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MG Midget and Sprite Technical - Piston spec 1275

Stripping down a Gold seal 1275 sprite engine and it has dished pistons . Is that normal? . The last 1275 I stripped had flat tops.

Also fancy repainting it gold when I refit it, anybody got any ideas what paint to use . Is Engine enamel available in Gold?
R Mcknight

They're normally dished, I've never seen flat-topped in a 1275 and would expect that to give far too high a CR - unless the pistons are decked so the top sits lower down the bore.
David Smith

Its very rare for them to be flat topped... there probably very expensive ... not something BMC would have done in there gold and silver rebuild service

In the states mini mania sells a set of flat tops, ill bet a wooden nickle that there sister site in the UK sells them also....

Is it ... Minispares.com ???

Prop
Prop and the Blackhole Midget

I just looked

The pistons are called mega. And cost around $600+ USA money for a set

Im not sure what you have ... but its a starting point
Prop and the Blackhole Midget

just my opinion but a gold engine looks shocking. . .
P Bentley

. . . unless it's an original Gold Seal reconditioned engine.

When did that service stop, I wonder?
Nick and Cherry Scoop

will have to agree to disagree. . :)
P Bentley

"When did that service stop, I wonder?"

When customers realised that "Gold" or "Silver", most stuff rebuilt by BMC and sold as reconditioned, wasn't that good. My experience of it on g/boxes was crap. As for engines, you do a better and cheaper job yourself.

I wouldn't be impressed or think I was getting anything special if I found a gold or silver seal unit g/box or engine, in a car. I certainly wouldn't pay extra for a car with one. Esp since BMC are very long gone, a gold or silver seal unit, isn't exactly going to be a new build is it.
Lawrence Slater



I want to say the gold and silver seal was lost to the MG world in the early 90s, maybe up to the last day of BMC existance.. it certianly had a long successfully run of stoggies and victims


Prop
Prop and the Blackhole Midget

"you do a better and cheaper job yourself"

True - but these cars were owned by ordinary people in those days, not by spanner maniacs like us.
Nick and Cherry Scoop

Back to the pistons ...
Original pistons were dished, flat tops used as high CR version, about 10:1. By the time the MG Metro came along, they realised that extra squish around the edge would give better combustion characteristics, so 10:1 was achieved by retaining a dish and raising the whole piston 1mm.
I still get people who insist on flat top for high CR, but the Metro spec is a better option.
Paul Walbran

"True - but these cars were owned by ordinary people in those days, not by spanner maniacs like us."

What am I then? I've been driving Austin/Morris stuff since 1970. I bought my sprite in 1977. Just an ordinary bloke, not a 'mechanic', with no money to waste. Needs must, so I did it myself when I could, and wasn't pleased when I wasted my money on silver/gold seal crap.

And as much as some might not like to think it, these are ordinary cars. Nothing special about them other than age. Very basic in fact. Also, I'm still ordinary, very ordinary. Certainly not a "spanner maniac".
Lawrence Slater

Ehhhhh... sorry lawerance, I have to disagree with you,

You may not like it, but you are one of use... a spanner maniac, you have given great advice over the years and have worked thur some difficult issue weather its the oil circulation or rebuilding that red spridget and getting the body realined... sorry buddy but you have fhe mind of an engineer, and your certisnly not afraid or above getting your hands dirty

Not to mention you have alot of tenacity, where many of us would simple throw out a burned out drill you cound away to rewind the copper core and give life back to it

Sorry .. you, ordinary hahaha... sorry, that just makes me laugh... lawerance, your as bout as ordinary as a blind race car driver, and your never going to be ordinary...

Lets face it lawerance you are one of us, you live by your own code, you think with your own mind, you live a life thats independant

The bottom line, ordinary people dont do what we do, they dont think like us, and they certianly wont rebuild a carb, or diagnoise an electrical issue, ordinary people not like us take there cars to a pro for fixing, they would simply never even attempt to service there cars let alone rebuild a starter

Im sorry lawerance, I dont mean to laugh, or burst your bubble about your life ...but you simple do not qualify as ordinary people...your stuck buddy your one of us

The eagles sang it best... you can check out anytime you want, but you can never leave... welcome to the hotel MG
Prop and the Blackhole Midget

Well if any of that were true Prop, then the human race must have changed an awful lot in the last 60 years or so.

Just about every kid I knew, had the same approach to cycles/bikes/cars, and anything else that we couldn't afford to buy a new replacement for, or steal (lol).

Nah. People today have so many choices in front of them, that they don't know how clever they really are when push comes to shove.

But thanks for the praise anyway. LOL. :). I'm sure there are plenty here who disagree. LOLOLOL.
Lawrence Slater

Back to what Paul said about the dished pistons, a dished piston should be more thermally efficient. i.e. a smaller surface area of combustion chamber to volume ratio, therefore less heat (heat = energy) lost during combustion. So what he says sounds good to me.

Similar to why the Dodge Hemi is so powerful despite two valves per cylinder. Hemispherical chamber shape = large volume with small surface area= excellent thermal efficiency and why the Mazda rotary engine is flawed, long thin combustion chamber = large surface area = poor efficiency.

Malcolm
M Le Chevalier

*note: other Hemi engines are avalible :-)

Also, before being shot down, yes this is a very simplistic view of totally differing engines, and I am talking about combustion chamber shapes etc. but the piston is just the bottom of the combustion chamber really!
M Le Chevalier

"What am I then?"

An ordinary bloke who could only just afford a car, or maybe even not quite. But there were loads of bank managers, doctors, architects, teachers, lawyers, district nurses etc. etc. who could, and who chose to spend the afternoon bathing in costly bath-oils and getting dressed ready for the dinner-dance, rather than lying underneath an oily car and barking their knuckles on sharp metal.

How comforting for them, the Gold Seal promise!
Nick and Cherry Scoop

as you know I don't know much about mechanics or other technical stuff but if I did I'm sure I'd still prefer doing most other things than doing anything but driving the car, it's always a dirty business on a car that used regularly
Nigel Atkins

Go on then, I'll bite

1966 Hemi engine 61bhp /litre

1967 NSU ro80 rotary engine 113 bhp / litre


Malc
Malc Gilliver

Malc, W*nkel engines are so different from the norm that it's hard to make a direct comparison of swept volume, thus the Motorsport authorities used to publish a multiplication factor to decide which capacity class they fit into, just as they still do for petrol engines with turbos.
David Smith

David,

I know they are different , so different that I wouldn't compare them, but

Malcolm did :-

"Similar to why the Dodge Hemi is so powerful despite two valves per cylinder. Hemispherical chamber shape = large volume with small surface area= excellent thermal efficiency and why the Mazda rotary engine is flawed, long thin combustion chamber = large surface area = poor efficiency."

Hence "go on then I'll bite".

to claim a Hemi engine is so powerful and a rotary engine is so inefficient leaves one open to the simple point that the rotary engine produces more power per litre.

Malc
Malc Gilliver

"But there were loads of bank managers, doctors, architects, teachers, lawyers, district nurses etc. etc."

Well except for the district nurses, I would describe many of those as no more than ordinary blokes too, albeit with different opportunities, interests, and perhaps abilities. But think on, there are ex dentists, bank managers, teachers etc, who now own and repair/maintain their own old cars. Are they "spanner maniacs"?

Please define what exactly you mean by the term.

"How comforting for them, the Gold Seal promise!"

How comforting for them, that they might have earned a sufficient amount to afford to be ripped off by a "gold" seal. lol.
Lawrence Slater

Ach, I was in two minds about posting that as I knew I couldn't actually be bothered defending it properly! Especially with crap examples I picked (I am not saying that theory doesn't still hold!). But I shall make half an attempt before letting it lie as another pointless piece of unsubstantiated internet waffle! :-D

1. Shouldn't really have confused power/efficiency although they are somewhat inter related.
2. Specifically on the W*nkle engine...
2.a. Mazda RX-8, ~200hp, 24 mpg, 299 g/km CO2.
2.b. BMW 328 of similar vintage (just as an example), ~200hp, 31 mpg, 216 g/km CO2.

50% improvement, although still comparing apples to iphones really!

The hemi head has generally gone the way of the dinosaurs anyway as it was flawed in other ways, but still, circling back round to the original point, dished pistons should still be advantageous even if my reasoning comes from the rear end of a cow! :-)

Laters,
Malc.
M Le Chevalier

Another example is probably the V12 Jaguar. The original version had a Heron head - no chamber with all the volume on the piston. With such a short stroke engine (70mm) the chamber was wide and shallow. The engine delivered 12 MPG, 13 if you were lucky, no matter how hard you tried to economise.
The HE version had amongst other things a chamber in the head and delviered 20mpg.
Paul Walbran

"The eagles sang it best... you can check out anytime you want, but you can never leave... welcome to the hotel MG"

Prop - brilliant! Sums up our predicament perfectly :-)
Do you mind if I use it?
Paul Walbran

Sure paul,

Go for it... I listened to that song way to much as a child, just the mention of the word hotel brings back that song in my head

I also loved desperadeo

Prop
Prop and the Blackhole Midget

Come on chaps.

Take it easy. Don't let the sound of your own wheels drive you crazy!
Lawrence Slater

Well too late for that :-)
Paul Walbran

This thread was discussed between 08/06/2014 and 12/06/2014

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