Piston pin offset question.

[C Body Bob]

Ok guys the question is about flipping pistons in the bore. Yes there’s a reason for that from a performance standpoint. Yes I’ve done my research & asking here is part of that research because several of you know your way around a hot rod. Yes I’m aware of the benefits & drawbacks of doing this. The engine in question is the motor home 440 I’m putting together for my Drag car FURYIOUS. As you can see in the pic it’s a flat top no valve relief. See the two notches on the top edge. Yes I’ve talked to MOPAR engine builders. On the phone last night with one. Yes I’ve talked to my machinist. It will cost me $100 to have the pistons flipped on the rods. So I’m interested in the opinions of you guys who understand what I’m doing here. Has any of you guys done this & your results where?
As a side note I’ve learned that shops that cater to stock eliminator engines (he knows tricks) is not the same as a shop that works with top sportsman engines (he’s working with race engine parts & knows completely different tricks) or a shop that rebuilds the engine in the wife’s Camry ( he knows of no tricks)

 

[Wollfen]

From what I understand, there isn't even a need to flip the pistons on the rods, simply put the whole assy in upside down, the rod itself is the same shape regardless which way you put it in.

 

[Davea Lux]

I have never tried this with a big clunky truck piston. Changing the offset is a proven way to reduce friction and slightly increase the piston height. Putting a used piston back in the engine in the flipped position is going to move the thrust surface to the opposite side of the piston and might lead to some increased slap, it will depend some on how much the piston and the bore are worn. The truck engine is also a low compression application to start with, so there might not be all that great of a performance boost. You would probably be better served to start with a fresh bore on the block and spec a higher compression piston with the offset that you desire. If you are just experimenting with an engine that is expendable, by all means try it, otherwise start fresh. My 2 cents.

Dave

 

[furious70]

From what I understand, there isn't even a need to flip the pistons on the rods, simply put the whole assy in upside down, the rod itself is the same shape regardless which way you put it in.

I was trying to figure out why there would be a difference myself.
I would also suggest that the max effort tricks done to those types of engines are unlikely to be noticed within the margin of error in the bullet you're assembling, coming from the humble rv.

 

[C Body Bob]

From what I understand, there isn't even a need to flip the pistons on the rods, simply put the whole assy in upside down, the rod itself is the same shape regardless which way you put it in.

No it has a chamfer on one side. That give you the side clearance & oil control. Race engine rods are chamfered on both sides

 

[C Body Bob]

I was trying to figure out why there would be a difference myself.
I would also suggest that the max effort tricks done to those types of engines are unlikely to be noticed within the margin of error in the bullet you're assembling, coming from the humble rv.

No it’s worth doing.

 

[furious70]

I guess I'd like to see them test it on engine masters to see. On a used stock engine.
Might as well get a 4/7 swap cam too. Somewhere along the line a budget bullet stops being one.

 

[C Body Bob]

I guess I'd like to see them test it on engine masters to see. On a used stock engine.
Might as well get a 4/7 swap cam too. Somewhere along the line a budget bullet stops being one.

I’ve seen builds where this was done but it wasn’t the only thing done so it’s hard to say how much it helped. The general consensus is about 15 horse power. Uncle Tony did it to Project Bottle rocket but also did other neat tricks too & he’s nowhere near had the car dialed in yet. Seems like the guys that know about this say it works & the guys who don’t know about this have never heard of it.

 

[furious70]

15hp on a stock rv engine for $100? That would be a no brainier

 

[70bigblockdodge]

It works, I don't know how good, not 3/10ths worth. If you look at the side of the piston it's pretty obvious which side the piston pin is offset to. You want the skinny side toward the drivers side as looking from timing chain end rearward. By move #1 to #2 it will keep the oiling notch in the connecting rod facing the opposite cylinder, even though most new bearings do not have the notch to match the rod, so point is moot.
The notches on the piston should face forward toward timing chain end, stock. They need to face the rear to put the pin on the correct side of centerline to not have push across the centerline to the crank pin. You could just flip them over in the bore they are in, the oil squirt hole does not matter, but the chamfer in the rod is the real reason they have to go to opposite bank. Chamfer needs to be opposite on rod pairs, not facing each other

 

[C Body Bob]

15hp on a stock rv engine for $100? That would be a no brainier

Well mine won’t be stock. But yes even on a Ford Fourm I was reading last night an engine builder FINALLY chimed in & said 15-20 on a 302.

 

[C Body Bob]

It works, I don't know how good, not 3/10ths worth. If you look at the side of the piston it's pretty obvious which side the piston pin is offset to. You want the skinny side toward the drivers side as looking from timing chain end rearward. By move #1 to #2 it will keep the oiling notch in the connecting rod facing the opposite cylinder, even though most new bearings do not have the notch to match the rod, so point is moot.
The notches on the piston should face forward toward timing chain end, stock. They need to face the rear to put the pin on the correct side of centerline to not have push across the centerline to the crank pin. You could just flip them over in the bore they are in, the oil squirt hole does not matter, but the chamfer in the rod is the real reason they have to go to opposite bank. Chamfer needs to be opposite on rod pairs, not facing each other

Yep I measured mine & the offset is .200 thou. I don’t want to swap rods as that would put the piston in a different hole. That was discussed yesterday on the Ford Fourm. I think that’s if the cylinder was bored & piston can be hone fitted.

 

[70bigblockdodge]

Yep I measured mine & the offset is .200 thou. I don’t want to swap rods as that would put the piston in a different hole. That was discussed yesterday on the Ford Fourm. I think that’s if the cylinder was bored & piston can be hone fitted.

Just swap the piston to the opposite bore, the piston is worn enough to provide ample clearance. The new rings will seat to the cylinder, piston just needs to keep them straight enough and transfer pressure to the rod. Turn them and drop them in the other bank.

 

[Big_John]

Yep I measured mine & the offset is .200 thou. I don’t want to swap rods as that would put the piston in a different hole. That was discussed yesterday on the Ford Fourm. I think that’s if the cylinder was bored & piston can be hone fitted.

Measure the pistons and I'll bet there isn't enough difference to worry about.

 

[413]

@Davea Lux The stroke is not changed! The stroke is set by the crankshaft.

What it does is raise the piston higher in the bore at TDC Which is more compression for you.

Yes it is worth doing, but not if you are using thick head gaskets, then it’s all for nothing.

just swap to the other side bank and leave them on the rods, then it’s FREE POWER.

 

[C Body Bob]

@Davea Lux The stroke is not changed! The stroke is set by the crankshaft.

What it does is raise the piston higher in the bore at TDC Which is more compression for you.

Yes it is worth doing, but not if you are using thick head gaskets, then it’s all for nothing.

just swap to the other side bank and leave them on the rods, then it’s FREE POWER.

I left the part out about the compression increase on purpose. I did that to see if anyone would pick up on that. So hone the cylinders & new rings then swap left bank to right & vise Versa ? Turning them around as I go ? Is that the correct recipe. As for a head gasket the thinnest one I can find is a .027 compressed MLS Cometic for $160 a pair. I have 72 cc heads. 2.14 intake valve

 

[C Body Bob]

Just swap the piston to the opposite bore, the piston is worn enough to provide ample clearance. The new rings will seat to the cylinder, piston just needs to keep them straight enough and transfer pressure to the rod. Turn them and drop them in the other bank.

So 1-2, 3-4, 5-6 & 7-8 turning them all around backwards as I go ?

 

[70bigblockdodge]

So 1-2, 3-4, 5-6 & 7-8 turning them all around backwards as I go ?

Yep

 

[Davea Lux]

@Davea Lux The stroke is not changed! The stroke is set by the crankshaft.

What it does is raise the piston higher in the bore at TDC Which is more compression for you.

Yes it is worth doing, but not if you are using thick head gaskets, then it’s all for nothing.

just swap to the other side bank and leave them on the rods, then it’s FREE POWER.

Your right, I fixed the post.

Dave

 

[C Body Bob]

Yep

Excellent! I feel better about trying this now.