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MG Midget and Sprite Technical - Piston decision

I have the engine out now to investigate the clattering. Louder than usual ticking started in the summer, before the overheating episodes I mentioned before. After the last one, when the heater valve dumped all the coolant, it became much worse. Sure enough there is a broken piston ring. The pistons are 998 A-Plus flat tops, made in Australia as it happens. They have done about 25k miles. It is getting difficult to find non-oversize pistons - the bores are standard.

The ring groove clearances are way too high, which accounts for the clattering. I'm surprised at this level of wear at moderate mileage, but of course the car has been mainly used for short trips. There is little evidence of damage from the overheating. Bores are ummarked, just a bit of scuffing on the pistons, and slight erosion from detonation on the land above the top ring, only on 2 pistons.

I have a set of Hepolite rings but I now find they are thinner than the ones on the pistons. In any case the groove wear varies around the piston.

The dilemma I have is that I would like to put the original engine back in, so that I have a matching numbers car. But that will take time as it needs boring to 998cc, cam bearings etc, and I'll need to transfer all the performance goodies from the current engine. So I'm looking at a stop-gap, and as it happens there is a set of used pistons on eBay:

https://www.ebay.co.uk/itm/MINI-998-FLAT-TOP-STANDARD-PISTONS-CONRODS-MINISTOX-RACE-RALLY-A-A-ENGINE/273553290936

The idea of installing used pistons horrifies me, but would I get away with it in the short term? The seller can't be bothered to identify the make so I don't know what rings they will need. I'll have to suck that and see. Risk is that they are too worn to use and again the seller won't tell me the ring clearances. But if they are OK, and Hepolite (likely) I can use the rings I have. I am sorely tempted to take a chance.

Les
L B Rose

Not meaning to sound over critical but why on God's green earth would anyone consider using used pistons from an unknown source? Oh, by the way, I have a charming little bridge I would love to sell you! But seriously, I would steer clear, this just seems to be one of those "penny wise - pound foolish" endeavors. Imagine an undetected, hairline crack in a gudgeon pin boss and the havoc it would wreak. Maybe this would help you thru the short term but for me, I just wouldn't risk it - too much to lose, too little to gain.
Stan Kowznofski

Agree with Stan.
David Smith

I sold the bored to maximum 948cc engine from my car and am now fitting a 1275. But I can still make it matching numbers again if I want to ever go back to a 948 - I kept the engine number tag! So I wouldn't worry too much about that side of things.

I can't remember where I read it but the golden rule with engines is 'there is no substitute for new bores and new pistons'. And its even more important with a performance engine.

The thing that would make my mind up though is think of all the time and effort involved if you end up doing it twice!
John Payne

Les

Try giving F W Thornton a ring: http://www.fwthornton.co.uk/2.html (or Minispares). The former sometimes have new old stock pistons as well as new ones - why not just put in standard 948 Sprite pistons if you do not want to rebore to fit 998 pistons?

Cheers
Mike
M Wood

Les

See various 998 Pistons here: http://www.minispares.com/search/classic/Pistons.aspx
https://www.minispares.com/catalogues/classic/Classic~Body~Parts~Manual/Engine/Piston~Rings~and~Sizes.aspx?1~2~103

Or 948 Sprite: https://www.ebay.co.uk/itm/AUSTIN-A35-A40-HEALEY-SPRITE-MG-midget-PISTONS-STD/320299783913?hash=item4a935ad6e9:g:jYsAAMXQHxVRhNq7

Cheers
Mike
M Wood

At that price, I'd be tempted to take a punt - even just for the conrods.
Dave O'Neill 2

One thing to be aware of - later A+ 998 engines used a different piston/rod set-up, with an interference fit pin. When buying pistons, you need to make sure get the right ones for your rods.
Dave O'Neill 2

I have bitten the bullet and ordered new pistons from Somerford Mini. Minispares don't have circlip type flat tops that are not oversize.

I was tempted by the used pistons but unless the seller can tell me something about their condition it's not a goer. Yes the rods alone are worth £20, but I already have 1098 rods in the engine and a spare set.

Re engine numbers. I used to think that all the 948 blocks were identical, but not so. My 9C block has 09C cast into it. My gold seal block has something else. So swapping engine number plates can be detected.

Another reason for pulling the engine is to fix the rear crank bearing oil leak. I have the Morris Minor Centre seal kit and as I have said before I'm not madly impressed with the design. The lipseal I have just removed is worn out. It's a huge bearing surface and running at high revs. I have spent some time restoring and polishing the flange, which has a Speedi-Sleeve on it, so I hope that will improve things. I have also added a screw that holds the lower half of the seal housing to the bearing cap. Not much more I can do.

What do folks think about running-in oil? I will of course glaze bust the bores, gap the new rings etc. The seal kit instructions say that running-in oil MUST be used for 500 miles after fitting new rings or crankcase pressure will blow the seal. This oil seems to come as either 20/50 or plain 30. Which to use?

Les
L B Rose

Les
The size of the shaft seal isn't really an issue, lots of late model cars run large diameter seals--but-the quality of the seal could be an issue, not all seals are equal-
For a crank seal you need a high temp rated seal and ideally one with a directional sealing surface (anticlock) and have a built in dust lip.
Your local bearing/seal shop should be able to supply the correct seal for you and plenty of lube (vaseline is good) on the shaft and on the sealing part of the seal when you do the final assembly to stop the seal grabbing/burning at startup-
On the running in oil choice between 20/50 and 30 , either would be fine, it's not as if you're going to be driving it hard during the runin time, and don't be in a hurry to change it out,I'd treat the 500mls as a bare minimum
willy
William Revit

I have searched for months for the correct seal and it's only available from the Morris Minor Centre. My local bearing/seal shop? No such thing here in medieval Salisbury!

Les
L B Rose

The engine is back in with new pistons. I honed the bores of course and filled up with 20-50 Dynolite running in oil. The pressure is slow to come up when cold but it eventually does. No oil leaks so far, and the terrible clatter is gone, leaving just the usual valve noise. Very pleased to find that A-Series guru Steve Harris is just down the road in Wilton now, so I got him to skim the piston crowns to get them flush with the decked block.

I also found that one of the crank oil restrictors had slipped and scored the bearing shell. I punched it back in and Steve had some old stock shells at the right undersize. Just as well I found it.

The work was as ever more protracted than expected. After torquing down the head the very last nut, on the rocker shaft, stripped the stud. I could not get the stud out (I had a spare), so off came the head again to deal with that.

New clutch friction plate while I was at it - I had forgotten what wheelspin is.

Les
L B Rose

This thread was discussed between 12/11/2018 and 22/11/2018

MG Midget and Sprite Technical index

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