Midget 1275 rebuild

[Mark Wilson]

Last summer I acquired a Midget 1275 engine with no engine number and little known history. (Phil, if you are reading you may remember that you kindly looked at the eBay ad photos for me and confirmed that it was indeed a Midget 1275 with the thick flange block.) As far as I can tell it would have been a 12CE number (casting window under the centre main)

I've just stripped it down and I think it is quite an interesting engine in some ways. It looks to be a good one to me, but it is my first bottom end rebuild and I would like some opinions and advice, please.

The block and sump were very clean, but I suppose I still need to remove the oil gallery and core plugs. Are replacement brass plugs available? I've no experience of tapping cast iron for threaded inserts and don't expect to be cleaning inside again.

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It has an EN40B crank, which in theory shouldn't be in a thick flange block, and may not have been original. It has a few small chunks ground out of it here and there, which I assume are evidence of a balancing exercise. Although I know EN40B cranks are sought after in some circles, this of theoretical interest only as I'm definitely not going to be racing the Traveller its going into! The crank has been ground to - 30 on the mains and - 20 on the big ends, with virtually no wear on the big end shells and some light scratching on the mains shells. The journals look immaculate and measuring with a digital caliper I'm getting 1.971 - 1.976 on the mains (standard 2.0005 - 20010). I don't trust my measuring and suspect that the larger figures are more unreliable than the smaller. The crank spins smoothly on the mains.

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Big ends (standard 1.6252 - 1.6259) range from 1.604 - 1.606 with the same questionable measurement technique.

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To me this sounds like very little wear and I'm tempted to bung in new shells, but do the more experienced think that having the cranks checked professionally would be worthwhile?

The mains are secured by what I take to be high tensile bolts - it took a 4' scaffold pole extension to my socket bar to shift them. Can I re-use these (hope so - new hi tensile bolts are over £80 a set!). What torque do I need to apply - will the normal 60lb/ft plus Loctite be ok for non competition use?

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The conrods are AEG521 (or possibly 621 - casting indistinct) Three of the caps have a flat area ground out, but this hasn't been done on cyl 3 although its otherwise the same cap. Is this likely to cause inbalance or any other problem? I assume I should replace the big end bolts, again with Loctite.

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The cylinders have been bored to +030 and have a barely perceptible ridge at the top. They are very shiny, so I'm assuming that a light glaze busting hone and new rings should be sufficient.

The camshaft appears to be a standard Midget spider drive, which slightly surprised me as the EN40B balanced crank and the high tensile strength bolts suggest to me some competition background. Quite happy with this - I didn't want a lumpy high performance cam! No offset key or vernier valve timing either, just standard duplex chain (untensioned) and slightly worn cogs which I will be replacing, along with the oil pump.


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Sorry to throw so much at you in one post!

Mark

[philthehill]

Mark
The oil gallery plugs are available from MOSS.
You are correct in that only the very early Midget engines had the EN40 crankshaft fitted.
The EN40 crankshafts were Nitrided and normally the crankshaft would only be re-ground to 10 thou before being discarded.
As you are not going to be using it for competition it will be ok.
It appears that the con rods are early 1275cc Cooper S ones, early Midget engines had the same rods. The forging numbers for the 1275cc Cooper S is AEG177 or AEG521 and are made in EN24V material so the best 'A' series rods available.
I have two sets on the shelf awaiting use.
Later Cooper S con rods were down graded to AEG625 which are the same as the later MG MIdget.
If the crank spins well there should be no reason to check for straightness/alignment.
The Main cap bolts can be re-used and torqued to 60 lb/ft and using the locking tabs.
The rod bolts can be re-used but the self locking nuts are once only use and they are special high tensile nuts so must be replaced.
The rod caps have been lightened on the big end by machining in the groves.
I would recommend that all four rods are checked for balance and to be absolutely certain get the crank checked as well.
Yes you may get away with a light glaze bust to remove the top ridge if it is as small as you say.
The cam shaft if standard and has no wear I would just re-use but with new cam followers.
I notice that the pistons have 4 rings. These are now obsolete and only 3 ring piston ring sets are available unless you can find a NOS 4 ring set or buy a set of replacement pistons with 3 rings rings.
Come back to me if you want any more information or I can be of further help.
Phil

[bmcecosse]

From what I can see there - just new shells and a glaze bust - although I would be v tempted to upgrade the cam shaft to one of AC Dodd's 'high torque' cams.... It will also get you away from the poor spider drive oil pump. Does it have a 'big valve' head ?

[Mark Wilson]

Many thanks to both of you, your knowledge makes all the difference to a beginner! Sounds like generally good news, but the obsolete pistons are a bit of a disappointment. There's a set of possibly suitable Cords rings on eBay

http://www.ebay.co.uk/itm/MG-MIDGET-127 ... 3641.l6368

I've picked up an impression that Cords were generally used for oil control to delay reboring a smoky engine, but is there any reason these wouldn't do the job in my engine?

The mains cap bolts didn't have locktabs - would I be better fitting these or using Loctite?

Roy - the head is a standard 12g940, but at the risk of offending purists I'm not using it (in the short term at least) as I picked up a brand new Mini mpi head with unleaded seats and all valves and springs fitted. I lose the bypass hose and the temp sensor tapping, but seemed too good a deal to miss.

I'm unlikely to follow your advice on the cam, simply because I've spent too much already. Is the spider drive really an issue? The cam followers look in very good condition with flat shiny tops - the photo isn't brilliant, but I can't see any wear on them.

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Won't be able to give you any feedback on the results of this until summer when I rebuild. I had to reorganise my garage to let a gas engineer repair my boiler, so thought I'd drop onto the mechanicals while I had the space with the car off the spit! I do envy some of the pictures of nice big workshops we sometimes see on here!


Thanks again

Mark

[philthehill]

Those cord piston rings will be more than adequate for your requirements.
I would want confirmation that they are for 4 ring pistons though.
I use the locking tabs for the main bearing caps but using Loctite machinery adhesive is ok so long as you use new Loctite and the threads are completely free of oil and grease. Loctite does have a shelf life.
As regards the cam followers - if there is no wear or pitting they will be alright to reuse but make sure that they go back onto the same lobe as they will have worn (sic) to the lobe.
As you will be taking some time over the re-build I would advise the use of engine assy lube. I personally would not build an engine without it and I plaster it on all moving surfaces that have an interaction.
That new MPI head will serve you well. I did look at purchasing one myself as they were a real bargain but having spare 940 heads on the shelf thought otherwise.
Having a big workshop is not always what it is cracked up to be. The space just gets filled with spares and equipment - mostly because I cannot say no to parts and equipment when offered to me.
Phil

[IslipMinor]

Mark,

If you are anywhere near Oxford I can measure everything for you as a check (bores, crank pins etc.) and also deglaze the bores 'while you wait'! The picture of the piston appears to show the top ring missing, or have you removed it?

Are there any markings on either end of the cam? If it is not standard, it could give an idea of what it might be?

As Phil has suggested, definitely check how many rings per piston the Cord set is for.

[Mark Wilson]

Richard,

Regrettably up in Leeds, not very convenient for Oxford, but a very generous offer nonetheless. The top ring on cylinder 1 emerged in two pieces, didn't feel it break but no marking on the bore. The vendor has confirmed that the sale is for 4 rings x 4 cylinders, so I've ordered. Well worth you both pointing out the possible error, though, I hadn't noticed the rather ambiguous description in my excitement!

No markings on the cam at all, but I've compared it with Google images for standard Midget cams and in terms of spacers, rings etc, it looks identical. I'll have to wait until the dry build to check on the opening angles. As I said, I think I'll be happy enough with a standard cam.

Phil - yes, I'll be slapping assembly lube all over as suggested! (not on bolts prior to torqueing, though....)

Mark

[IslipMinor]

until the dry build

Mark,

You were getting me a bit worried there, until the comment to Phil a little further down!

[bmcecosse]

With that EN40 crank you should really use Leaded Bronze bearing shells - and not the cheaper Aluminium /Tin shells. I'm not sure the MPi heads have the larger inlet valves ????

[philthehill]

As regards crankshaft bearings:
I think that you may have to use what you can get as the better quality main bearings @ 30 thou under do not seem to be currently available.
The supply of good quality big end bearings @ 20 thou under is better.
Mini Spares do main and big end bearings that are more than suitable for your engine re-build but so far I have been unable to find both main and big end bearings of the type as recommended by bmc.
As you are not going to be undertaking any competition work I would suggest that the bearings you have if you consider that they need replacement would be ok to be replaced with like for like bearings or enquire at Mini Spares.
With the mains @ 30 thou under - the original Nitride surface will be undercut anyway but still leave you with a very good and strong crankshaft.
As an aside top quality Mahle bearings if were available are in the region of £100 per set for mains and £100 per set for big ends - so good quality does not come cheap.
Phil

[Mark Wilson]

Roy - I'm a bit out of my depth with you competition boys! It's going to be an everyday driver, my only ambitions are to get up a long hill near home without dropping to second and occasionally cruising on the motorway at 65-70 without bursting a blood vessel, so the EN40 crankshaft is a bit of a red herring. The mpi head doesn't have large valves.

Richard, I think the term dry build came from Vizard, which I keep reading to make me sound more knowledgeable than I am. I think I mean that I'll put the head on and check the cam timing before I lube up and torque all the gaskets down

Phil, I was thinking there was so little wear that I might even keep the present shells but the scratches near the centre groove on the front mains are easily felt with a fingernail, so I think they'll all have to go.[frame][/frame][frame][/frame]

Sussex Classic MG Parts do the sizes I need at a very reasonable price, but don't indicate the manufacturer or material.
http://www.sussexclassiccar.co.uk/shop_ ... 71_01.html

These are lead/copper "uprated" at twice the price, but a lot less than Mahle! No manufacturer name given.
http://www.ebay.co.uk/itm/MG-MIDGET-SPR ... 1508131525

Any preferences or other recommendations?

Many thanks to all of you for taking so much time to help.

Mark

[bmcecosse]

Definitely don't re-use the old shells!

[IslipMinor]

Surprising the number of suppliers that quote shells for reground 'undersize' crankshafts as '+' sizes!! Moss do and now Sussex Classic MG as well - have they found the engineering holy grail - the 'putting on machine'??

If seriously considering the lead/bronze version, Peter May do them, but are slightly more expensive than Sussex. They are extremely knowledgeable, and can be trusted to supplied top quality parts.

This is the link to the relevant page in their catalogue:

http://www.petermayengineering.co.uk/ca ... catalogue7

[Mark Wilson]

Thanks Richard, certainly reputable but pricey with VAT on top. I'll have to think about this, always tend to pay a bit more for quality, but suspect I'm in danger of over engineering for my intended use. I do know what sort of money you and Phil must have spent on your engines, but horses for courses.....

I took it that shafts are ground to minus figures, but as the shell is thickened it is correspondingly positive. Its radius is minus, though!

Mark

[philthehill]

Mark
If you can feel a ridge with your finger nail I personally would replace the bearings.

Having looked at the products listed via all of the links above - any of those suppliers products will be suitable for your requirements.

Richard
I did look at the Peter May product list but the description was not sufficient to determine the bearing composition - but it does state heavy duty and the price is competitive.
I have over the years purchased several items from Peter May Engineering and I also found them knowledgeable and providers of good service.

As regards making things bigger i.e. building up bearing surfaces - a process of hot metal spraying can be used to build up the bearing surface and then grind to size.

Regarding dry building - you should still lubricate any interacting surfaces i.e. when you install the cam - oil the cam bearings.
When you have checked that all is right strip it out again, wash/clean everything spotless and then carry out the final assembly but this time with engine assembly fluid.

Phil