Race Engine Forum UK

I have a set of NOS VX pistons 84mm bore, these are modial, two of them have 8440 cast inside the piston skirt and the other two have 8420 cast in but otherwise all markings are the same I measure the piston diameters as

3.3065"
3.3060"
3.3055"
3.3055"

I have a used standard bore block from an argenta, that I would like to use but I'm unsure if the piston-bore clearances are ok/will be ok after it has been given a light hone.

It has no wear ridge and the bores are all class D,
I measure them as

1: 3.3098"
2: 3.3095"
3: 3.3010"
4: 3.3095"

Matching the larger pistons to the larger bores I get piston to bore clearances of 0.0035" to 0.004", I realise these will increase with a light hone by some amount, maybe 0.0005" ?

In your workshop manual you state up to 0.075mm (0.003") clearance is acceptable. Elsewhere on this site you have said up to 0.006" is ok, but you were referring to forged pistons.

So getting to the point, will it be ok to use these pistons in this block after it has been given a light hone?

Regards
Tom

Good post.

Replacement pistons were sized (always - in my exp) Class A which is - from OE data:

83.930 - 83.940mm or 3.3043 - 3.3047"

Class A bores would have been piston size plus OE running clearance of 0.060 - 0.080mm, so taking the smallest piston/smallest clearance and biggest piston biggest clearance the range for Class A bores would have been 83.96 - 84.02mm.

Your pistons measure:

3.3065" - 83.985 (which is class E)
3.3060" - 83.972 (E)
and
2 at 3.3055" - 83.96 (top limit class C)

If you measurement is right (gauge point distance is 25mm from the end of the skirt) it looks like you have a mixed set of pistons that were never available in the aftermarket, maybe taken from the factory line, because to the best of my knowledge nothing other than Class A was ever sold outside of there.

Leaving aside whatever is stamped on the crankcase skirt (D in this case) your bores in used condition measure:

(1) 3.3098" (84.069 mm)
(2) 2 at 3.3095" (84.061 mm)
(3) 3.3010" ¢‚¬Å“ assume that is a typo - do you mean 3.3090 - 84.049mm?

You'd tend to match biggest piston to biggest bore, so:

Largest piston first
83.985 goes to bore (1) above clearance = 0.084mm (3.3thou)
83.972 and 1 x 83.96 pistons go to bore (2) giving clearances 0.089/0.10mm respectively (0.10 being 0.004")
and
1 x 83.96 goes to bore (3) giving 0.089mm clearance.

That looks OK to me, but a lot depends how big the bores go after honing. You should also check all the top ring end gaps, you want to be looking for say, 14-16 thou. 18 is OK but much more and I would be for getting some 0.4mm oversize top and second rings and gapping them down.

I URGE you to buy a 180 grade silicon carbine Flex Hone to do this because it will glaze-bust far more effectively than any other method without increasing the bore diameters hardly at all. If you go shoving a parallel rigid hone down there you could end up with way bigger bores than you expect. Rigid stones are for final finish-to size after reboring, not for glaze busting.

GC

Thanks Guy, I'll get the honing done and measure it up again.