Engine: why didn't Stude have piston 'slots' symmetrical? Seems backwards...

[bsrosell]

I'm slowly but surely working on my '57 289 V8, and a few weeks ago had discovered the machinist assembled the rods backwards on the right bank side.
Have not had time to get back in and 'un-do' and 're-do' but spent some time looking at it today to be sure I have orientation right for both rod, piston and piston pin screw... (have hypereutectic pistons with NO slot, and arrow for orientation, no 'slot in skirt' to reference).

After studying the shop manual, it appears to me that the original Stude pistons had the slot in the skirt facing the outside of block for one bank, and facing towards the camshaft for the other bank.
One cut-away view of the whole engine shows the slot on Left side of engine facing OUTSIDE, away from camshaft.....
(other text indicates the Right side pistons have the skirt with slot facing towards the camshaft, based on photos and orienting with the connecting rod numbers..). Yet all of the piston-pin nuts are supposed to be on the slotted-side of piston; which means one bank has those pin nuts on outside, the other bank has the piston-pin nuts facing inside. I would think you would want the nuts on the "opposite, symmetrical" sides for balance, and that you'd want the skirt slits the same direction (which was the case for my Model-A and Model-T pistons).

I have not pulled them and disassembled and reversed the rods yet, and I don't have 'slits" in my new pistons, but I plan to assemble the piston pin nuts as manual seems to indicate; outside facing on left bank, inside-facing on right bank..... Odd (assuming I've finally got it right).

And what is different with the newer pistons that they have an arrow for which side faces front? They don't specify which SIDE of the engine to install them in, so seems they should be identical except for the arrow. Is that just an assembly aid for the mechanic after he has rods and such correctly assembled (which ARE different side to side?). In that case, maybe I don't need to disassemble my pistons, simply spin 180 deg to get the rods oriented correctly, and let the arrows face backwards; less monkeying around with the now-oily assembly, less risk of debris. Otherwise not afraid of a little work....
Am I right? And if so, anyone explain the slits in the skirts and why they wouldn't be installed originally 'symmetrically", slots facing out, or in, on both sides??
Thanks!

[PackardV8]

maybe I don't need to disassemble my pistons, simply spin 180 deg to get the rods oriented correctly, and let the arrows face backwards;

Short answer, no, you shouldn't. Since an engine rotates, there is a thrust side of the cylinder; it's on the outside of one bank and the inside of the other. Thus, the orientation of the slots on the OEM Stude pistons.

Your hypereutectic pistons most likely have the pin bore offset from the center line because that lessens cold piston slap on startup. The arrow is to orient them so the offset will be in the correct orientation to the thrust side.

jack vines

[bsrosell]

Short answer, no, you shouldn't. Since an engine rotates, there is a thrust side of the cylinder; it's on the outside of one bank and the inside of the other. Thus, the orientation of the slots on the OEM Stude pistons.

Your hypereutectic pistons most likely have the pin bore offset from the center line because that lessens cold piston slap on startup. The arrow is to orient them so the offset will be in the correct orientation to the thrust side.

jack vines

Sorry Jack, I'm not quite understanding your statement "no, you shouldn't'. Do you mean "no, don't remove and rotate the pistons on the rods', or "no, don't leave them as-is, take pistons off and rotate, before I reinstall (and rotate) entire assembly of piston & rod?"

I can't believe the pistons, even though marked with a "forward" arrow, can be any different which side (and therefore direction) they are mounted, unless they were also marked for "left bank' and "right bank"; otherwise, they have to be symmetrical to be used in any of the 8 cylinders.... right?? (and there are no other marks on them, just the arrow stamp).

Thanks for your help! (explanation about slits in skirts makes sense...)
Barry

[doofus]

After all is said and done you still must be absolutely sure pinch bolts are seated in wrist pin notch. i would check all 8 pistons now,before you go further. Id like packard V8's procedure doing this. Luck Doofus

[bsrosell]

After all is said and done you still must be absolutely sure pinch bolts are seated in wrist pin notch. i would check all 8 pistons now,before you go further. Id like packard V8's procedure doing this. Luck Doofus

thanks for the reply; I've never had Stude rods apart (remember, I sent block up to machinist complete; he re-bored and installed the new pistons on the rods (sent back loose, as I asked); he simply had the arrows pointing opposite the way the rods need to be assembled in the block (and I followed the arrows, not the rods, until I noticed BEFORE TORQUING thank goodness, that the rods were backwards.

That said, he has the pinch-bolt for each piston pin in the 'correct position' per the manual, in that when I turn the rods around, the pinch-bolt nut will be facing 'out' towards block on left bank, and facing in towards cam on right bank.

I may take one apart to see how they work, I'm curious, and if as I can see from cross-sections in Shop Manual, should be pretty obvious to an experience machinist to align tapers.... The rods being 'turned 180deg" is a lot harder to keep straight without the 'skirts/slots' to reference as the Shop manual does.... So I can forgive him that.

And THAT is what I'm trying to figure out; do the stamped arrows in the Silv-O-Lite hypereutectic piston domes really make any difference whether they point front or back, since all eight pistons are identical and can't (unless I'm really missing a simple concept) be skewed in how they are made?
If they WERE, they'd have to tell you which side of the engine to mount them in too, not just which direction front/back....... Right?

In which case I'd be better off just rotating the piston and rod to correct orientation, never even take them out of the cylinders, and torqueing them down.... (with arrows on right bank pointing 'back'....).

I've researched on internet, and surprisingly not much on the arrow direction, only comes up in motorcycle forums, where it has to do with how it is oriented vs exhaust valve ports and such for 2 cycle engines.....
Jack appears to know, I just didn't understand what he was saying "no, don't do that" to..... :-)

thanks again; I'll check a pinch bolt and make sure all the rest are the same extension (and therefore would be seated properly , same depth, etc...)

[Dan Timberlake]

"do the stamped arrows in the Silv-O-Lite hypereutectic piston domes really make any difference whether they point front or back, since all eight pistons are identical and can't (unless I'm really missing a simple concept) be skewed in how they are made?"

Disregarding split skirts, which are also a quieting feature, and must be installed on the non-thrust side of the engine, pin offset is common, and important.

I'd use a vernier/dial/digital caliper etc to measure from pin to skirt. If there is pin offset ( for quieter running ) it is likely about 1/8" measured this way. (I/16" off center)

http://images.thesamba.com/vw/gallery/pix/833800.jpg

A bunch of years ago a friend had a reputable shop assemble the press-fit rods and slightly used stock type pistons for his big block Chevy V8. Upon start-up There was a slight knock or rap that concerned him.
After trying a bunch of external diagnostics, he suspected maybe some wrist pins were too loose. He took the engine apart with the intention of measuring all kinds of things. The symmetrical looking flat piston tops had identify dimple marks, not on the front or rear, but toward one side. The shop had assembled the rods and pistons so the marks on the piston tops were all toward the center of the V engine. This resulted in the pistons on one bank being in backwards, so the pin offset was toward the noisy side, not the quiet side. When engine was assembled with the 4 backwards pistons installed on the rods, and in the engine, correctly, the noise was gone.

[PackardV8]

Yes, the arrows must point to the front of the engine. As previously stated the wrist pin bores are offset in most OEM and cast replacement pistons. The arrows make sure you can tell at a glance you've got them in correctly.

Yes, the rods must be assembled and installed per the directions in the shop manual.

No, don't even think of trying to cheat on either.

jack vines

[bsrosell]

Yes, the arrows must point to the front of the engine. As previously stated the wrist pin bores are offset in most OEM and cast replacement pistons. The arrows make sure you can tell at a glance you've got them in correctly.

Yes, the rods must be assembled and installed per the directions in the shop manual.

No, don't even think of trying to cheat on either.

jack vines

OK Jack, I'll pull 'em and rotate them then; the key must be the rotational direction of the crank then, as that is the ONLY thing they'll have in common between right and left side banks.... but then that is what you were trying to explain earlier, I just wasn't getting it. :-)
Thanks for all the replies. Side benefit is I can check the taper pins (HAVE to...) without being tempted to just assume he got them right. :-)

(THAT still puzzles me a bit, since rods have to be different, why wouldn't they have designed so the piston pin clinch-nuts were either "both facing in" or both facing 'out', instead of all facing to the Left? oh well, not a lot of mass there I guess.... (or, same argument, in the direction of the momentum of rotation maybe.....)

Thanks again! I sure do appreciate the advice, and explanations. I like to know WHY I'm doing something. (Dad never appreciated that fact for some reason; all I'd get was 'because I told you so!!!!" on the farm. :-)
Barry
PS: the rods won't even FIT if wrong; I knew they were wrong as soon as I started to tighten the rod bearing nuts and they were off-center in the piston, so absolutely no question about THAT getting corrected; that's what started this whole conversation...... the piston symmetry is what had my puzzled.
That's why I do as much as I can myself even though not an experience mechanic; I CAN see someone just continuing to torque those rods as they are now, and force them into place, and ruining the engine almost immediately upon startup..... At least I STOP when I notice something doesn't seem right or the Shop Manual has me confused. The slow way, but I'm only doing it once, and keeping the car 'forever'. I sure hope at least one of my kids appreciates it someday!

[Dan Timberlake]

"why wouldn't they have designed so the piston pin clinch-nuts were either "both facing in" "

Look at how the rod bearings are offset from one edge of the rod. That is to make room for a nice radius on the rod journal fillet.
That would mean there would need to be dedicated left hand and right hand rods.

I'm sorry Mr Brosell, we have plenty of the left hand con rods, but the RH con rods are no longer available.

I think manufacturers prefer to make 2X as many of one part, and certainly prefer to deal with stocking one part number instead of 2.