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Sat Jul 26, 2008 5:04 pm

What's the reasoning behind the arrow pointing toward the flywheel?
Just curious.

-M

Sat Jul 26, 2008 5:23 pm

To orient the piston to the rod,re:wrist pin offset.

Sat Jul 26, 2008 5:30 pm

Points toward the flywheel.

Sat Jul 26, 2008 5:45 pm

Read his question...he knew that.

Sat Jul 26, 2008 5:57 pm

It's because the piston pin is offset from the centerline of the piston, due to the crankshaft being off center. They are offset to couteract piston slap that is caused by the piston rocking in it's bore when it goes from TDC on it's way to BDC. I used to know where an illustration was, but I can't find it now.

Sun Jul 27, 2008 12:43 am

The Noof wrote: Read his question...he knew that.

d'oh #-o

Sun Jul 27, 2008 6:59 am

Works for me...thanks.
-M

Tue Aug 12, 2008 11:16 am

I just got my 85.5 "B"s back from Rimco. Had them dished, which erased the arrows (doh!).

I don't want to call Rimco and sound like an idiot, so I thought I'd ask you kind, gentle folks:

How do you tell which side to point toward the flywheel if the arrows have been dished out?

I knew you'd understand...

Mike

Wed Aug 13, 2008 10:44 pm

For lack of response, I did some measurements.

I have both standard 85.5 "A" pistons, and a set of 85.5 "B" pistons. Assume the flywheel is "to the right" when facing the 1/2 cylinder head. Assume that you are looking at these pistons while they are laying down (ready to roll across the workbench), straight at the top of the piston, with the arrow "to the right". Thusly, the wrist pin would be perfectly horizontal.

I measured the distance from the edge of the wrist pin hole to the "bottom" arc (where the piston touches the workbench), and to the "top".

"A" Piston wrist pin hole to:
Top=1.202" / 30.5308mm
Bottom=1.316" / 33.4264mm

Which means the "bottom" has more leverage during combustion, and will get "pushed" with more force, cocking the piston slightly.

"B" Piston wrist pin hole to:
BOTH top and bottom=1.257" / 31.9278mm

So what's with the "B" piston not having an offset?

AND, answer me this. We mount the "A" 1/2 arrow "to the right", meaning the "bottom" will get pushed harder during combustion. BUT we mount the 3/4 arrow "to the left", meaning the "top" will get pushed harder. What's that all about?

Mike

Thu Aug 14, 2008 4:20 am

It's not a top/bottom thing.. it is a dynamics thing. Think of it in terms of the rod angle with respect to the crank. Remember that the crank on the 1/2 side is heading down at combustion, and on the 3/4 side is heading up at combustion. The offset changes the way the rod is loaded under strain, as well as the net effect of the piston side loads.

Other pistons don't take that into account.

Thu Aug 14, 2008 4:26 am

visket wrote: AND, answer me this. We mount the "A" 1/2 arrow "to the right", meaning the "bottom" will get pushed harder during combustion. BUT we mount the 3/4 arrow "to the left", meaning the "top" will get pushed harder. What's that all about?

Think about it: It's all about where the throw of the crank is when the power stroke occurs. On the 1/2 side, the crank pin will be at about the 4 'o clock position, whereas the 3/4 side will be at around the 10 'o clock spot (looking fwd on the engine from the pulley). Thus, the 1/2 pistons must offset the small end of the rod more to the top of the bore, to avoid slap. Same goes for the other side, but in reverse due to the position of the crank pin- rod small end positioned downwards, because the crank is moving in an upwards clockwise direction.

If the pistons weren't offset, the piston pushing on the crank pin would tend to push itself to the opposite side of the bore. This is pistons slap. Not all engines have offset pistons, though. If I'm not mistaken, the Fiat 500 inline twin I had in my Bianchina had the entire crank centerline slightly offset from the bores, which achieved the same result. Pistons had no offset in that engine.

Thu Aug 14, 2008 9:59 am

Wow - good explanations. I guess I never gave it that much thought before. Sometimes it's hard enough to remember to put the arrow

So, the 85.5 "B" pistons I'm putting in the 1883 don't have an offset. Yet, before I had Rimco dish them, they did have an arrow on them.

From observation, I can see (and now have measured at least the offset..) no symetrical differences on the piston - nothing that I can see that would hint at "this thing toward the flywheel". On my old set of "A" pistons, each piston has a "nub" on the bottom side that indicates "front".

Do you suppose the arrows on the "B"s were just there to eliminate confusion, even though they *could* be mounted either way?

Mike

Thu Aug 14, 2008 6:32 pm

Put it together and if it makes a slap, swap some pistons around. :D That's how I adjust bucket style (non-hydro) tappets. Just listen to them through a metal rod, and re-shim the ones that make noise. Works every time.

Thu Aug 14, 2008 9:45 pm

A man after my own heart...

"Raise the CR until the heads wind up down the street, then back it off a hair."

They are COFAP, KY1143, 211 198 069. The skirts are 85.47mm and the crowns 84.65mm.

A friend of mine thought perhaps they were lifted from another application. Anybody know what kind of pistons the aircraft VW folks use?

Mike

Thu Aug 14, 2008 10:00 pm

Here's a picture of the bottom of the B.

http://home.ca.rr.com/savethebays/Temp/Cofap_B.JPG

Fri Aug 15, 2008 12:37 am

visket wrote: A man after my own heart...

"Raise the CR until the heads wind up down the street, then back it off a hair."

Hehe, I wouldn't go that far, but a lot of the things I learn are by trial and error. With the tappets, I just got sick of trying to do it with the feeler guages- the way it's 'supposed' to be done. :roll: Lots of wasted time there, that's for sure. Now I shim by feel, listen for the tapping of a loose one, and add shimstock as required by my calibrated ear. :wink:

So, as I said: if you can't figure out which ones are offset (if they are at all), just throw them in, and see what happens. Worst case, they make a bunch of noise, and you pull the heads to figure it out. Good job investigating the offset issue BTW, most people wouldn't have caught that. 8)

Good luck!