Re: pistons

[bearbvd@domain.elided (Greg Hermann)]

>In message , Greg Hermann <bearbvd@domain.elided> writes
>Snipped loads of good stuff
>>
>>The place where there is GREAT room for improvement in performance and
>>durability is in more accurate machining of liners (like with them
>>installed in a block with a torque plate installed and warm water
>>circulating in the water jacket--and in use of sophisticated coatings to
>>both reduce wear and friction and to reduce heat transfer into the pistons.
>>(Among other benefits are tighter allowable piston to sleeve clearances,
>>and still lower leakdown rates. Most Alfa motors have attrociously high
>>leakdown rates!!
>>
>Is this for real?
>People actually put them in the block to do a re-bore like in a mini?
>This is a new one on me, and I thought I'd heard all the ideas about
>engines, from the Alfaesque to the downright sidevalve. Where would you
>attach the torque plate, and how do you keep the steam around the liner,
>or do you just have HOT water?

A torque plate goes on, just like a head, but has holes in it where the
chambers would be so that the boring bar/honing stones can get at the
bores. The idea is that the liners are thus subjected to the same
stress/strain (deflection) as they are when the engine is fully
assembled--thus you get bores in the sleeves which are round and straight
in their as operated condition.

Yes--circulating hot water (about 160 degrees) in the block cavity when
doing the machining helps get closer to duplicating the deflections that
the sleeves see when the engine is assembled and operating. This is
particularly true with an aluminium block, because of aluminiums greater
thermal expansion. Blocks for small block fords are often machined using
such tricks for high performance work--so there ARE machine shops out there
who would understand.

The point of all this is that the sleeves ARE NOT particularly rigid
pieces--they deflect under the stresses imposed when the head is torqued
down, as well as from thermal stresses.

If you doubt this, try assembling a block with sleeves and a properly
torqued head, but without crank, rods, and pistons--then check the bores of
the sleeves for roundness and straightness with a bore gauge inserted
upward form the crankcase.


>Reducing the heat transfer into the pistons pushed me into "this is a
>wind-up" mode so bear with me here guys and gals, I thought that the
>heat transfer was from the pistons, via the liners to the coolant.

It is. Sorry--I was not too clear. A ceramic coating on the crowns of the
pistons will help keep heat in the chambers--thus both improving the
thermodynamic efficiency of the engine AND keeping the pistons much cooler.

I am partial to a teflon impregnated hard anodizing on the rest of the
piston--skirts and ring grooves and pin bores. Such coatings have a very
high surface hardness ( actually a bit harder on the surface than a file),
and, as you might imagine, are also rather slippery, which helps improve
power output through reduction of friction. Less skirt friction, a harder
skirt surface which is much less prone to galling, and a much cooler piston
to begin with all add up to allowing one to set the skirt clearance MUCH
closer than otherwise possible with a forged piston. This closer fit allows
much tighter ring sealing to develop, Less motion of the rings in the
grooves (as the pistons no longer rock in the bores), together with a much
harder, better wearing surface inside the grooves all add up to a much
longer wearing , better sealing piston/ring/sleeve assembly. Of course,
having a better ring seal means less (hot) blowby gas, which further helps
keep things somewhat cooler.
>
>
>>The other problem you will get into with compression ratios above 10 on
>>forged pistons in an Alfa is extremely rapid wear of the top ring groove.
>>The right coating can help this, as can a true synthetic oil.
>>
>Wonder why the ring groove would wear? Anybody know?

2618 Aluminium (the strongest of the piston forging alloys) is a fair bit
softer than the usual piston casting alloys. Also, a higher compression
ratio means that the piston temperature will be higher, particularly at the
top ring groove area. These factors  work both to reduce available
lubrication AND to make the piston material softer still. Add in the fact
that a forged piston normally has to be fitted with substantially more
skirt clearance than a cast one, and that this leads to more motion of the
rings in and out of their grooves. Hence the rapid groove wear.
>
>>Close attention to the design of your ring pack is also VERY important.
>>Personally, I would not take a gift of Deves rings for any engine! My own
>>preference is for a ductile chrome top ring, Total seal second, and a
>>chrome plated iron, H section, oil ring with a coil spring expander.
>>
>>I am also partial to hard surface conversions on the sleeves.
>Such as chrome, or what?

Chrome has been used for this (as in the "chromal" barrels for Porsches),
but my own preference is for "Bore Tech"--which is a proprietary treatment
which leaves some variety of a carbide surface on cast iron bores. The
surface of cylinder bores given this treatment are harder than a lady of
the evening's heart!! And  consequently, they last three to six times
longer. LOTS of motorcycle folks use Bore Tech.
>
>After reading this post, as I rarely do before spouting, I can see that
>it looks like a Gregflame. This is not the case, as I have found some
>new ideas which is, IMHO always a good thing. I also found several
>questions implicit within the original post, because of my ignorance.

No prob--I just sort of threw a few of the thoughts out there without a lot
of detail, to see if there was any interest. Now you have more detail.

But--sheesh--what's gonna happen when I mention that I like to rifle drill
the con rods so as to get pressure oiling  to the small end bushings??  Or
hand lapping the rings to pre-seat them to pre-polished bores???
Meaning--no more break in time or waiting for the rings to "seat" !!! :-)
>
Regards, Greg

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