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Thread: How to turn a piston?
03-07-2006, 08:21 PM #21This was done to make the piston bigger, not smaller. It was like knurling on the skirt at the major diameter of the piston. Hone the cylinder, resize the piston new rings and SELL the car.
03-07-2006, 08:49 PM #22
Actually, knurling used to be really standard stuff, In the days of "overhauls" instead of "rebuilds".I worked for an automotive jobber with a machine shop back in the mid 60's and we had the cam grinder for semi sized pistons and the knurling machine for worn ones and the valve stem knurling tools and used most of the stuff daily. Of course, those were the days when your engine was only expected to make it to maybe 80k miles before it's first major overhaul. Some of the shops I called on used the Hastings oil stopper rings that were so severe they would dry up an oil burner that had the cylinders all whopper-jawed and the pistons worn out. They lasted about 25k miles on the outside, and when you tore the engine down the NEXT time, there was a ring ridge at the top and another at the bottom of the ring travel, and you either had to bore the thing way oversize or give it up for lost. I had one customer who used to have us knurl the pistons heavily, then he'd drive them in with a wooden section of fence post and crank each one around and around 'till it loosened up somewhat. He always seemed to get by with it, but then EVERYBODY gets away with that stuff except me....Joe
03-08-2006, 12:39 AM #23
Tom, what Joe was referring to was a totally different process than piston knurling. James
03-08-2006, 07:54 AM #24
This thread is bringing back memories of my friend's automotive machine shop, as I wrote above. That same old automotive machine shop had a piston-knurler as well as a tools for knurling the nsides of worn valve guides. The knurling of piston skirts was originally done to get a little more life out of worn pistons. As the men in that automtoive rebuild shop told me, during the Depression and into WWII, auto parts were hard to come by. During the depression, it was a matter of not having the money to buy them. During WWII, it was a matter of total unavailability of new parts. During WWII, used parts were reconditioned. Junkyard parts from similar engines were sometimes machined and adapted to run if all else failed.
By the 1950's and 60's it was matter of some smaller US car makers going out of business and parts unavailable. The result was the automotive machine shops had their ways to get some more life out of worn-out engine parts. Of course, the engines were fairly low performance with fairly loose tolerances. Tricks like knurling piston skirts to take out piston slap or knurling worn valve guides in place on the engine were routinely done. Consider that many "side valve" or flathead engines were still in use at that time. Working on the valve guides meant doing it in the block, not taking the heads to a bench. If the valve guides were worn, counterboring them and collapsing them to get them out of the block, in the vehcile, could be a real nasty job. Knurling the bores of the valve guides was a quick and dirty way to get some more life out of things.
A few old mechanics told me they routinely used to make their own valve guides for various engines. they used to keep some "Ampco Bronze" stock around for making guides. During WWII, they told me they used to take busted camshafts, which were a fairly hard cast iron, and make the guides of the iron from them. These same guys told me that people wanting to build stock car engines, dragsters and similar routinely came in to have the pistons knurled to hold oil.
As for getting a rebuilt/tight engine ready to go, there was some folk-wisdom around about that when I was a kid in the 1950's. Guys would put together an engine and mix some scouring powder (like "Bon Ami") with the oil. The idea was the scouring powder contained fine pumice. This would polish the running surfaces and fits, but would break down as an abrasive in a short time. Even as a kid, I tended to discount this idea. Later on, I found about "timesaver". This was a fine soft abrasive powder which was mixed into lubricating oil on new engines. Mostly, it was used on big stationary and marine engines and compressors to final-polish the running fits.
With today's engines, I think there is such sophisticated geometry and such close tolerance fits on parts that very little can be made up in a local shop. My buddy and I were repairing a 1975 BMW R 90/S motorcycle with a holed piston (pre-ignition damage). The bore was soemthing on the order of 3 5/8". BMW's spec for piston side clearance was only 0.0014". That works out to 0.00038"/inch of diameter. Piston weights had to be within ten grams of each other. Bear in mind that that is a piston for an aircooled motorcycle engine that is thrity years old. Tolerances have not gotten any looser in the thirty years since the motorcycle was built. The higher performance engines of today will have even tighter tolerances and more complex geometry to compensate for dimensionsal changes due to loads and thermal expansion. For this reason, I think turning or grinding the pistons down to size for a modern light truck engine is not a good idea.
03-08-2006, 09:41 AM #25
I had to square up the ring grooves in some Model A Ford pistons; and, like you I didn't want to risk damage by pressing out the wrist pins.
I substituted a piece of flat stock for the bearing cap. It was center-drilled for the tailstock center.
To hold the piston, I took a 1-1/2" thick piece of aluminum plate and chucked it in the 4-jaw. I machined a 1/8" deep recess in it slightly larger than the diameter of the piston crown.
After this recess was machined, the surface of the aluminum was "true," so I didn't disturb it before the next step.
Then, I Super-Glued the crown of the piston to the aluminum plate.
It was a simple matter to center the piston with the 4-jaw. The adjustments in the jaws should be made in tiny increments. If not carefully adjusted, the axis of the piston might get knocked off true.
After I was done machining the grooves, I took the combined piston/aluminum and set it upright on the heating element of an electric stove. About the time the oil on the piston starts smoking, I'd grab the rod and knock off the aluminum plate with a block of wood.
The remains of the glue will need to be removed before re-using the aluminum. Acetone works well.
Before gluing the next piston to the aluminum block, take a skim cut to true it up, first.
More and more I use this same work-holding technique whenever I need an extra bit of accuracy or the work-piece doesn't lend itself to being chucked or clamped.
So far, the glued piece has never come loose. Of course, common sense dictates that heavy cuts should be avoided.
03-08-2006, 11:01 AM #26
Chances are, his piston will have two different
measurements across the skirt, down near the
pin. The larger one will be the nominal one.
It will be eliptical near the skirt.
But it will be round up near the rings and
crown. The number he reads up there will be
substantially less than the nominal value.
Joe M - say it's not so, did Ron's R90S
hole a piston?
If so he's found out about the big problem with
those bikes. Fuel today simply isn't high
enough octane. He needs to do one of two things
to prevent this from happening again: 1) dual
plug the motor, or 2) install thicker base
gaskets to reduce the compression. I went with
the second approach, the thicker gaskets are
available stock from Capitol Cycle or Bob's
BMW. It's a simple inexpensive fix that
retains the stock ignition components which are
quite reliable IMO.
If nothing is else besides replacing the
piston is done to the motor, the trouble *will*
Inspect both pistons carefully, what you are
looking for is a clean, sandblasted apperarance
on the crown with a complete absence of any
carbon on the crown. When it's bad you will
see what look like small termite holes in the
crown, in the clean areas. That this implies
is that pre-ignition and detonation are blowing
the gas boundary layer apart at the metal
surface and piston damage is starting to happen.
You cannot fix those motors using: stronger
advance springs, retarded static timing, richer
main jets, lead additives, etc. even though
folks will claim any and all of those.
Even if the piston did not come apart, inspect
it very closely. One of my bikes had cracks
on the underside of the pin boss once I cleaned
it up and got a good look at it. Those heads
with the two large valves and the plug way off
to one side really need 100 octane fuel. Before
I reduced the compression in mine, it would break
up and run ragged at WOT on a hot dry day. But
there was a station that sold CAM2 fuel near the
taconic back then, a half tank of that would
smooth it right out. [img]smile.gif[/img]
03-08-2006, 01:32 PM #27
I'm going to disagree with Jim about the thicker base gasket to reduce compression, at least for many engines.
The air-head BMWs I've seen have generally had open combustion chambers, and twin plugging does them more good than it does on a Guzzi which has a fair amount of squish area. If the chamber is completely open with no squish, then adding the thicker base gasket can work.
If you have an engine with working squish, adding a thicker base gasket just widens the squish clearance and makes it less effective (if effective at all).
If you want to decrease the compression on a head with squish you need to increase the combustion chamber volume either by grinding out the chamber (leaving the squish areas alone), welding all over the gasket surface to make the head thicker and the chamber deeper (retaining the squish and potentially needing some camchain mods on an OHC head), or go to a lower dome piston (but taking the metal out of the center so you don't change things in the squish area). Oh yeah, you can also go to a shorter stroke crank which will reduce compression due to the reduced cylinder volume.
Guzzi used the basic 850 head castings on the 950/1000 engines which gives them lots of squish area due to the increased bore size. I think it may be possible on a Beemer to use a smaller engine's head to do the same thing (and IIRC it was a common tuning trick to use a 350 Velo head on the 500s to get the small chamber and squish area).
If you get squish working you can run decent compression and be much less likely to have detonation problems. Generally, .040" is considered the maximium clearance for any squish effect, and as you tighten things up you've got to look at crank flex, rod stretch, cylinder growth, RPM, etc etc as you want to avoid having metal/metal contact between the piston and head.
Twin plugs will generally want less spark advance - 4-6 degrees less is a good place to start.
03-08-2006, 01:39 PM #28
I personally have never seen the bmw heads
tightened up to give any squish band. I do
know the thicker base gasket does reduce
compression and will eliminate octane deficit
detonation and preignition.
The benefits of dual plugging are indisputable.
It gives better performance, allows less advance,
the motors run cooler and make somewhat more
power. All on lower octane fuel.
However this is not without penalty, the main
one being added complexity in the ignition.
Also any failure of one system with the other
set of plugs still running will rapidly produce
burned valves or seats as the advance will be
too small for single plug operation.
03-08-2006, 01:47 PM #29
My experience with 356 Porsches mirrors yours. Engines will detonate at 9.25:1 with a .06" clearance in the quench area (small on these engines), but will tolerate over 9.5:1 at .025" or so.
The reduction in spark lead for a twin-plug installation also approximates what you found.
BTW, does anyone remember 'ridge-dodger' rings? They had a reduced diameter at the top so you didn't have to ream the ridge off the cylinders.
(edit: .06, not .60)
03-08-2006, 01:56 PM #30
It ain't so. Ron's R 90/S did not hole a piston. Another R 90/S which came along unexpectedly did, however. Last fall, I saw an ad in the Woodstock paper listing two (2) BMW bikes, a 1975 R 90/S and a 1970 R 60/5 at ridiculously low prices. I bought them both. Both bikes had a very checkered history with many, many repairs done by previous owners using BMW motorcycle delaers. A mutual friend of Ron's and mine wanted that /S so we sold it to him. Ron wanted to check the engine over and service it, but the guy was hell-bent on riding that /S. Inside of two weeks, he limped it back to Ron running on one cylinder. Ron pulled the engine down and called me. The LH piston was holed through with a hole in the crown you can put your index finger thru. No stock R 90/S pistons in the "A" cylinder bore range are available as new. I believe the R 90/S was set up for 9.5:1 or 10:1 compression. No R 90/6 pistons in the "A" cylinder bore range, which would have given about 9.0 :1 compression and maybe have solved the high compression problem.
We shopped high and low for pistons. The best we could find were a set of used R 90/S pistons for "A" range clyinder bores. The guy who had them in an R 90/S changed out to Venolia pistons since he was racing, so these are low-time pistons. Ron put one of those used pistons in on he LH cylinder with new rings. He put new ring son the RH piston that had been runnign in that engine, and used it again. Both pistons were within a gram of each other in weight when he checked them. Also went for lightened wrist pins.
We took a look at the holed piston and even had Oak Okleshen from Airheads BMW Club look at it. Oak said it looked like a case of preignition. You can see a kind of "termite hole" pitting on the piston crown that forms a widening streak that gets deeper and heavier and finally blows through. I think Ron held onto that piston.
I miked the cylinders two ways at top, midpoint & bottom. Cylinder bores were within a couple of tenths top to bottom, despite perhaps 60K miles on the engine.
The little R 60/5 was represented by the owner as having tranny problems, so he let it go really cheap. It turned out the rivets holding the spline cup to the rear hub had sheared. I am repairing the rear wheel in my shop, line reaming the holes & making fitted stainless bolts. Otherwise, it's a nice, clean little bike.
03-08-2006, 03:05 PM #31
OK, everybody seems to have their own idea of how to turn pistons, but how many have actually tried to do it.
Actually turned a couple of pistons for a NITRO HARLEY drag bike, used this real exotic alloy that was meant to be the ducks nuts.
These pistons were turned to the specs I was given and then fitted for a few trial runs.
Now here's the killer, whatever material, YOU, can buy over the counter, is the WRONG SH*T for pistons. Pistons actually need to be made from CAST, then worked on. Cast alloy is a whole lot stronger than extruded alloy and in the melting/casting proccess you can add all sorts of exotic materials to make it even stronger.
The pair of pistons that I made, just went ALL out of shape, from the heat and all other forces that are involved in the combustion proccess. OK, it was 'nitro' that was hittin' on 'em.
The wrist pin hole went all egg shaped and the crown even went and half melted and finished with a dip in it, a real mess to look at, hours of work turned to scrap.
So a word of caution, with whatever material, you decide to use.
03-08-2006, 04:03 PM #32
I always thought that piston blanks were forged, not cast? I saw a CNC lathe made by Okuma that was being used at a Nascar race shop to turn pistons. They had a custom expanding chuck that would grip the piston from the inside, this is after the inside of the piston blank was machined on a different lathe. Then the turret was on its own axis so that it could move to create the elliptical shape. I was told that the turret could keep in time only up to about 1350 rpm. Faster than that and the turret couldnt move fast enough to keep timed to the spindle.
They also had a dedicated milling machine to cut the valve reliefs in the top of the piston. All eight were put into a tombstone and a macro was used to custom cut each one individually for each cylinder.
03-08-2006, 04:14 PM #33
If I understand what you are saying it seems that you would be unable to machine the top 1/8" of the piston. How did you/would you do that? Also, what detrimental effects, if any, do you think occurred from not making the pistons elliptical?
03-08-2006, 05:57 PM #34
Charles, I think the high-silicon alloys (which have less expansion) are typically cast instead of forged.
Jim, if the Beemer heads have completely open hemispherical chambers(which I believe some of them do, and a cutaway drawing of an R100 appears to support that) there's no squish area to tighten up. Adding weld into the chamber at the sides is one way you can get some squish.
Twin plugging gives less benefit if there is working squish in the top end. Given my druthers I'd rather modify a head for squish and then put in a pretty flat crown piston. That both gets the squish benefits, reduces surface area to suck up combustion heat, makes for a lighter piston, and lowering the dome usually helps air flow and flame propagation (the latter is improved by the turbulence from the squish).
So if you've got an open hemi chamber with the bottom of the chamber the same diameter as the cylinder, add all the base gaskets you want to lower compression as there is no squish present (unless you do like Triumph did on some of the race engines and use a really domed piston that has the sides of the dome closely matching the combustion chamber bowl, in which case you can get some squish, but it doesn't do the rest of the power making aspects of the head much good).
Twin plugging definitely helps, and I've done it to Guzzis and sold ignitions to both BMW and Guzzi owners for use with twin-plug heads. But if you've got a head with good squish at work, twin plugs don't give as much benefit as they do on the heads with a poorer design.
03-08-2006, 06:07 PM #35
Well I'm glad it wasn't Ron's bike that
grenaded. He would have been heartbroken.
I'm really really really suprised that the
600cc /5 bike could shear those rivets on
the spline coupling on the hub. Those are the
same couplings and rivets that they use on the
1000 cc bikes! You can still get those rivets
Joe I do have a set of engine cases from an
R60/5 that I had hot-tanked and bead blasted,
from a motor that siezed a crank. At this
point they're just about in the way so contact
me via Ron or Glen Stubbs if you are interested
03-08-2006, 07:54 PM #36
Pistons in the recent past were "cam & barrel turned", as rods have got longer & ring stacks have got shorter I think that the "barrel" aspect has become redundant (or unmeasurable)
The reason, in my experience, for pistons being machined as they currently are (and minutely but constantly evolving) are twofold, the dissproportionate mass of metal across the piston pin line requires compensatory machining to arrive at a lump of metal that will be something close to round at working temperatures. Here hypereutectic casting methods have greatly reduced bore clearances over the very best that could be achieved with machined forgings.
Next in terms of buggeration factor is the four cylinder head clamping bolts that normally surround the cylinder bore, to the extent that several manufacturers have tried a fifth bolt (to the detriment of exhaust port shape) to minimise the "squaring" effect that the bolts impart to the cylinder.
Piston design is an enigma to most of us, including a good number of piston manufacturers, but then "cnc'd billet forgings" sell very well (and profitably) to the unknowing majority.
To 300TD4x4's question regarding turning down some oversize pistons with the rods installed, I don't know the engine in question, but something to be wary of is that many pistons, particularly in older American designs, had the pin bore biased towards the major thrust face, making life somewhat difficult in terms of balance, should one try to machine the piston & rod as an assembly. As mentioned earlier just bore the block to suit the pistons ideally, if you do insist on "turning" down the pistons at least with a forced induction engine one can get away with larger clearances, the more constant ring land sealing seems to damp out piston rock somewhat.
Joe....I believe that the piston machining equipment was indeed "TobinArp", I had heard of this equipment many years ago & was much intrigued, on a visit to Ohio an old automotive business colleague took me out to a machine shop near Akron to satisfy my curiosity, alass the machine shop owner had succumbed to influenza that very morning & was unavailable to demonstrate.
03-08-2006, 09:24 PM #37
03-09-2006, 12:09 PM #38
If I understand what you are saying it seems that you would be unable to machine the top 1/8" of the piston. How did you/would you do that? Also, what detrimental effects, if any, do you think occurred from not making the pistons elliptical?"
If a person wanted to reduce the diameter of the piston all the way up to the crown, there's no harm done in machining away the aluminum block that the piston is glued to. The shallow recess was only there as a centering aid. It is no longer needed once the piston is securely fastened to the aluminum block.
In my experience, the Model A Ford pistons I cleaned up were circular in the ring area, not eliptical. After I had them centered in the lathe my test indicator barely wiggled as I rotated the pistons a full turn. It would have been all over the place if the pistons were eliptical.
03-09-2006, 03:08 PM #39
Got it. Thanks. However, I think I'm going to abandon this idea because I found a good (won't know for sure until they arrive, of course) used set for $40.
03-09-2006, 08:23 PM #40
About knuling valve guides...
In my Diesel Engine Rebuilding class we covered knurling valve guides in great detail, and went through the process on at least three of the engines we were overhauling for customers. Our instructor explained that it was pretty common practice in the industry. Really straightforward, I'd say it took less than 5 minutes per valve guides. Popping out the old guides and pressing in new ones took considerably longer.
Of course, this was back in the fall of 2005, so times may have changed.