13 replies to this topic

[whistler]

I have 3 good pistons and 1 bad. Numbers are 21268 G? J52. I think Cooperman used some of these back in 2009 so maybe he'll be able to answer my query. Can't find any information about dish size for these. I have a 21253 as a spare and am hoping to use the 21253 to make up the set. I know they were fitted to Allegro/Montego/Metro 1.3 but no info about the dish size.

[Cooperman]

If you want to use one 21253 you will need to measure the dish of the 21268 And the gudgeon pin centre line to the top of the piston. These need to be absolutely identical.
Then the weight needs to be established for each piston. So long as the 21253 is not significantly different from the other three any difference can be cut from the heavier one(s) until they are equal.
Those are the critical things really. Whilst not ideal to mix-and-match pistons, there is no reason why it won't work.

[whistler]

If you want to use one 21253 you will need to measure the dish of the 21268 And the gudgeon pin centre line to the top of the piston. These need to be absolutely identical.
Then the weight needs to be established for each piston. So long as the 21253 is not significantly different from the other three any difference can be cut from the heavier one(s) until they are equal.
Those are the critical things really. Whilst not ideal to mix-and-match pistons, there is no reason why it won't work.

Thanks. I've actually got 3 good 21253 pistons as well so maybe I'll just buy a new one of those and swap the rods over. I did do a depth of dish measurement and the 21268 is quite a bit deeper so a lower CR.

Edited by whistler, 14 May 2018 - 05:55 PM.

[Cooperman]

Unless the wrist height and the dish volume are identical you can't mix them. A slight weight difference can be corrected.
Get a 21253. What size are they. I may have one you could have.

[whistler]

Thanks Cooperman, standard size.

[whistler]

Unless the wrist height and the dish volume are identical you can't mix them. A slight weight difference can be corrected.
Get a 21253. What size are they. I may have one you could have.

Sorry Cooperman, I have just checked the 3 spares I have and they are 21962 not 21253 (they're what I replaced them with). I understand that the ring sets are the same for both numbers but are they compatible otherwise?

[Cooperman]

I don't think they are compatible. You would need a 21253 to compare the critical dimensions and weights.

For the cost, I would simply buy a set of 21253, do a dummy build and deck the block to suit, then you know all will be well. The 21253 is a very good piston and will take moderate sustained high revs.

[whistler]

I don't think they are compatible. You would need a 21253 to compare the critical dimensions and weights.
For the cost, I would simply buy a set of 21253, do a dummy build and deck the block to suit, then you know all will be well. The 21253 is a very good piston and will take moderate sustained high revs.

Thanks, I'm taking the block to my local machine shop tomorrow to have it looked at and measured. Then see if it will take standard pistons and rings otherwise I'll have it rebored and new pistons.

Edited by whistler, 15 May 2018 - 09:14 PM.

[Cooperman]

Sounds like a good plan.

[whistler]

Engineer says block needs a good honing and the crank a good polish and we're ready for new standard pistons.

[Cooperman]

Just make sure you do a dummy build and deck the block to bring the new pistons right to the top at TDC.

[whistler]

Just make sure you do a dummy build and deck the block to bring the new pistons right to the top at TDC.

If I buy the 21251 pistons any idea how much to skim the block to bring the CR up to 9.5:1? The block might need skimming anyway. It's for a road car and will be fairly standard.

[wile e coyote]

I'd still go for 21253 regardless  - not too expensive and worth it...(if you're searching :modern iterations under "Nural" Branding - part no : 87-524114-00) as once you've cut the deck down that's it - bearing in mind your block seems in remarkable order with plenty of scope for many rebores would be a shame to go cheap & compromise now....

[Cooperman]

The 21253 is a much better piston than the 21251.
Whichever you choose you must do a dummy build in order to complete the necessary CR calculations.
Basically you need to measure how far the piston tops are down the bores at TDC and the volume of the combustion chambers.
Once those figures are known the necessary machining for the desired CR can be established.