10 replies to this topic

[1963MKI]

Need new pistons for my 1275 engine. The block needs a rebore but originally came out of an Austin 1300gt. People have recommended the plus 20 21253 pistons but it only has two compression rings whereas the ones that came out of the engine had 3?

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[Cooperman]

The 21253 is the recommended piston for the 1275 engine. The alternative is the lower compression 21251, but they are not so strong.

I have used the 21253 in engines revving to around 7000 rpm with no problems at static comp ratios of up to 10.75:1.

For sustained very high revs the Omega or Karl Schmidt pistons are stronger.

[1963MKI]

So no one runs three compression rings anymore ?

[Cooperman]

The 21253 is well designed and is a low drag slipper piston and is ideal for a standard or mildly modified 1275 engine.
It's what we all use.

[slidehammer]

You must remember that the 1300GT engine whilst good in its time was a long time ago (around 50 years ago, as it is the MK3 Cooper Engine)and technology and thinking has moved on a lot now. You only have to look at how short the current pistons are in modern cars (all to reduce drag and internal weight)

[1963MKI]

Yes, but just because the engine is old doesn't mean the engineering principles have changed. Only advances in manufacturing and materials really has. Most power gains in the last 10-15 years in consumer cars has been from advances in forced induction specifically turbines. Now people can make turbines that are light, low inertia, high tolerance and reliable. So yes its an old engine and a smaller lighter piston is probably better but I really do not see how two compression rings is sufficient. These aren't alloy blocks with centrigually cast steel liners, its low quality cast iron.

[Spider]

The factory had a 'bad run' with rings and gettig them to seal in the early days of these engines and one of the 'fixes' they had was to introduce a 3rd and on some engines (IIRC) 4th ring + Oil Control Ring. Better honing methods and ring technology has come a long way along with Oils since those days.

[carbon]

iirc some A-series engines had 5 rings: 3 compression plus one oil above gudgeon pin, and another oil ring in the skirt.

 

But as Moke Spider says, for a 1275 engine with accurate rebore & hone to correct clearances a 2 compression & 1 oil ring piston works just fine.

[Retroman]

I can agree the engineering principles have not changed and as you rightly say the main advances have been in materials, also in the production engineering of all parts. This applies to the seriously well developed A series engine more than most, we have a much better understanding today of what is needed to create better and more efficient engines. 2 compression rings are all thats needed today. The highest compression I have run was a championship winning 998 at +040 with no issues with Powermax pistons and compression at 13.5:1, and big bore engines to 12.7:1 (Omega and Russell types) Modern alloys used are more stable and some expand at lower rates which means bores can be tighter, which gives better ring stability. Also better ring materials / profiles, oils, more accurate machining of all components including the block all help. Very few use 3 compression rings now, it makes for a heavier piston with more friction..I haven't used them since the early 90's even in standard type engine builds. If you go with the +020 21253 pistons they should be perfectly good for your application.

[FZRMADMIC]

Hi,

 

Sorry to Hijack this post but it's relevant to me.

 

I'm rebuilding a 1275 'A' series engine, it's either out of a 1275 or 1300 GT (pre A-Plus).

 

I'm also looking to use the 21253 piston, but the engineering shop has said these are for fitment to an A+ only.

 

Is there any reason why I cannot use this piston on the 'A' series engine? I was under the impression these were interchangeable.

 

Thanks in advance

 

Mic

[FZRMADMIC]

Sorry to hijack this thread, but it's relevant to me!

 

Is there any reason why 21253 pistons cannot be fitted to 'A' series (pre A+) rods? I was under the understanding that they are interchangeable, yet the local engineering shop I have spoken to said they are listed for A+ engines only.

 

Thanks

 

Mic