32 replies to this topic

[JonnyAlpha]

Hi;

 

What pistons do I need for a 1293 build. I am getting a quote for machining work and a rebuild of the short engine and poss gearbox from a local reputable Engineering Company but just want to make sure I know what I need to ensure that they follow suit   

 

I believe I am aiming for a 10:1 CR and the 1293 will be a +20 rebore so needs matching pistons.

 

I have read several threads recommending the 21253 pistons and have found these on MiniSpares offering a 10.3:1, which are AE Hepolite Pistions, they also do an Evolution versions suggested as a higher compression ration at 9.73:1 here.

 

Minisport do a Standard Compression and a High Compression AE Hepolite Pistion here and here

 

Any recommendations on what should be fitted and what CR I should be asking for?

 

Thanks 

 

 

[nicklouse]

not quite that simple. as normally you would work it all out for your set up and machine the block to suit. but to do that you also need to know the CCs in the head.

[JonnyAlpha]

Ok so where do I start?

 

As you know from here I am rebuilding an early MG Metro 1275 to replace the engine in my 1982 HLE 1000.

 

Rebored to 1293

Evolutuion 001 Cam unless MG Metro Cam proves to be in good serviceable condition

Poss some basic work on the head such as smoothing the valve seats and grinding back the Valve Guide bosses

Lightened Flywheel

HIF44 Carb

Stage One Kit

Electronic Ignition (Staring with standard Metro Electronic Ignition)

3.44 Dif Ratio

Edited by JonnyAlpha, 11 February 2019 - 09:05 PM.

[Cooperman]

The 21253 pistons will be ideal. However, the only way to achieve the CR you want is by accurate measuring and calculation.

 

First of all have the machining done and do a 'dummy build'. Measure how far down from the block deck the piston crowns are at Top Dead Centre (TDC). Then strip it all down and have the block deck machined down until the piston crowns come to between 0.005" and 0.002" of the block deck at TDC.

 

As a guide, for every 0.010" of distance of piston crown down the block, the capacity above the piston increases by 1 cc. Thus at -.005" down the bore, there is 0.5 cc plus the piston dish volume above the piston.

 

Allow 3.7 cc for gasket volume.

 

You can then calculate the ideal volume for the combustion chambers in the head. It is all on here somewhere, just do a search for how to do the calculations.

 

Once you know the desired volume, assemble the head and measure how much needs to be skimmed from the head surface. After this is done, re-assemble the head and do the final assembly of the engine.

 

It may sound a long job, but it is all necessary to get it right. That is why engine building is not a cheap job if done by a professional building a good, reliable and powerful engine.

[ACDodd]

Having recently had to rework an engine using an evolution 001 cam that had 10.2:1 and a pinking problem, I can suggest that a compression ratio of 9.5:1 is much better suited. Happy to do your compression calculations for you. I do need to know what head chamber volume you have. ( Standard is not a good enough answer). I can also measure yours if you can get it to me.

Pistons can also be machined to decrease the CR if needed. Here below is a set of 21253's being adjusted.

Ac

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Edited by ACDodd, 12 February 2019 - 07:08 AM.

[Retroman]

The link is useful for working the compression out

 

https://jscalc.io/ca...RNEglVkKLQir11T

 

I did supply the admin various other piston sizes and cc's but he has not listed them

[gazza82]

Found this one is pretty good .. and works in Imperial the way all good Mini owners should!! ...

 

http://www.gtsparkpl...pRatioCalc.html

[JonnyAlpha]

Having recently had to rework an engine using an evolution 001 cam that had 10.2:1 and a pinking problem, I can suggest that a compression ratio of 9.5:1 is much better suited. Happy to do your compression calculations for you. I do need to know what head chamber volume you have. ( Standard is not a good enough answer). I can also measure yours if you can get it to me.

Pistons can also be machined to decrease the CR if needed. Here below is a set of 21253's being adjusted.

Ac

 

Nice work Mr Dodd. When I asked about the CR the machining company said "The CR we aim for is somewhere between 9:1 to 10:1 it is a bit unknown as we  are facing the block and cylinder head. If you want a specific CR additional cost will be added to allow for calculations"

 

So unless I specify it will be hit and miss?? 

 

 

 

The 21253 pistons will be ideal. However, the only way to achieve the CR you want is by accurate measuring and calculation.

 

First of all have the machining done and do a 'dummy build'. Measure how far down from the block deck the piston crowns are at Top Dead Centre (TDC). Then strip it all down and have the block deck machined down until the piston crowns come to between 0.005" and 0.002" of the block deck at TDC.

 

As a guide, for every 0.010" of distance of piston crown down the block, the capacity above the piston increases by 1 cc. Thus at -.005" down the bore, there is 0.5 cc plus the piston dish volume above the piston.

 

Allow 3.7 cc for gasket volume.

 

You can then calculate the ideal volume for the combustion chambers in the head. It is all on here somewhere, just do a search for how to do the calculations.

 

Once you know the desired volume, assemble the head and measure how much needs to be skimmed from the head surface. After this is done, re-assemble the head and do the final assembly of the engine.

 

It may sound a long job, but it is all necessary to get it right. That is why engine building is not a cheap job if done by a professional building a good, reliable and powerful engine.

 

I have received the quote and at first I thought they were using cheap pistons as they have quoted £75. I have just looked at the quote again and the cost of each piston is £37.50 so that should be £150. Still very cheap I thought. I then asked what pistons they use and the answer was "County". A little searching and there are a couple of old posts on here saying that they are good Taiwanese AP copies?

[JonnyAlpha]

The link is useful for working the compression out

 

https://jscalc.io/ca...RNEglVkKLQir11T

 

I did supply the admin various other piston sizes and cc's but he has not listed them

 

Nice calculator - thanks 

[gazza82]

 

I have received the quote and at first I thought they were using cheap pistons as they have quoted £75. I have just looked at the quote again and the cost of each piston is £37.50 so that should be £150. Still very cheap I thought. I then asked what pistons they use and the answer was "County". A little searching and there are a couple of old posts on here saying that they are good Taiwanese AP copies?

 

 

Sign up to TMF+, get the 21253's from MiniSpares and get a 7.5% discount!

 

Any you having to say what CR you want doesn't sound great ..

Edited by gazza82, 13 February 2019 - 09:55 AM.

[ACDodd]

County are a trade brand and ok for road use, just don't expect top power figures. You get what you pay for with engine oarts.

Extra cost is required to set the compression ratio as a dummy build has to be performed to determine the deck height. From the the block can be skimmed to set the optimum deck height for your application. What most people don't realise is when you got to a trade engine builders they do not do most of the checks and adjustments that are talked about in forums. Consequently, when asked to do them the price goes up considerably. That's why a specialist builder can work out a better deal. By that I don't mean cheaper, I mean you get a better product.

My worry here is they are using the low compression county piston. These are not designed for high compression use and have no place in a performance based build.

Ac

Edited by ACDodd, 13 February 2019 - 10:44 AM.

[Retroman]

 

 

I have received the quote and at first I thought they were using cheap pistons as they have quoted £75. I have just looked at the quote again and the cost of each piston is £37.50 so that should be £150. Still very cheap I thought. I then asked what pistons they use and the answer was "County". A little searching and there are a couple of old posts on here saying that they are good Taiwanese AP copies?

 

 

Sign up to TMF+, get the 21253's from MiniSpares and get a 7.5% discount!

 

Any you having to say what CR you want doesn't sound great ..

 

 

And  we  can  give  you  10%  off

[JonnyAlpha]

 

 

 

I have received the quote and at first I thought they were using cheap pistons as they have quoted £75. I have just looked at the quote again and the cost of each piston is £37.50 so that should be £150. Still very cheap I thought. I then asked what pistons they use and the answer was "County". A little searching and there are a couple of old posts on here saying that they are good Taiwanese AP copies?

 

 

Sign up to TMF+, get the 21253's from MiniSpares and get a 7.5% discount!

 

Any you having to say what CR you want doesn't sound great ..

 

 

And  we  can  give  you  10%  off

 

10% off of what exactly? Parts, machining, build?

[JonnyAlpha]

Having recently had to rework an engine using an evolution 001 cam that had 10.2:1 and a pinking problem, I can suggest that a compression ratio of 9.5:1 is much better suited. Happy to do your compression calculations for you. I do need to know what head chamber volume you have. ( Standard is not a good enough answer). I can also measure yours if you can get it to me.

Pistons can also be machined to decrease the CR if needed. Here below is a set of 21253's being adjusted.

Ac

 

So which pistons would you recommend? 

Edited by JonnyAlpha, 24 February 2019 - 05:09 PM.

[ACDodd]

You need to see what the combination you have is. So you need to know the head chamber volume and the deck height. Then I can work out a combination that will work for your application.

Ac