4 replies to this topic

[SolarB]

I have a small problem with my engine rebuild at the moment.

My 1275 A+ block is at the machine shop waiting for a re-bore. Apparently it's border line if a plus 20 thou bore will be OK so I'll probably ask for a plus 40 thou re-bore. I need to buy a new set of pistons and get them to the machine shop to have them measured before final honing of the block, hence my choice to go 40 thou rather than buying a set of 20 thou pistons and finding that the bores are no good at 20 thou.

Now, to work out the compression ratio I need fit the pistons in the block, but I don't know which pistons to buy because the piston obviously affects the compression ratio, and to work out the compression ratio I need to trial fit the piston.

Engine is an early 1275 MG Metro. I have a later un-leaded MG head which has had the combustion chambers cleaned a little but not much material has been removed, though it does require a skim. Camshaft is an SW5-07, ignition will be Megajolt.

Compression ratio is undecided at the moment. Swift tune recommended a little over 10:1 for their camshaft and the MG engine was originally 10.4:1 but other sources tell me 10:1 will be a struggle with regular pump fuel. I don't want to build an engine that's permanently on the verge of self destruction and/or requires octane boosters, I just want an engine that will be reliable and economical and will last for 10,000s of miles.

Minispares have these clicky and the price looks good but the compression ratio is typically 8.8:1.

So (I think) my questions are:

How do I choose a piston without building it into a block and measuring crown to deck height?
What compression ratio should I be trying to achieve and if I ended up with 8.8:1 will this loose me a lot of power and economy?
If I was to aim for something like 9.5:1 and bought the 21251 pistons at Minispares will a head skim of a mm or so bring compression to around 9.5:1?

Any advice and experience would be greatly appreciated.

[Cooper-202S]

Were the origional Metro pistons Dished or flat topped to achieve 10.4?

The 21251 Piston is at the budget end of the scale, from the other bits you have listed it sounds like you have a decent budget for your build so it would be a shame to compromise it with a cheap piston.

IMO The 21253 would be a better buy or the hypertec pistons from minisport australia though they take a couple of weeks to get here with customs. both run about 9.75-1 which should be fine with the MJ.

Its tricky to work out the C/R without a dry build but if the block hasn't been decked then the new pistons should deliver there spec C/R no problem.

If you PM with an accurate height of the block (Bottom Flange to top of deck) I will check it against the 4 I have and try to give you an idea if its been skimmed.

As you also have a little to play with on the head it certainly would be better to shoot for the optimum C/R

[icklemini]

and bought the 21251 pistons

If you are intending any type of performance work - dont use the 21251 pistons...

[SolarB]

Thanks for the replies.

I'd assumed that anything produced by AE would have been reasonable quality but I guess you get what you pay for. The machine shop say that if I buy an AE piston they will bore and hone the block from the piston specification. With any other piston manufacturer they want the piston in the workshop to check the dimensions, so it looks like it's the 21253 for me.

Cooper-202S - Thanks for the offer, as far as I know the block is standard but it's at the machine shop so I can't measure it at the moment. I'll give them a call and ask them to get me the dimension.
I'll also have a look tonight at the original pistons and see what they are.

[Cooperman]

The 21251 is not a bad piston, it's just that the 21253 is better in the design of the oil return holes under the lower ring.
In fact, you can use a 21251 up to around 10.2:1 and 6000 rpm without risk of failure, but over that a 21253 is recommended. In my Innocenti I have 21251's running at 10.4:1, but that's just how it's worked out as I bought it with a rebuilt engine and it had those pistons (we shall see with time!).
If you have a block which has been machined down too much, you can find that 21253 pistons are too 'tall' from pin centre line to crown. when this happens you are really stuck with 21251 or you have to machine the tops off the 21253 to get the comp height correct so that the piston does not stick out the top of the block. I actually have this issue with a current engine build. Unfortunately the 21250, which was a good piston of the right height is no longer available. The height from gudgeon pin enter line to piston crown is 0.0625" more on the 21253.