Wrist pin offset question

[satellite65]

Just received my SpeedPro L2355's from Summit. The pistons have 4 valve reliefs, all symmetrical and arrows on the top to indicate (I'm assuming) the front of the engine. The pins are offset quite a bit from center but all 8 pistons seem to have the offset facing in the same direction. If I were to install these with all the arrows facing forward the offset would be opposite from one bank to the other. Which way is the offset supposed to face? Also, the pistons they are replacing are TRW 2388's, low compression flat tops w/o valve reliefs. These also have arrows but when I origanally built the engine I was told that a good, cheap old school trick was to hang them on the rods backwards to lessen the side loading friction on the skirts so thats what I did. I had no real issues with the engine (other than 7.9:1 CR) and noticed no scuffing on either the skirts or cylinder walls when I disassembled it.

All thoughts and advice will be greatly appreciated.

Thanks in advance,

Steve

[satellite65]

bump

[68_Val_Sedan]

install them so one set points front and one set points back ... the offset will change the installed hight of the piston.  install them  so that you wind up with the installed hight closest to what you want for static compression

[bOb shingler]

install them so one set points front and one set points back ... the offset will change the installed hight of the piston.  install them  so that you wind up with the installed hight closest to what you want for static compression

respectfully i believe this is wrong. all the notches, arrows etc. face forward. this has nothing what so ever to do with installed heigth or compression ratio. they are offset to reduce piston slap on cold start up by introducing fiction on the cylinder wall. by reversing them to face the notch, arrow etc. to the rear it reduces the friction causing horse power to increase slightly along with slightly more noise at cold start up. 

[william]

install them so one set points front and one set points back ... the offset will change the installed hight of the piston.  install them  so that you wind up with the installed hight closest to what you want for static compression

respectfully i believe this is wrong. all the notches, arrows etc. face forward. this has nothing what so ever to do with installed heigth or compression ratio. they are offset to reduce piston slap on cold start up by introducing fiction on the cylinder wall. by reversing them to face the notch, arrow etc. to the rear it reduces the friction causing horse power to increase slightly along with slightly more noise at cold start up. 

[Mopar451]

Bob has it right   notches face forward

[68_Val_Sedan]

So your are saying they are all offset the same way because of the direction of the crank rotation ?   So on the drivers side the offset goes towards the out side of the motor and on the pass side the offset is towards the lifter valley ?   I still would think that offsets to different side would affect the final height of the piston in the bore.

[satellite65]

Thanks for the replys. Like I said in my first post, The original pistons were installed backward and I noticed no ill effects. I didn't even notice any unusual noises on cold starts and I used this car a lot in the winter. Just had to let it warm up good before it would run well. Just wondering if the small (if any) power gains are worth the possibility of increased wear.

All the parts are going to the machine shop next week to have the pistons pressed on and rotating assembly balanced. I'll get his opinion too but I would like to hear everyones thoughts on this.

[Steve DeTar]

I have done this before....you are not going to see a 30hp increase in doing it. 

[satellite65]

Well to be honest, I haven't had the opportunity to try it both ways in the same engine to actually experience the difference. Steve, in your opinion, how much is it actually worth in terms of performance and how much reliability do you trade off? Is it worth it?

[oldkimmer]

I have done this before....you are not going to see a 30hp increase in doing it. 

....................On a std bore 10.5-1 factory piston 1970  340  which u have to swap pistons from bank to bank when u reverse the piston on the rod so the notch is to the rear of the cyl  we gained 25 hp and 20 ft lbs torque......kim...........

[satellite65]

That much?

[Dodgeboy]

Wow, that is surprising! I have never seen this done before, my best guess would have been maybe 5 HP. However, I would still install the pistons with the arrows forward. I hate noisey engines and the purpose of the offset pins is to reduce piston slap. And by the way the pin offset direction is determined by the direction of crank rotation, not which bank they are installed in.

Roger

[satellite65]

So your are saying they are all offset the same way because of the direction of the crank rotation ?   So on the drivers side the offset goes towards the out side of the motor and on the pass side the offset is towards the lifter valley ?   I still would think that offsets to different side would affect the final height of the piston in the bore.

Chad,
I didn't see this post of yours yesterday. Gotta admit, I was thinking along the same lines as you. I thought that the offset would be opposite on each bank but after sketching a couple of doodles I can see that they're right, it has to do with crankshaft rotation. Also, I'm not sure it affects the height of the piston in the bore. At TDC, the rod would be at the same angle either way, just in opposite directions. Would be pretty cool if it did though huh? Two different CR's from the same piston.

Still interested to find out how much power this mod is really worth if anything, and at what part of the powerband the benefit would be seen. And also if there are any bad long term side effects that I may not have noticed because my engine only had 15,000 miles on it.

[jyrki]

It's a trick forbidden in NHRA Stock rules, so it must be worth something. I have doen both ways, and really didn't hear much difference in hte noice, couldntä say of two running engines which one has got the pistons "right", and which one "wrong". In my opinion, it really isn't worth that much powerwise either, not a trick that makes or breaks a good engine. Most aftermarket forged psitons have the pins on center. The pin position doesn't affect the compression height or things like that, and the offset has to be on the same side on both banks. That's because, if you have the offset on the right on the other bank and on the left on the other, the difference of the TDC positions of ecah banks isn't 90 degrees any more. Of course it's a good idea to check the TDC positions against the dampermark in any case, but especially when reversing the pistons, because that will change it.   

[chryco]

I did that a number of years ago on a mild 70' 340 , with no ill effects . Go for it if you want to.
   Chryco

[satellite65]

It's a trick forbidden in NHRA Stock rules, so it must be worth something. I have doen both ways, and really didn't hear much difference in hte noice, couldntä say of two running engines which one has got the pistons "right", and which one "wrong". In my opinion, it really isn't worth that much powerwise either, not a trick that makes or breaks a good engine. Most aftermarket forged psitons have the pins on center. The pin position doesn't affect the compression height or things like that, and the offset has to be on the same side on both banks. That's because, if you have the offset on the right on the other bank and on the left on the other, the difference of the TDC positions of ecah banks isn't 90 degrees any more. Of course it's a good idea to check the TDC positions against the dampermark in any case, but especially when reversing the pistons, because that will change it.   

I'm going to be doing a little measuring for my own knowledge. Before I have my old pistons pressed off the rods and new ones pressed on I'm going to check actual piston TDC against the mark on the damper with the pistons foreward and reversed. I'm going to use a degree wheel and record my findings. I'll post back with what I find. This should be interesting. Could this possibly be the reason I had to throw 46* of advance at this thing to get it to idle?

[bOb shingler]

It's a trick forbidden in NHRA Stock rules, so it must be worth something. I have doen both ways, and really didn't hear much difference in hte noice, couldntä say of two running engines which one has got the pistons "right", and which one "wrong". In my opinion, it really isn't worth that much powerwise either, not a trick that makes or breaks a good engine. Most aftermarket forged psitons have the pins on center. The pin position doesn't affect the compression height or things like that, and the offset has to be on the same side on both banks. That's because, if you have the offset on the right on the other bank and on the left on the other, the difference of the TDC positions of ecah banks isn't 90 degrees any more. Of course it's a good idea to check the TDC positions against the dampermark in any case, but especially when reversing the pistons, because that will change it.   

I'm going to be doing a little measuring for my own knowledge. Before I have my old pistons pressed off the rods and new ones pressed on I'm going to check actual piston TDC against the mark on the damper with the pistons foreward and reversed. I'm going to use a degree wheel and record my findings. I'll post back with what I find. This should be interesting. Could this possibly be the reason I had to throw 46* of advance at this thing to get it to idle?

         you may just find the top speed secret of the mopar world but i'm a thinking you will not find any difference. 

[Thin White Duke]

If it was such a big thing, it would be common knowledge by now. 

[satellite65]

I'm sure you guys are right but i have to see for myself. For the couple of hours spent farting around with a degree wheel I feel it's worth the effort. I'm not expecting to find anything of a revolutionary nature.

[bOb shingler]

otay

[satellite65]

It's a trick forbidden in NHRA Stock rules, so it must be worth something. I have doen both ways, and really didn't hear much difference in hte noice, couldntä say of two running engines which one has got the pistons "right", and which one "wrong". In my opinion, it really isn't worth that much powerwise either, not a trick that makes or breaks a good engine. Most aftermarket forged psitons have the pins on center. The pin position doesn't affect the compression height or things like that, and the offset has to be on the same side on both banks. That's because, if you have the offset on the right on the other bank and on the left on the other, the difference of the TDC positions of ecah banks isn't 90 degrees any more. Of course it's a good idea to check the TDC positions against the dampermark in any case, but especially when reversing the pistons, because that will change it.   

I'm going to be doing a little measuring for my own knowledge. Before I have my old pistons pressed off the rods and new ones pressed on I'm going to check actual piston TDC against the mark on the damper with the pistons foreward and reversed. I'm going to use a degree wheel and record my findings. I'll post back with what I find. This should be interesting. Could this possibly be the reason I had to throw 46* of advance at this thing to get it to idle?

Well, here's what I found - not much.

Set the piston/rod in the #1 bore correctly (arrow facing forward), put on the timing cover, balancer and degree wheel, put a dial indicator on the piston and rotated to true TDC. Set degree wheel to zero. Mark on balancer does NOT line up with tab on timing cover. Turns out the piston is actually 2 degrees BTDC when the marks line up. With the piston reversed, it's about 3.5 degrees.

Didn't exactly find any hidden speed secrets, nor did I expect to.

[oldkimmer]

..........The reason 4 swapping the pistons from bank to bank in a 70   340 is because the valve notches r only on 1 side of the piston.....a flat top lower compression piston can b reversed in the same hole...........on pressed pins u can swap the piston and rod assambley to the oposite bank ..this trick is 4 factory offset pistons........Dont knock this trick till u try it.....Y do u think some factory super stockers ran deep under the index even back in the day..........kim.............

[chryco]

If it was such a big thing, it would be common knowledge by now. 

I thought it was .

[satellite65]

The reason I did my little TDC check is because I thought that I might see a considerable difference due to the offset being in the opposite direction. I did see a difference, only not as much as I expected. I would imagine that it would show up more with a shorter connecting rod, where the piston doesn't dwell at TDC as long.

This "trick" was brought to my attention a long time ago. I did it (flat tops, no valve reliefs to worry about) and always wondered if it really made any difference. Turns out, my engine had other things wrong with it that hurt my performance, things that required attention. This time around, I'm going with flat tops that have 4 valve reliefs (higher compression distance) and wondered if using that old trick would be worth it.

Like I said in a previous post, upon disassembly I didn't notice any undue wear on the skirts but then again the engine only had about 15K miles on it. I know it can reduce drag but at what cost? Do I run the risk of snapping a skirt off or increased bore wear? If it's only going to offer a 5-10 HP increase I would say it's not worth the risk. It's a street engine, I just want it to throw me back in the seat when I stand on it. If I ever see the track with this car it will be for fun only, not in a competitive environment.

[375InStroke]

I'll say that pin offset has a greater affect on TDC shift with a short stroke engine, not short rod engine.  Compression won't change.  The pin offset from center would shift TDC the same amount around the circumference of the crank pin travel.  That same linear motion on a longer crank throw occurs in a smaller angular motion.  The offset needs to stay on the same side of the crank rotation.  Either leading or trailing.  If not, the angular offset, times two, would be the amount of degrees from TDC the right bank would be different from the left side.

Here's a new question:  If one runs offset pistons with a forged piston, does the does the motor need to be warmed up completely before getting on the gas pedal?

[CHAPPER]

 Confucious say.."some people more handy with computer than with motor"

[satellite65]

Confucious say.."some people more handy with computer than with motor"

What?

[satellite65]

I'll say that pin offset has a greater affect on TDC shift with a short stroke engine, not short rod engine.  Compression won't change.  The pin offset from center would shift TDC the same amount around the circumference of the crank pin travel.  That same linear motion on a longer crank throw occurs in a smaller angular motion.  The offset needs to stay on the same side of the crank rotation.  Either leading or trailing.  If not, the angular offset, times two, would be the amount of degrees from TDC the right bank would be different from the left side.

Here's a new question:  If one runs offset pistons with a forged piston, does the does the motor need to be warmed up completely before getting on the gas pedal?

According to all of my sketches and calculations (and I feel I've wasted enough time worrying about it) the TDC shift will be less with a long rod and less with a short stroke. We're talking about minute incremental differences here, maybe something NASCAR engine builders lose sleep over but not anything that I'm all to concerned with. Any noticable gains would come from reduced friction, not from any TDC shift.
I definitely agree with you that the offset needs to be in the same direction on both banks. That's what got me overthinking all this in the first place. When I took my new pistons out of the box I expected to see 2 sets of 4 with opposite offsets, how wrong I was.
As far as letting the engine warm up? That's always a good idea no matter what pistons you use or how they're installed but I have to tell you, I used to use this car as my daily all-season driver and I'm quite sure there were many cold winter mornings where I threw it in drive as soon as the engine would run on its own without the choke. Never had a problem.

[Vernmotor]

If it was such a big thing, it would be common knowledge by now. 

I thought it was .

It is .. old trick been around forever