21 replies to this topic

[samt88]

I need to buy some new pistons for my engine rebuild. I was suggested these circlip types http://www.minispare...ADU3490-00.aspx which are about half the price of any other pistons I've seen. Could someone explain why this might be and just what issues there might be with circlips?

 

Cheers,

 

Sam

[Cooperman]

There are no issues with circlips. Literally thousands of BMC A-Series engines used circlips to retain the gudgeon pin.

Those pistons look good, but I notice they are standard bore. Checking the Mini Spares web site it seems they are available in +020, +040 & +060 sizes.

If you are currently at standard bore you would be advised to go to +0.020".

[samt88]

Oh okay cheers but if I may ask what's the advantage of that? 

[Cooperman]

You most definitely don't want to put brand new pistons into worn bores. If you do that it will soon need re-building again.

I can say that in 50 years of building engines for Minis & Fords I have never, ever fitted a new piston set into old bores. It represents no saving as the new pistons may well not seal into old bores, which will be worn most at mid-stroke. Then you have to buy yet another set of pistons and get the block re-bored.

Do it right first time, then you don't have to re-do it any time soon.

[samt88]

So I should rebore then? Just that I went to a rebore shop and asked their opinion and they told me the bore was absolutely fine - just needed new pistons.

[Cooperman]

So I should rebore then? Just that I went to a rebore shop and asked their opinion and they told me the bore was absolutely fine - just needed new pistons.

 

Well, if they are prepared to not re-bore it's up to them, but most good engine builders I know would never do that. New pistons need freshly machined bores if the re-build is to be done properly. If a set of pistons needs replacing, it shows the bores are worn as well. If the bores are more-or-less OK, then get the bores honed and fit new rings to the existing pistons. Old bores & new pistons don't mix.

[Ethel]

A +20 bore shouldn't cost you much more than gaskets to put it back together afterwards. I'd agree it's a no brainer if you are buying pistons anyway.

[Gr4h4m]

If using the circlip pistons make sure you put the circlips in the correct way around.

[samt88]

So a +20 bore and the +20 pistons is the consensus then? What would that increase the capacity to?

[Ethel]

1014cc

[Carlos W]

Also, a good machine shop will want the pistons so they can bore to exactly the right size due to tolerances in producing the pistons

[samt88]

How does buying the standard pistons sound and then honing the bores? This is what I've been recommended by my local reborer

[Ethel]

As Cooperman said, cylinders don't wear evenly, and honing won't correct that. It seems odd that you could have written off your pistons, but have left the bores that perfect. Checking the bore wear is more time consuming than recutting them, you'd need a good reason not to rebore - like saving 150 quid for new pistons!

 

Specialised are quoting £21+vat, including honing, on their site. It's a no brainer if you can match those prices. It would be worth looking at your crank journals while you're at it - and replace your rod bolts regardless.

[Vipernoir]

At this point someone really ought to mention to the OP that pistons have to match the rods.
Press-fit rods have a different little end to fully-floating rods (circlip type) and they are not really interchangeable.

 

Don't confuse an old bore with a worn-out bore.
If you have your existing bores checked for taper & ovality by a reputable machine shop and theyare still acceptable, then a re-hone will suffice.  Indeed, if the pistons aren't knackered then you will only need a set of new rings instread of pistons & rings.

 

On a budget build, get everything stripped, cleaned and measured before you spend any money on anything.

[Cooperman]

If the pistons need replacing it's virtually certain that a re-bore will be necessary.
It truly is false economy to buy brand new pistons and then fit them into worn (as in barrel shaped, ovalised or scored) bores.
The accepted best practice is: home and new rings on the original pistons if the bores are not too bad. New bore & pistons if engine is well worn generally.
However, no-one is obliged to take the good advice offered on here. Just please don't come back on here after not listening and say in a couple of months time, "my engine with new pistons in old bores seems to be burning oil and is smoking, what can I do?"