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MG Midget and Sprite Technical - Main bearing clearance

Hi all
Working through the strip down of the Marina 1275 engine that will be going in my midget.
Checked bores with a bore gauge and the look within spec.
Check the main bearing clearance with some plastiguage last night and they range from 2 to 2.5 thou. According to the book this is at the upper end of the scale.
So should I try some new bearings and seeif this brings things into spec. Or should I get the crank reground? It's going to have to be machined anyway for the flywheel and I am also getting the whole lot balanced.
Was also wondering about the conrods. They havev large balance weights. Looking at vizard he suggests grinding them off and balancing them end to end (did this on my lotus). Or is it better to get some mg metro conrods ?
Cheers
Tim
T Dafforn

Check with new bearings.
You will always want to delay crank grinding.
As for the rods I don't have a clue as I am spoilt with only midget and cooper s rods
Onno K

Tim, look on ebay. S Rods come up from time to time, and might be cheaper than getting yours done.
Lawrence Slater

S rods have 1-5/8" big ends, unlike the marina which is 1-3/4". This of course is a plus as you can offset grind the bigend journals to the smaller size and get another 50cc or so from the extra stroke :-)

A point to watch with the Marina cranks: They are not cross drilled, and the steel is not as good as the Midget crank. Up to being a gently driven car but if used enthusiastically they have been known to run the bigends. Cross drilled, and especially if nitrided, they work well.
Paul Walbran

Don't know for sure about Marina, since we didn't get those, but as far as I know the balance weight rods are for 1 3/4 journals, but the S & Midget rods are for 1 5/8". Not sure about latest Midget for that matter.

FRM
FR Millmore

Am I correct in thinking that the mg metro conrods are better than the Marina and a direct fit. The ones I have seen pictures of seem to have less of a balance weight.
Cheers
Tim
T Dafforn

Is cross drilling and nitriding something that my machine shop could do?
T Dafforn

Tim, Google "nitriding a crankshaft", it will give you an idea of whats involved.

Alan.
Alan Cotterill (1972 standard 1275 )

The A+ rods (Metro and Ital) will work with the larger journal Marina crank and are a better bet for a modified engine because they weigh substantially less than the Pre A+ balance weight rods. The standard rod bolts are however of marginal quality and I would suggest ARP rod bolts for tuned engines.

Cooper S/Midget rods are much stronger than the larger journal rods and are the ones to go for (with a Midget EN17T or EN40B crank) if there is a choice..
James Bilsland

I should add that the only engine that I've built that threw a rod (On Kemmel straight at Spa) was running A+ conrods with a nitrided cross drilled Ital crank...
James Bilsland

Tim

Cross drilling is something your machine shop should be able to do. Ask them to copy the drilling pattern of the Midget crank and you can't go too far wrong.

Nitriding is a process carried out by heat treatment specialists. You may be able to track one down yourself, if not your machine shop would be likely to know where to get it done/arrange it for you. (My one is a bit far away!)

You are aware of the different flywheel fitting for the Marina crank?
Paul Walbran

Yes, know about the flywheel issue. This is why I decided to take the engine apart.
will look out some heat treaters!
Tim
T Dafforn

Tim,

IIRC nitriding is only suitable for specific steel alloys and I'm not sure the Marina crank is made from such. You're more likely to need tuftriding which is a process that increases the carbon content of the steel surface and to some extent the nitrogen as in nitriiding, the carbon penetration being significantly more than the nitrogen as nitriding is typically a very shallow treatment which may need to be redone to preserve the benefits on a regrind. The benefit is in increased wear and fatigue resistance of the treated crank.
David Billington

Standard EN16 Midget cranks weren't cross-drilled. EN40 cranks were.
Dave O'Neill2

With some combinations of Marina/Ital crank and a different flywheel you may end up with the flywheel in a different place than before - as it did for me. This can result in the starter bendix throwing out and bashing into the ring gear which isn't very nice. So it may be worth checking the flywheel position will remain unchanged.
F Pollock

Dave - good point, I was thinking of the earlier cranks.

David - we have had good success nitriding them and also MGB cranks. The heat treatment company we use does what they call a deep nitriding process which penetrates about 25 thou they say. However much it is, the effect was certainly enough that when one customer's engine ran a bearing as a result of oil surge (oil level too low!) the crank escaped damage in circumstances where a normal crank would not have done so.

Early attempts to use Marina cranks by owners here resulted in run bearings in engines which were competed with (fast road engines sprints etc rather than full race engines) with the crank in largely original form. Hence the move to crossdrill and nitride them. Since doing that therre hasn't been a problem.
Paul Walbran

Reading vizard last night his comments on conrods were interesting.
While he confirms that S or midget rods are the best he thinks that all the rods are over engineered. I like the idea of the metro rids as they are about 100 grams lighter according to vizard.
Couldn't find much mention of cross drilling the crank, but ran out of time.
With regard fitting the flywheel. Any idea why machining cause it to change position? I thought that the flat faces at tge end of the crank and flywheel weren't touched
Cheers
Tim
T Dafforn

If your crank/flywheel mounting face remains in the same place you may be OK. I used an A Plus crank with a 1098 flywheel and the position of the ring gear moved about 5mm rearwards, so may be worth checking the throw of the bendix before you put the motor back in. Seem to remember the top dowel locator on the backplate fouled on the gearbox mounting flange - again something to check.
F Pollock

Paul,

I had a similar result when my 1215 overheated due to an unknown water leak and partially seized the engine. The crank ended up with a tiny blemish which polished out and was pronounced good to go and has done about 65k miles since, that one was tuftrided. Also seen similar on a Lancia Beta crank that ran a bearing and the bearing was totally trashed but the journal still looked very good. The Beta cranks were factory nitrided IIRC, at least the 2 litre engines.
David Billington

David

A relief isn't it to find the crank has survived :-)
We just went by the advice of the local heat treatment metallurgist in nitriding, either way it looks like a good result.
Paul Walbran

Paul,

I seem to recall you did mechanical engineering, as I have, but metallurgy is another field in itself. It may be touched upon in mechanical engineering, at least the basics, but it is another detailed field of study all it own and I know a guy that has done it. A mate in engine development has mentioned that they deal with metallurgists regularly as they work out the best comprimises for the production bods.

Regarding the Lancia crank I was amazed to see that most of the steel backing of the bearing shell had been extruded around the crank pin but the big end bearing journal itself looked OK. Another mate ran a bearing on a Ford X flow and the journal was trashed. That X-flow did get him most of a 100 miles home with no oil pressure when driven gently, it was when he tried to speed up the last 10 miles that the engine failed and the bearing gave up. That rod did not look pretty as it had obviously partially seized to the bearing and crank.
David Billington

This thread was discussed between 06/03/2012 and 07/03/2012

MG Midget and Sprite Technical index

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