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MG Midget and Sprite Technical - Matching, mis-matching parts

I am selecting parts to go with a 1098cc engine to go into my Frogeye so its a matter of selecting what will fit with this engine.

First, the dizzy. I have a DM2 40561 which I think is from a 948 engine. I also have 25D 41270 that I am not sure about I presume it is for a 1275 engine though it isn't marked as a 25D4 type.

At this stage I just want to select one that will get it up and running. Tweaking or rolling road set up would come later. Which is likely to be the better option to get started (lterally!)
GuyW

Guy,
according to the list on my 123 dissy instructions -
40561 - midget MK I, 1961, 948 cc, Engine no. 9CG; distributor-type DM2P4

41270 Midget MK III, 1969-1974, 1275 cc, Engine no. from 12CC/Da/H11639 on; 12CE & 12V; distributor-type 25D4.

As far as I can remember from a JT vid (about B dissys IIRC) and if I understood correctly, the later dissys were more concerned with the emissions and fuel savings so earlier ones were more about performance but I can't remember how much earlier is best. Obviously there'll be different curves and different age of style of make up of parts with different age of dissys.

Both dissys will have the car starting and running, bear in mind that their original settings could be effected by a lot of wear in their parts and with possibly different, other than original, parts that could have been fitted.

According to Moss (but I've not cross-checked) 25D4 40919 - from (c) H-AN6-24732, (c) H-AN8, from (c) G-AN2-16184, (c) G-AN3, 1098cc.

Nigel Atkins

"..the later dissys were more concerned with the emissions "

Certainly for the US market, where emissions were more critical. Home market cars had different carburation, ignition, etc.
Dave O'Neill 2

Dave, Guy, I think by performance JT also meant different carburation, ignition, etc. (which also involved emissions to some extent and the crankcase venting etc.).

From my 123 dissy list of curves -

40561
Advance 500-1000 rpm* 10.0
Advance @ 2000 rpm* 23.95
maximum @ 5000 rpm* 34.0

41270
Advance 500-1000 rpm* 10.0
Advance @ 2000 rpm* 14.5
maximum @ 5000 rpm* 28.0

* degrees advance and engine speed both relate to the crankshaft
Nigel Atkins

I was puzzled that the later dizzy is just marked 25D - without the 4. But after posting realised that they are probably all like that and the 4 is taken as the first digit of the longer reference number, in this case 41270. - Dohh!

I had also forgoten that this one will be from my 1971 1275 Sprite, which currently has an MG Metro electronic dizzy in it.
The 1098 engine has more in common with the earlier 948 than the 1275 so I think I will start off using the early DM2 one.
GuyW

Guy

The '4' in 25D4 is for 4-cylinder, but even the 25D6 (6-cylinder) distributors seem to have the 5-digit code number beginning with a 4, so it does look like your '4' is just missing, for some reason.
Dave O'Neill 2

Oh so it doesn't take the 4 from the 5 digit code? The 25D I am referring to is the raised lettering on the top edge of the dizzy, next to the vac advance diaphragm.
GuyW

Ah, I see what you mean.

Some dissies have ‘25D4’ or ‘23D4’ stamped onto the side, where the 5 digit code is.

As it’s cast into the body, it’s probably because they used the same casting for the 4 and 6 cylinder versions.
Dave O'Neill 2

Considering the following
Fuel has changed since the 60’s needing updated ignition curves.
In 60 years of use the springs in the dissy might have stretched a bit.

So take the one with the least play etc and don’t worry about the curve
O K

Just the 5 digit, plus a letter and the date stamp on the side, and the raised numerics on the upper edge.

I have decided to use the earlier DM2 to start with. Mainly because although filthy inside, it shows very little signs of wear.
GuyW

The next match is I am using a 1098 engine with a MM ribcase. The engine has a thick type backplate though the flywheel is a 2 dowl version for a spring type clutch cover. I think it should all bolt up ok but am wondering if the recently mentioned modification to fit a diaphragm type clutch is worthwhile considering I need to buy a new clutch anyway.
Thoughts?
GuyW

Guy. Are you thinking of using the 1275 diaphragm clutch which is 6.1/2 inches, or buying a diaphragm clutch of 7.1/4 inches to fit the 1098 flywheel? If using the 1275 one, you would need to have the 1098 flywheel machined to take the 1275 clutch.

Bernie.
b higginson

Oh, have I misunderstood? I thought the other discussion was simply about fitting a 1275, 6.5" clutch cover to the early two dowel flywheel. Would the friction surfaces not line up then?
I need to do some measuring up to check.
GuyW

You can use a 1275 clutch on a 948 flywheel if you redrill some of the holes.

Whether or not you can use a 948 flywheel on a 1098 crank, I couldn’t say. I believe that the 1098 had two different flywheels, depending on which size the crank main bearings were.

I know I used a 1098 flywheel on a 948 engine, but I don’t know which 1098 engine it was from.

That probably doesn’t help much, other than to suggest that it isn’t as straightforward as it might seem.
Dave O'Neill 2

I do have both a 948 flywheel that takes the early 6 1/4" clutch disc, and a 1098 flywheel that takes the larger 7 1/4 clutch.
I do need to buy a new clutch cover and plate. The options seem to be to stick with the 1098 flywheel that goes with this engine and just buy the standard coil spring clutch for it. Or to see if the 948 flywheel will fit the 1098 crank (both are 4 bolt designs) and then modify the dowels on the flywheel to take a later 1275 diaphragm clutch. The latter is appealing in that the clutch is stronger and lighter and the flywheel is also lighter. But the easy option is just the standard 1098 clutch.

One other thing, looking at clutches for 1098 Morris Minors I have found one that illustrates a diaphragm type clutch. I didn't know they were available and maybe its just a generic photo, but if not than that might be option 3. I have emailed to check.
GuyW

The 948 and 1098 flywheels are interchangable. I fitted a 1098 flywheel(from a Minor) to a 948 engine when I fitted a ribbed box.I used the 1098 clutch cover as it was easier!
Bob Beaumont

Yes David, I was following that recent thread with interest. It probably wouldn't have occured to me otherwise that a 948 flywheel could be adapted for a 1275 clutch. Presumably if one only needs to drill 2 extra holes then the PCD of the 948 dowels is the same as for the 1275 clutch cover so one of them is reused.

I think I was maybe also wrong earlier in thinking that the 948 flywheel was lighter than the 1078 one. Even though the clutch is smaller, the overal flywheel is the same diameter so I doubt there is much if any difeference in weight.

Still considering the pros and cons though! Drilling two dowel holes sounds easy but in reality there is a lot that could go wrong. It would be different if I was doing an engine rebuild as I think it would be essential to get it all rebalanced after the modification. What with swapping flywheels around and then drilling for a new cover position I doubt very much that the balance wouldn't be put out.

You can tell what the weather is like "up north". I could do with fewer heat waves like this one.
GuyW

Guy,

Yes the PCD is the same as is the bolt pattern. I used a HD 6.5" diaphragm cover on a 948 flywheel for my 1420cc A series, a big mains 1098 crank so a slightly stroked 1380cc.

IIRC it's only the small mains 1098 crank flywheel that will fit the 948 crank, the large mains 1098 has a larger PCD.

In the old days the normal way to uprate the 948 clutch was to fit the 9 spring Spitfire 6.25" clutch cover, IIRC a direct swap with no flywheel mods.
David Billington

It's not just a case of drilling two dowel holes, as the mounting bolt positions are different.

I don't have any parts to hand, in order to compare the differences, only photographs and my memory.

It's 30 years since I fitted a 1275 clutch to a 948 flywheel, so can't remember all of the details.

I seem to recall using three of the original bolt holes and drilling and tapping three more. What I can't remember is whether one of the dowels lined up with one of the holes, or whether they all had to be redrilled.

I do still have the lightened and redrilled 948 flywheel somewhere, as I came across it recently. It's just a question of where.

(948 flywheel photo borrowed from ESM Morris Minor Spares website.)

Standard 1275 on the left, 948 on the right.



Dave O'Neill 2

I used to use the Spitfire 1200 clutch that David noted on a Mk1 lightly tuned sprite engine in my old Moggy Minor.
Its the original Spitfire type - sometimes called Mk1 or Spitfire 4 (not mk4). Just bolted straight on. The Moggy having a rod clutch made the pedal a little harder ISTR.

They were hard to get for a while i understand but I see Rimmers have them in stock.

R.
richard b

Looks like you also tried fitting the 1275 flywheel to the 948 crank as an alternative to achieve the same outcome.
GuyW

Looks like you also tried fitting the 1275 flywheel to the 948 crank

No, that's standard 1275 6-bolt and 2-dowel crank mounting.
Dave O'Neill 2

Sorry Dave, it's a long time since my 1275 flywheel saw the light of day! I had forgottten about the two crank dowels as well as the 6 bolts!

I have decided to stick with the original 1078 flywheel on the engine it came with. Less risk of getting things out of balance!

Is 1.5 thou (0.0015") acceptable for flywheel face run-out for a standard tune non-race engine?


GuyW

Do the 1098 and 1275 fitted with a ribcase box both use the same clutch lever arm?
I have one with part number, which I think it is 22G769 but cannot remember what it came off.
GuyW

checking, it could be 22G169. Its definitely a 9, not an 8 on the end
GuyW

think so Guy - the Morris Minor one is different but all the Spridget ribcases I've seen had the same arm.
David Smith

Before David posted I was going to put that David previously posted a photo of different fork arms (three I thought) but after his post and not being able to find the photo it must be another false/wrong memory of mine.
Nigel Atkins

Thanks David. Actually comparing this to Mark's recent photo I think mine is bent slightly, near to the pushrod end. I wonder if it would straighten with some heat. Or would that just soften it?

Nigel, I thought that too which made me think there were 3 types, 948,1098 and 1275. But I couldn't find it in the archives either. I did find the one of the different gear lever ends, but that's another story, of a different missing part! Perhaps I should make a list!
GuyW

Something weird happening with the messages here, or is it just on my computer?
GuyW

Sometimes happens when archiving is taking place, I believe.
Dave O'Neill 2

Just found this on gerardsgarage.com

I thinks it’s what has been posted in the past.


Dave O'Neill 2

Guy, there must be two of you!
Bill Bretherton

Bill, as well as apparently two threads with the same title, one has repeated itself 3 times over so every posting (not just mine) gets 3 places and the discusion is 80+ messages instaed of the 30 or so that it should be!

This is the 1275 lever arm I have. It is slightly bent towards the outer end - between 4 and 3 on the straight edge. Would that straighten with some heat or is that not enough to worry about? Better to leave well alone?


GuyW

Guy, you are probably right that its the three gearlevers I was thinking of rather than fork arms.
Nigel Atkins

Guy,
Its not that bad, looks like you will lose 3/16" on the throw but that should be compensated by the position of the piston in the slave.
Bob Beaumont

This thread was discussed between 25/05/2019 and 15/06/2019

MG Midget and Sprite Technical index

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