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MG Midget and Sprite Technical - Piston Decisions

I had a spot of bother at the weekend.

The Sprite was just warming up nicely for its first run up Loton Park when it started misfiring on one cylinder. Blipping the throttle to clear it caused it to back fire as well.

Plugs removed and Plug No4's electrode was bent closed and the porcelain damaged. A dash home to pick up my Christmas present endoscope showed there was a foreign body in there.

Diagnosis: I found, on fitting the matched pair of SUs that I bought on Ebay following their rebuild to replace the original non-pair that came with the Sprite, that the outer bolt holes for attaching the air filters were butchered: miss-shapen and threads stripped. There was just enough thread behind the carb's flange to take a nut, so all went well last year. With the engine out this winter I filled the bolt holes with plastic metal, drilled and tapped them and all seemed OK. However, the rear repair failed and I assumed that a piece of plastic metal had found its way into the cylinder, so no hill climbing for me at all this weekend.

With the carburettors off today, a couple of bits of metal in the inlet port spelt trouble and you can see the result in the photos. That's a piece of 5/16" spring washer sitting on top of No.3.

Showing my problem to a couple of folk has thrown up two options:

Option 1: Buy two new pistons and get the job done straight away.

Option 2: If the bores aren't scored and the valve seats aren't damaged (I'm pretty sure they're OK but will investigate further before deciding) put it all back together and finish the season and then pull the engine and fit a set of flat topped pistons. I will need to to some homework on what else I would need to do alongside that. However, I have also just been offered a Weber and manifold that has just come off a friend's Frogeye, so the options seem to be multiplying.

My basic question: Which would you recommend, Option 1 or 2?

Sod's law has also come into play as I have just booked my first rolling road session with Peter Burgess at the start of July.

Colin






C Mee

If there is no damage to the bores and head then I would be inclined to clean it all up (remove any odd bits of washer etc!) and put it back together. The crowns of the pistons are pretty thick so should be ok.

If in any doubt then replace all 4 pistons. Flat tops are not as efficient as dished. Omega with an offset dish are my preferred option. Cast should be ok as the forged ones are for out and out racing
Bob Beaumont

Thanks for that Bill. I assumed flat tops would be better due to the higher compression ratio but I guess the dish has an effect on the gas flow and combustion process.

More homework required.

C
C Mee

Colin, what's the exact spec of your pistons - size, make, number etc? Might have a couple of odd spares you can have....
David Smith

David

My 1275 Sprite has been re-bored + 0.040"

These pistons are Nural 87-521711-00 (21251 040) 12H

I would certainly be interested if you happen to have a pair.

Colin
C Mee

Disaster!! Hope you can fix it promptly.

As long as there is no damage to valve seats and the pistons are "OK" (I can't really tell how bad the pistons are from pictures) I can see a case for just clean it up and stick the head back on! It ain't perfect but it will work! Don't tell Peter (shhhh! he might be reading!) get it on the rollers in July with your prefered SUs or Weber. Then sort it all properly over the winter. Swapping like for like won't throw out the setup work done on the rollers.

But then again... it's not *too* much more work to drop the sump, pull the rods and put some fresh pistons and rings in. You are in pretty deep already, do you want to have to do it all again in six months time?

Have you ever had the rotating and reciprocating masses balanced? It would bug me that new pistons would throw that out slightly if you have. But you might be able to find some that weigh pretty much the same.

Best of luck,
Malc.
Malcolm

Colin, sorry - no luck. I can only find 3 +30s, 3 +60s and a few 1380s
David Smith

David, Thank you for looking, and for being prepared to pass on some of your stock.

Malcolm, I'd already had to change my date with Peter once so I thought it better not to mess him about any further and have cancelled for this year - so there's a cancellation slot for someone.

The combustion chambers have been full of paraffin all day and enough to dampen a paper towel on the end of my finger but has got through so far but no dribbles so I think the valve seats are going to be OK for now. The pistons just appear to be pitted with no holes or sharp bits so, if Bob is correct about the crown thickness, I think they're usable in the short term.

I didn't have the rotating bits balanced when I did the rebuild during Covid - the bill was already several times more than I was expecting so I had to draw the line somewhere. My plan, now, is to reassemble things and see out the season, possibly with the Weber in place on a 'try before I buy' basis, and then have the engine out over the winter. I don't have the means to raise the car to a decent height and I'm not really flexible enough to do it by dropping the sump. I will also get the head skimmed again as there is a bit of pitting on the flat areas outside the combustion chamber.

C Mee

Bad news Colin and you now have me worried about mine. One of my cap head bolts in my K&N doesn’t really tighten up either . I hadn’t thought about the spring washer getting sucked in I assumed the bolt wouldn’t if it came loose . Does anyone know what thread that is in the carbs ? It’s quite coarse . Is a longer bolt and nut the solution or I suppose a helicoil but I have never used one .
Mike Fairclough

Can’t really help to make your decision but I once had a Mk2 Golf 1.3 that ingested a spring washer and caused similar damage to a piston. I removed it, filed the top smooth and refitted it. It was still going 50 or 60k miles later at about 150k miles.

Hope you get it sorted.
John Payne

Thanks John, that's quite reassuring.

Mike, I believe the filter to carb flange bolt should be 5/16" UNC. It must have been the bolt bouncing around inside the K&N that made the rattling sound that we couldn't identify.

I'd forgotten the spring washer was there when I assumed it was plastic metal that had been sucked in.

When it goes back together I will use a bolt that's long enough to take a nut on the back of the flange, though it might need to be a half-nut similar to the ones holding the carb to the manifold.

I wondered whether Lumiweld or something similar could be used to fill the bolt hole to be re-drilled and tapped but I would always worry that that might fail or the thread strip like the plastic metal did, so it will be the nut and bolt route for me.
C Mee

Thanks for the bolt size Colin I think I will go the same route.
Mike Fairclough

Colin
Bad luck there buddy-
Don't know if it'll help but some 1 3/4" su carbs have a bolt that comes in from the back side into the carb with a sort of knurled shank that holds it there then a nut on the outside (nylock)
I've also seen some with a flanged/threaded steel insert that goes in from the back as well but that'd mean a bolt in the aircleaner so no help really
William Revit

It sucks you have hit trouble, but sounds like you have a way forward and you will sort it out. Good luck Colin! :-)

Malc.
Malcolm

Yes, Its a real pain when these things happen but I think you have got away with it! A clean up of the crown a new HG and bolt in the carb and you should be good to go. Save up for new pistons in the Autumn!
Bob Beaumont

Mike

The thread is 5/16" UNC, as Colin said.

I was thinking of buying a helicoil kit, as I have some carbs that need repairing.

Are you sprinting this year?
Dave O'Neill 2

If all goes to plan Dave I should be at Curborough in August for MGCC.
Mike Fairclough

Great, I'll see you there.

I couldn't make the April event, unfortunately.
Dave O'Neill 2

Colin

Bad news. Hope the just replace the broken piston replacement goes well.

I had the same problem with broken flanges on an HS2 carb housing (and also incorrect and miss-matched carbs). I tracked down a secondhand carb housing - AUC 871 - which is common with Mini Coopers as well as Spridgets and rebuilt the carb around that. If you buy a complete Mini carb you will need the midget plastic spacer that sets the angle of the float bowl. If you do track down an AUC 871 you might need to put in a new throttle spindle and butterfly as well as replace the jet. I paid about £12 for a secondhand AUC 871 a few years ago, on eBay they unfortunately seem over double that now e.g.
https://www.ebay.co.uk/itm/334514128849?epid=1147894960&hash=item4de29887d1:g:TlAAAOSwGZBi4BTz&amdata=enc%3AAQAIAAAA0Pj07mYgIX2oZt%2BL8JGHUkEmYo1WlKBBsub368EDWJkx4H7iDD76Y2Pw4sMQyt9AnU6%2F0nU8UGd0GZF5

I have a rare few days in the garage - I’ll see if I have anything of use in terms of HS2 carb bodies.

You mention a potential switch to a single twin choke sidedraft Weber (a 40 or a 45 DCOE?). If you want something different there are manifolds around for twin HS4 carbs as well as single HIF44. The twin HS2 carbs a might still have enough flow for your set up?

Cheers
Mike
M Wood

Mike

Thank you for your comments.

One frustration is that the carburettors have been doing their stuff really well. They are a matched pair that I bought a couple of years ago and re-built with new jets, needles, spindles and butterflies. It's just the outer filter to carb bolt holes that are stripped, the two inner ones are fine.

The previous carburettors were mis-matched, with the rear one being ex-mini. Fitting the correct spacer levelled the float chamber. They were running OK but I was never 100% happy with them. One reason for swapping them was to get rid of the PCV and have the breather take-offs from the bodies.

I had been giving a single HIF4 some thought but, since the weekend it's a friend with a Frogeye who's offering me his Weber on a try-before-you-buy basis so I'm intending to do just that.

The head and pistons are now cleaned up and a couple of head gaskets arrived today so, other commitments permitting, I should be able to have it all back together shortly.

Colin



C Mee

This thread was discussed between 15/05/2023 and 19/05/2023

MG Midget and Sprite Technical index

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