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MG Midget and Sprite Technical - Engine rebuild or refresh

Hi all
I'm getting to the stage of looking at the engine in my long term restoration. The engine is a 1275 A series out of a Morris Marina. I have all the bits to convert to fit in the midget.
I have had a quick go at measuring for wear and found the following.
The bores are STD and show no signs of a wear ridge. The pistons also look in good condition.
The main bearing jounals measure as standard and the bearings I took out looked good ( not down to the copper). The same is true of the big ends.
My plan is to
1) change the bearings and thrust washers
2) replace the conrods with some lighter ones from an A+ engine. Question here, As there is no apparent wear ridge at the top
of the bores would there be an issue with using the replacement con rods complete with the pistons the came with. The pistons look pretty good. Would there be a risk of cracking a ring even if I cannot feel a wear ridge. I take it I should replace the rings? Should I also deglaze the bores?
Then build up the engine.
Good idea?
Tim
T Dafforn

K series????
Hugh Alison

The A+ rods don't fit the A crank.

If you had done a compression test before disassembly you would know if the rings would need replacing.......

Just keep the rods and pistons and replace the rings.
Change the bearings as said.
Get a mild cam (266)with oil pump and followers fit some good double valve springs and time it properly with a new double chain

Then just enjoy the engine


Onno K

Got a duplex chain kit and the lighter conrods have the correct size big ends for the Marina crank which has larger journals than the midget.
Sadly didn't check the oil pressure before dismantling as head was already off when I got the engine.
Tim
T Dafforn

I would choose to get the crank,damper and flywheel balanced, and at least select the rods and pistons too to get the best matched weights. All easy to do at this stage.

When I got my engine balanced he also chose to line bore the crank journals as well although I am not sure why, or how helpful this was.
Guy W

Good point. May as well get it balanced.
Tim
T Dafforn

I think it's all covered but my recommendation would be new big end main and thrust bearings, new rings and new oil pump and deglaze the bores. As you are changing rods I think the balancing is a good idea too.

Guy, where did you get your engine done?
N McGurk

Do you have A+ conrods and pistons?

When the engines were originally built, the pistons would have been selected to have the correct clearance to fit the bores. If you have another set of pistons, they may be looser or tighter in the bores, even if they are standard.

When you have an engine rebored, they will machine the bores to have the correct clearance for the new pistons.

If you do get some A+ rods, there's no guarantee that the pistons will be any good. One option would be to use your existing pistons on the A+ rods, although it's not always possible to remove them without damaging them.
Dave O'Neill2

Hi Tim
Have you measured for wear in the bores, top, middle and bottom, using one of the rings and feeler gauges in the gap?

New rings and hone too - well worth it.

My 1275 had a genuine 60K and was using a pint of oil every 150 miles, only 1 thou. of wear but the rings were shot.

Regards Tony
Tony G

N McGurk, (sorry, I don't know/ remember your name)

Hi. I get my machining done at Lancaster Engines. The workshop is definitely "old school" engineering (!!) but Malcolm there is very knowledgeable and does a good job. Just be aware they do machining and supply the parts back to you for re-assembly. They don't assemble engines themselves as far as I know.
Very reasonable prices too, compared to what I see others post on here.

http://www.lancasterengines.co.uk/
Guy W

Yes have two sets of larger diameter big end conrods.
The original Marina ones have big balance weights and Vizard says that they ate some of the heaviest.
I also have some a+ series with virtually no balance weight.
Have the original ones marked to ensure they could go back in the correct cylinders if I reuse them.
Have measure the bores with a bore gauge. Is it better to use an old piston ring?
Tim
T Dafforn

Id just buy a new piston set...they come with the rings and appear to be good quality...i want to say rings alone are around $100 for deves and a 21253 piston set from AE is around $300

If you go just the rings...id go all in and get gap-less rings esp.considering these engines love to do blow by often and soon

Prop
Prop and the Blackhole Midget

This thread was discussed between 27/11/2012 and 28/11/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.