Xflow, forged pistons to bore clearance

[Olgawld]

Hi. Just having my 1700 rebored to fit 83.5mm accralite forged pistons. Chap's asked what the piston to bore clearance should be. Any ideas? Also will any other machining to pistons be needed? Refitting 234 cam, standard rods etc. Cheers

[Englishmaninwales]

Try Omega Pistons Limited (incorporating Accralite) 0121 559 6778. I can't help with the latter question, hopefully Roger King will be along shortly to answer.

[Roger King]

You should run .0045" to .005" clearance for normal use. I can't answer about the machining because it depends on your engine, but the things you need to look out for are that the valve pockets are of sufficient diameter for whatever valve size you are running and once that is sorted, what extra (if any) you need to take out of the bowl to give you the compression ratio you are wanting. This will vary according to block height, etc. It is rare to need to increase the depth of the valve pockets. I am assuming that you are running proper Crossflow pistons and not flat top pistons. 

[Olgawld]

Thanks for that Roger. I am using the standard xflow pistons yes not the flat tops. Cheers

[auyt]

Olgawld, hang on a sec, classic forged pistons ran large clearances due to the allow please check with the manufacturer other wise the engine will rattle all the time.
The alloy used in new forged pistons especially JE and I suspect other manufactures do also, have the same expansion rate of cast pistons, and there for a .0025" skirt clearance.
Trust me I got caught on a classic Maserati V8 fitted with JE pistons. I finished having the pistons stuff coated to reduce the clearance to 3 thou. 

[Roger King]

Well, I confess that I haven't used any Accralites for some years, but unless they have changed the material (I believe they still use RR58) my figures still apply. And it won't rattle either although I have found some forged pistons from the USA rattle like a bag of spanners.

[Olgawld]

Thanks Auyt. I did check other places and they said the same sort of figure as Roger, which i thought they would.

[auyt]

Pleased you checked.

[AnkerB-S]

Are the 83.5mm accralite forged pistons available from Burton Power direct replacements for the 1700 Supersprint after boring out to 1700 cc? Or does it entail more work?

[Roger King]

It depends on the compression ratio you are aiming for, the height of your block, your valve sizes, etc. In general, for road use you will probably want to lower the compression ratio to suit your fuel octane and intended use; You need to do the measurements first but this is usually just a case of machining the bowl a little deeper. Valve size is down to the spec of your engine; you can get a good idea on this by removing the head of an old valve, turning the stem to a point and then using it down the valve guides as a punch to very lightly mark the piston crown. That will give you the valve centre within the piston cut-out which will allow you to work out if the pocket is large enough diameter. You need to allow a little more clearance for the fact that the piston will rock a little when running, particularly when cold. It is 17 years since I last built a Crossflow so I really can't be sure what diameter valve cut-outs the pistons come with these days; in my time, you could generally expect the cut-out diameter to be fine for standard size valves, but need enlarging for big valves.