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MG Midget and Sprite Technical - 1275 Gearbox - Rear Extension

I did actually make it past step 2, but I'm surprised by what I'm seeing.

The rear extension had a LOT of metal filings. So much that I figured it would be easy to find that something has been destroyed. Between two gearbox oil changes and what I found in the extension, there's probably one to two tablespoons of filings.

When I removed the side panel it all looks great. No destroyed gears, anyway, and I don't know enough yet to recognize excessive wear. And NO filings. Well, practically none. Not what I expected.

Made me stop and think, what can go wrong in the rear extension?

The metallic bits are attracted to a magnet. Not sure if it’s all, or some, because the oil makes them act as one, but I’ll try to figure that out.

I see two areas in the extension itself that are shiny. Where the three fork rods match up, there’s not only a shiny spot, but there’s a ridge within the spot as well. Maybe a fork isn’t anchored as it should be. I should get a feel for that as I remove them. There’s also a part in the casting where the main shaft passes by, above the speedometer connection. Even though it's an inaccessible area I'm guessing it was machined to clear the shaft, and I would see some evidence of wear on the shaft if it somehow was touching something it shouldn't. Either of these could account for steel filings, and the first and second speed fork rod is flush at the top, where the other two have areas that stand proud. There’s even what looks like a bit of iridescence like from high heat on that rod, but I don’t know if it shows up well in the picture.

Speaking of pictures, when I use to frequent this forum years ago the option for pictures didn’t exist. Is there a thread with guidelines about posting them? I’m assuming it’s one picture per post, and the size limit is around 640x480.

This picture shows the three forks, and the marked one sure might be worn. I’d appreciate any advice to know if that’s what it should look like. I realize the chances are slim that someone has their gearbox open, especially since it’s a great time of year to be driving, but sometimes I get lucky.

Thank you.

(Sorry for the wordy posts. Hope I get better as I go along.)



Mark 1275

I think the iridescence you mention is probably as a result of grinding during manufacture.

There isn't really much that can wear in the rear extension, other than the bush that the prop-shaft rides in, unless there is any wear in the linkage.
Dave O'Neill 2

Thank you for the reply, Dave. I do see filings now inside the front cover so they're not restricted to the rear.

Only mystery so far is the plunger that should be at the front end of the third and fourth selector rod just wasn't there. The bolt (plug), washer and spring were there, but no plunger. Not available from Moss, but that's something I'll want to find. Might account for my nasty rattle?

I should mention the rattle was really bad in third, and didn't happen in fourth. If I start understanding how this gearbox works, it might make sense that the missing plunger is connected.
Mark 1275

IIRC the plunger holds the selector rod in place when in neutral, third or fourth. Without it the selector would be rubbing on the synchro sleeve you'd think.
Bill Bretherton

Check if the plunger is still in the case. they don't always drop out. Also the filings in the rear extension i think is normal. I have seen it quite a few times before. The metal comes from the baulk rings and the brass selectors during normal usage. It all gets mixed up in the oil and whirls around inside the casing. When you stop and leave the car the metal drops out of the oil and hence the deposit. The fine metal particles remain there becoming like a metallic paste. They gradually build up until the day the gearbox is renovated!
Bob Beaumont

I guess they could be concentrated in the rear extension more than in the main box due to the greater agitation by oil in the main box conotinuing to lift them back into suspension. Whilst in the quieter backwaters (or should that be backoils) of the rear extension the settled debris is allowed to accumulate.
GuyW

Really glad to see the responses.

Definitely can't find that plunger anywhere, and I presume I'll end up making something that will be a close approximation. If anyone has one out, or even a good photograph I could use to figure out the length, I'd appreciate it.

Once I got past that seized stud in the remote control housing, everything comes apart just fine.

Facing two hurdles now.

How the heck do I measure the end float of the laygear? I can see exactly where I should put the feeler, between the rear thrust washer and the laygear, but I can't get any size feeler into that seam. I must be missing something, because I don't see other people asking the same question.

Second, not sure how that main shaft is going to come out. The layshaft will come out easily, so I'll have the laygear out of the way as soon as I figure out how to measure the float, but I'm not sure what part of the main shaft to tap on.

Thank you for the continuing help.



Mark 1275

It's easier to measure laygear end-float with the input and main shafts removed. In any case you may need a new laygear. The main shaft is drifted out via the bearing from the inside of the g/box. You're probably renewing the bearing so it doesn't matter if you damage it. Also try to wiggle the tail of the shaft as it starts to move.
Bill Bretherton

When I say bearing I include the bearing housing.
Bill Bretherton

The other way I have used is to hold the end of the main shaft in the air with one hand and tap the casing with a plastic mallet. The shaft and the carrier usually start to come out. Once it starts moving you can lay the box down and whilst holding the shaft continue to
tap the casing until the carrier is clear. You then have to wiggle it clear of the reverse rear and out. Make sure you have drifted out the layshaft and the laygear is no longer meshing with the main shaft gears before you attempt to remove the mainshaft.
Bob Beaumont

Bob,that sounds a good method as there's probably less chance of damaging anything.
Bill Bretherton

There doesn't appear to be much 'crash' damage to the first speed gear or reverse idler, which is promising. Hopefully, the laygear will be in similar condition.

I may have a spare plunger somewhere.
Dave O'Neill 2

Missed the edit window.

I'm a little confused by your missing plunger. There should be a plunger for the reverse selector rod which is accessed from underneath after removing a plug. The plungers for 1&2 and 3&4 are accessed from the side cover. Their retaining springs are visible as soon as you remove the side cover.

There is another plug underneath the casing which appears to have nothing above it, but it is just an access hole for fitting/removing one of the interlock balls between the selector rods for 3&4 and reverse.

Maybe that's why you think the plunger is missing?
Dave O'Neill 2

I'll measure the float after the main and input shafts are out of the way. Should be easy then.

I like the idea of holding the shaft and tapping the casing. That way I'll know what's supposed to move.

I've probably been mis-identifying the missing plunger, but I just learned a lot. It's definitely the one accessed from underneath. It's #78 marked with red in the attached image from Haynes. And I was going to mention that the plungers accessed from the side cover are present and accounted for, and lo and behold they're also #78, so I know what to fabricate if it comes to that.

The other thing I learned was about the ball #75 above the plug #76. I was wondering why the ball didn't come out with the plug, but showed up later. Now I know.

Thanks very much.



Mark 1275

Also, keep an eye out for the interlock plunger (74) which sits between the 1&2 and reverse rods, at the back end of the 'box.
Dave O'Neill 2

Glad you mentioned it. It was just sitting there, but who knows when it would have escaped if you hadn't pointed it out.

I went into this thinking as long as I carefully follow the Haynes manual, I should get through it. But there's clearly a lot of knowledge learned in the process that I'd be lost without. How do people do it without a forum like this.

The layshaft came out, but after sliding easily for the first few inches, it took a lot of strikes on the drift. Guess it's expected to replace the layshaft, but how do I make a decision about the laygear when it comes out?

(Got some pieces of needle bearing in the process. Hope that's not unusual.)



Mark 1275

You need to check inside the laygear for any damage to the bearing surfaces.

Also check for wear on the flanks of the gear teeth, particularly first.

First gear also often suffers from chipping, but the corresponding first speed gear and reverse idler don't look damaged, so hopefully it will be OK.
Dave O'Neill 2

You need a new layshaft and needle rollers! Could you post a picture of the laygear - as Dave says it's the straight cut first gear that suffers but your main shaft first gear looks ok. Also check the tip of the main shaft where the needle roller bearing runs. AIUI they typically wear there especially if oil has been low. I hope it's ok as main shafts are hard to get/ very expensive. A bit of pitting is to be expected.
Bill Bretherton

The layshaft/bearings and the spigot bearing on the mainshaft are always replace items in any box rebuild. They have a hard time. They were sized for the original A series engine producing some 30BHP. Pushing double that through them shortens their life.
The layshaft has to be properly hardened. The stuff from the usual suppliers was not and I ended up getting one from Peter May. I see however the AH Spares offering appears to be of decent quality.

The other thing to check is the wear of the spigot bearing in the crankshaft as this supports the input shaft. A worn one puts greater stress on the spigot bearing on the mainshaft.
Bob Beaumont

Also, thinking ahead, replace the input and main shaft ball bearings and the synchro (baulk) rings, whatever state they're in imv. The small teeth (dog teeth) on the synchro sleeves tend to wear but may be ok. If reverse was whining maybe change the reverse idler. Also worth replacing the seal for end of main shaft on rear housing.
Bill Bretherton

Main shaft is out!

Used your method, Bob. Held the shaft and rapped the casing with a hard rubber mallet. Over the course of a couple of evenings it wasn't moving, but at the end of my time in the garage tonight, I ran a heat gun on the casing for a few minutes, then figured I'd try just a couple of whacks before heading in, and out it came.

Pretty happy about it.

I'll get a better picture of the laygear once it's out.





Mark 1275

Well done! Reverse idler looks ok.
Bill Bretherton

Hey well done!Glad its all coming apart.
few tips, the Haynes manual says the input shaft taps out into the bell housing. It doesn't on the 1275 gearbox. It has to go into the casing and you have to remove the spring ring from the bearing first.
Do not disassemble the sliding selectors unless there is something wrong with them. there are 3 balls and springs which are devil to get back in.
Leave the gears on the mainshaft unless they are worn again can be tricky to get all the plungers and locating rings in place.
Bob Beaumont

This thread was discussed between 14/09/2017 and 21/09/2017

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