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MG Midget and Sprite Technical - Piston size, and type 19320?

Here's a pictue of my spare engine. I'm thinking of rebuilding it.

I bought and fitted these pistons sometime around 1984/5 (I Forget when exactly, it may have been earlier.)

I believe they are Hepolite 19320 Cooper S pistons, but might not be. I Lost the receipt, but have a note in my workshop manual that I bought 19320 pistons.

My questions are.

1) What do the characters -- 1L --, after the +030 signify?


2) I can't find much in the way of a reference to these pistons (19320) on the web (may be my search technique). If discontinued, would I still be likely to source new rings for them in +30 thou.

3) If I cant re-ring them, what's a good piston to go for. I'm not thinking of doing a rebore, as it's in excellent unscored nick, just a bit glazed, and barely has a lip at the top of the bore.

I will be bolting on the head I took off, which is an Austin 1300GT head.



Lawrence Slater

Don't know about the 1L

Key thinks to check are the deck height (the clearnce between the top of the piston [at the outer edge] and the top of the block) and the capacity of the dish. There were several variations of both of these and obviously they have an effect on compression ratio.

Capacity of the dish can roughly be calculated base on the cone formula (1/3 pi r2 h) if you take h as being the depth of the dish plus 25. (The radius is the dish radius not the bore radius).

Rings should be easy emough to get. Be aware there are two thicknesses of compression ring - 1.5mm and 1/16". They are aproximately close but not near enough to be the same here. Original midget would have been 1/16", while the Metros by at least the mid '80's had the metric rings and high compression pistons with a shallow dish.

One of the part no's is BHM1284 (plus the oversize) - I'm not sure whether 1/16" or 1.5mm but suspect 1/16".

1.5mm rings can be hard to find, if you get stuck let me know.

You need to check the clearance between ring and piston down the side of the groove. It usually starts at around .003", if worn the book figure is that if you can get a .006" feeler more than 1/16" into the gap between ring and land then you need to do something about it. This could be either new pistons (preferred) or by widening the groove to take wider rings or the original rings with a spacer. If you have metric rings, then taking the approx .004" out to fit 1/16" rings can provide a neat solution to moderate groove wear.
Paul Walbran

If you Google it there are a bunch of things like "Hepolite 19320 Powermax Cooper S pistons "

FRM
FR Millmore

Hi FR, I did google and all I seem to get is other bbs that mention the 19320, but no real info. One I hit though, did say he had the same problem. Same 19320 piston, same oversize +30, and couldn't find a ring set. Another said they were obsolete, and not available anymore.

Paul, thanks for that. Are you saying the BHM1284 3 ring set, is the same as for the 19320 Hepolite piston?

Until I pull the pistons, I have forgotten how many rings they have. I recall it being 3, but might have been 4.


And if I go for new Pistons, what are the ones to go for these days?

Lawrence Slater

From Deves listings, this piston takes 1.59mm rings (1/16"), a standard Mini item - but so are the 1.50mm ones!
3 ring pistons and pretty common ring set

http://www.deves.com/devesrings/cars_AUSTIN.php

FRM
FR Millmore

From what FRM notes from the Deves list, BHM1284 looks like the ones you want.

If you need new pistons, you need to decide what CR you want and whether or not CR is more important than money as the higher CR are more expensive. In round figures about 100 quid for std CR (about 9:1 on 030), 150.00 for 10.4:1 on 030 bore. That includes rings which run out at about 35-40.00 if you were just to buy the rings on their own.
Paul Walbran

FR, thanks for that link to Deeves listings , I've saved the page for reference.

Paul, I reckon I'd be happy to stay at 9:1. I seem to remember that number when I bought the pistons, and from reading around on here with the current octanes available going to higher CR seems not to be worth it for my road/touring use.

I'm amazed that rings are still so comparatively cheap. And those piston prices aren't that bad either.

When I get around to pulling the pistons out, I'll be able to gauge more accurately if new rings will suffice, or if it's a +40 re-bore and new pistons job.

Thanks very much for the info.
Lawrence Slater

This thread was discussed between 01/08/2011 and 02/08/2011

MG Midget and Sprite Technical index

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