPAG is somewhat tuned up so delivers more horsepower than standard, hence it can cope with the tall diff.

We've done 2 return trips to QLD from Melbourne in this configuration, and will do so again next year for the rally in Gypmie. As above, we cruise at 3800rpm all day, being just over 60mph.

Might start a new thread if you want to know more.
Regards,
Robert
R Ades


Robert

Yes please start a new thread. I am very keen on this topic and in the past just read about Morris conversions that needed welding and other big stuff.

Question: what are the spider gears. maybe you can refer to the Workshop manual in chapter Q (there is a cut away drawing.....

Your cruising speed sounds sensational.

Thanks
Anton
Anton Piller

Hi again Anton,

from my reading of "Lucas Technical Service
Overseas Technical Correspondence Course, Part2, p16 I have copied the following......
STANDARD GENERATORS TYPES C39, C45
AND RA5.
We can now consider modern production generators,
that is, the series of stari'dardised generators which
are fitted to practically all British cars, light commercials and tractors now being produced for both
Home and Overseas markets.
There are three main types : C39, C45 and RA.5.
These basic types comprise of 6 and 12 volt models,
fully enclosed or ventilated.
GENERATOR SYMBOLS.
But first, a word about the identification symbols.
You can see that the letter - prefixes refer either to
the positioning of the armature or to the shape of
the machine yoke. The numerals 39, 45, etc., as in
the case with starter motors, again refer to the diameter of the yoke.
Suffixes folIowing the numerals, as you will see
later, refer essentially to variations of the original
basic type.
These suffixes may be followed by numbers 2, 3, 4,
etc., which denote internal design modifications.
The symbols illustrated should be carefully studied
as they are of great assistance when handling the
units.
DYNAMO SYMBOLS
THE PREFIX
C' CONCENTRICALLY MOUNTED ARMATURE
RA' RECTANGULAR LAMINATED YOKE WITH
Dl E-CAST C. E. BRACKET
THE NUMERALS
39- 3.9' 0IA.YOKE
45.4.5" " "
5.5" - . '
THE SUFFIXES
P- LONG 2 POLE TYPE
V = VENT1 LATED TYPE
5 - SPECIAL EQUIPMENT


If you go to www.dbraun99.com and look there you can find the full pdf (18MB)

regards KGM

K G Mills


KG

Thank you for the generator topics. In my request of December 4th, I was actually referring to your diff conversion - alas the "Midget/Sprite 4.22 CWP set using the original Y-Type spider gear".

Please start a new thread :o)

Anton
Anton Piller

This thread was discussed between 07/11/2021 and 06/12/2021

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