Bore Hone Pattern and Surface Finish

[stevesingo]

Hi All,

I am rebuilding a BMW S14 engine (E30 M3) and I'm seeking advice on what is best for honing the bores.

The pistons are JE made of their 2618 alloy with a non permanent Moly Sulphate coating on the skirts claimed to aid break in. SC on this page http://www.jepistons.com/TechCorner/PistonCoatings...

The rings are also from JE. J62001-3740 on this page, http://www.jepistons.com/Catalogs/ProSeal/RingSear...

Top ring ductile iron with moly coating, 2nd ring Iron with phosphate coating and the oil ring is carbon steel.

The engine will see 8200rpm max approx 280bhp and 200lb/ft from a 2.5lt.

Any thoughts/advice appreciated.

Steve

[Pumaracing]

Whatever anyone's conclusions on optimal bore finishes I think your hardest task is going to be to find anyone who can actually generate or even measure a specific finish if you go as far as trying to specify one. Certainly most engine reconditioners are just going to say "well we do it like this mate and we don't get no comebacks". Of course anyone who used a double negative like that in polite conversation ought to be avoided like the plague anyway.

Specialist honing companies will of course be much more clued up and there are several about.

There's a very good guide to boring and honing practice available from Kolbenschmidt although I don't know whether it's available online. It's generally accepted that if you take the bore as vertical then the cross hatch pattern should be between 30 and 45 degrees to the horizontal.

As to surface finish this is mainly dependant on the rings and I suggest you ask the supplier of those for their recommendations. For race engines I always tended to use a somewhat finer finish that bedded in quicker but was maybe not so long lasting in terms of oil control and bore wear. I'd finish to size with a medium/fine stone like a 280 grit and then plateau or even take another fraction of a tenth out with a 400 or 500 grit. I could get engines to pump out within 5 psi of their final cranking pressures before they'd been fired up for the first time and be fully up to pressure within a few miles of starting.

For higher mileage road engines I would do most of the honing with coarse stones to get a good deep crosshatch for oil retention, take the last few tenths with a medium stone, maybe 200 grit or thereabouts and then perform a very light plateau hone with a fine stone for just a few strokes, 500 grit as I recall.

However you can worry too much about it all. You're never going to know if a slightly different finish might have given better or worse results anyway and I find that once an engine has bedded in it sorts everything out itself in fairly short order. As long as there's a decent crosshatch angle, not too much taper or ovality and the finish isn't godawful rough like some numpty's done the whole job with an 80 grit stone she'll be right mate.

[stevesingo]

Dave,

I was hoping you'd chime in, Thanks.

As far as taper and ovality are concerned, currently, the greatest difference between top and bottom is 0.001mm and ovality is 0.003mm as the bores stand having done 20000miles. I am looking to have a hone pattern put in and measure again. If it is within the recommended piston/bore tolerance from JE (0.089-0.14mm) fit new pistons & rings.

Given that the bores are in good order with the original hone pattern still visible, how much can I expect will be removed by potting the hone pattern back in? It has been suggested to me about 0.001" (0.0254mm).

Thanks

Steve

[Pumaracing]

stevesingo said:

Dave,

I was hoping you'd chime in, Thanks.

As far as taper and ovality are concerned, currently, the greatest difference between top and bottom is 0.001mm and ovality is 0.003mm as the bores stand having done 20000miles. I am looking to have a hone pattern put in and measure again. If it is within the recommended piston/bore tolerance from JE (0.089-0.14mm) fit new pistons & rings.

Given that the bores are in good order with the original hone pattern still visible, how much can I expect will be removed by potting the hone pattern back in? It has been suggested to me about 0.001" (0.0254mm).

Thanks
Steve

You have been very badly advised. Just light honing the bores to provide a new crosshatch surface for the new rings to run in against will remove an almost negligible amount of material at the actual surface of the bores if done properly. Maybe a tenth of a thou if that. Most of the crosshatch would be under the surface and not actually altering the basic bore diameter. Anyone who took out another whole thou just cleaning the bores up would be an incompetent nitwit and would basically bugger up the engine by taking it outside its tolerance.

If you are close enough to south bucks to get to me I could do this for you.

[stevesingo]

Dave,

In fairness, the guy who advised me was generally honing compressors where I work, and not an automotive engineer. But I'll tell him

I'm in Cumbria, so a little far. I might have to go to London for some other car related business next week so I might take you up. If not there is a machine shop that comes reccommended http://www.lancasterengines.co.uk/.

I'll let you know.

Many Thanks

Steve

[stevesingo]

Dave,

You have mail, RE Honing.

Steve

[E21_Ross]

OP, if needed, you could try my dads precision engineering firm based in southampton, don't know where you are.

place is called williams & co. if you want, i can PM you their phone number and my dads name (he's the MD there so that's who you'd need to speak to)

Ross.