Crossflow acceptable bore wear?

[dikko]

Measured at 8 thou..... is this too much?

 

It races it's head off and does 6.500..

 

1600 FF spec...

 

Will there be a need to re-sleeve & bore to ? thou or is there any magic available for zero power loss?

[Sheds Moderator]

0.2mm in new money. That's not very oval. I take it there isn't a noticeable lip. Are the honing marks still visible, or have they worn away?

 

Someone who knows the tolerance for this motor will be along shortly, but I'd say it was acceptable, hone and new rings at a guess. Meanwhile, normal procedure if it is worn is to go to 1 oversize and buy pistons to fit. You only need bore and sleeve if the bores are already at max oversize, or significantly scored, or the pistons are superduper forged unobtanium items that you can't replace.

[dikko]

Ah, not allowed to oversize...bores good...could even be the fault of pistons...just want to know what sort of power loss at 8thou as opposed to 2,3,4ish...

[ECR]

This might give some pointers Dick

"Although pistons are matched to bores by Vauxhall, Gary Dunn says that that is for engine life reasons (100000 miles plus) Swindon run clearances of 3.5 to 7 thou on racing engines with no performance problems and I should expect to see 4 thou maximum"

[Bio]

Would have thought 4 thou is a max for a road engine....2 to 2.5 thou from new / rebore.

The bigger clearance might cause loss of compression and oil burning.

[7 wonders of the world]

Are you looking for here guidance on piston to bore clearence of bore condition?

 

Get a bore guage down the bore to check for ovality and sizing, bear in mind crossflow take a hammering at the top due to piston weight and material / casting grade which 25 years ago was somewhat variable- if OK hone and check / mesaue again afterwards

 

Check your piston dimensions against new spec to establish wear, if both pistons and bores are within tolerance then measure in the normal place to reveal piston to bore clearence.

 

General 4 thou race and 2 thou road but check with the piston manutacturers as these vary a little if using forged items - your probalby restricted to cast items though for FF ?

 

 

[oilyhands]

Are you talking bore wear or piston clearance? 8 thou is excessive, this will add 24 thou to the ring gaps and the oil ring especially will lose a lot of authority on the bore. The pistons will be noisy too.

 

If you are going to reuse the bore, either remove the ridge at the top or use some ridge dodger rings. You may be able to use some +.010 rings properly gapped but check ovality carefully. Make sure you glaze bust the bores.

 

Oily

 

Edited by - oilyhands on 26 Jan 2012 08:39:07

 

Edited by - oilyhands on 26 Jan 2012 08:40:14

[Sheds Moderator]

OOh, ridge dodger rings, that takes me back. I remember listening to tales of engine rebuilds when I was a kid in the 70s, that was of course back in the days of poor metallurgy and worse lubrication science when things like Minis needed rebuilds every few years and a car that had done 100k miles was an old car. I always thought things like Cords rings were a bodge designed to get a half-knackered engine running again, secure in the knowledge that after another 30k miles the car would be dropping to bits, big ends clanking away, and the by-now-rampant bore wear would be the least of your worries. Do they have a place in a correctly engineered engine rebuild then?

 

Re the wear you report, I understood the 8thou to be an ovality measure, ie the bore measured 8 thou more one way than at 90 degrees. Or are you speaking of something else?

 

Edited by - batteredoldsupersport on 26 Jan 2012 16:50:05

[dikko]

Piston hard over = 8 thou...any point.

[Sheds Moderator]

Ahh. I see. I'm not sure if that is a valid measure, not sure what it tells you. AIUI the normal measures (at least on the engines I have stripped down) are the ovality of the bore and the gap between the ends of the piston ring(s) when normally installed. These have published tolerances, I don't think that the measurement you are talking about does. I'm no expert though.

[oilyhands]

I assume you are measuring skirt clearances..?

 

Have the bores and piston skirts properly measured.

 

Then you will know if the bores/pistons are serviceable.

 

Oily

[Roger King]

Dikko,

 

You need to measure the pistons close to the bottom of the skirt, perpendicular to the axis of the gudgeon pin. Anywhere else will be significantly smaller due to the complex shape of pistons (oval, tapered, barreled) which is necessary to ensure they come true when up to running temperature.

 

Bore should be round and parallel within around a thou if bored and honed by someone with good equipment.

 

Standard road pistons run much tighter clearances than forged race pistons (from memory I think the spec on a Crossflow is .0008" to .0015" or thereabouts), but you would normally expect to loosen things up a bit for a FF1600 - perhaps around .0025".

 

If you really do have .008" that is far too much.

[dikko]

Cheers Guys, much appreciated, as always there's more to it than one thinks. Will go back to the shed and do it properly.