I did a search and there isn't much about full floating pistons in the 250-292.
I've never been a fan of press on pistons. Does anyone run full floating pistons in their motor?
If I decide to go with full floating pistons do I need to buy full floating pistons or can they machine the grooves in the press pin pistons?

Dear Vince;

The pistons don't float. You need to look up 'full floating wrist pins'.

This term is used when the item has a bearing surface on both sides and moves (floats) freely etc. Flathead Ford V-8 rod bearings do this too.

The GMC is the only one one I know of that has 'full floaters' in there pistons/rods & It's because the pin is almost 1" in diameter (.990).

It doesn't really matter with the small pins.

Larry P. might have more info on this topic.

Oops sorry thats what I meant full floating pins.
Around here everyone just says full floating pistons,which I guess is technically wrong.

Not all pistons have a ring groove in them. As for the room to do so? Again I'm not 100% sure. My
307s did have the ring groove all ready in them.
My rods were machined to let the Pin float The were also Drilled so that Oil could lube the pin.

Personaly this extra work/money may not?? be worth it for a daily driver. And Maybe not enough
gain in a daily driver. That would ever see Or feel. If it is The first motor you ever built,
Or if it is a different Rebuild(combo built meaning Not the same build with The Pins being the only change in the Build)

If using floating pins do you have to use a bushing in the rod? Or is steel on steel ok if oil hole and clearance are figured in.

Mine were done with a oil hole and clearanced.

what clearance did you ude?

I do not remember how much the machine shop gave it.And I did not spend the extra bucks to have this Build blue printed.

the advanage of floaters is you can put them together your self. also you can get all the center to center distances the same.

floating in steel? some have done it with good resaults. but there are a lot that don't like the steel on steel. 2 like metals running together will gaul or seaze.

I don't really see any difference (strenght wise) as there isn't much movement there.

Does anyone know of wrist pin failures?? There's a huge amoung of oil there already that the oil rings provide.

The GMC engines were hugh & built 'tougher' for that (post Korean war) era. Remember they had to compete with Ford, Dodge, International and Chevrolet.

I guess (it wouldn't hurt) for an 'all out' racing engine.

Thanks, just looking at different options.If I go this route the rods will be bushed and drilled for pin oiler.
So if the 307 pistons already have the grooves in them,I don't have to change the pins?

John the motor I just took apart had a wrist pin that was failing and wore the bore pretty good.Waiting to see if it can be cleaned up or if I need to find a new block.

I believe stock 307 pistons do not have grooves in them.At least the ones I've seen.
I believe Twisted6 I.I 307 pistons were aftermarket forged racing pistons???

My Forged TRW 307 flat top w/four valve reliefs pistons , had no grooves in them for spiral locks.

So only the real expensive pistons have them.
Did you have the groove but in or did you go with the press pin?

A slight twist on floating pins, when I had my 292 balanced the small end of the rods were to heavy and since I had selected rods with minimal balance pads (since GM aims to get the rods within a given weight tolerance a small balance pad means that their is more metal in the beam and less in the balance pads) we had to take the weight out of the pin by shortening the pin in order to balance the rods as there wasn't any metal left in the small end blaance pad. So even though the pistons were set up for floating pin we put them in a pressed pins. Just made sure we had good pin to piston clearence.

Thats pretty wild,but why not full float them.I would think the bushing would weigh less then the metal it replaced in the small end of the rod.There for making the small end a little lighter.

I had used press pins.
My TRW pistons had no groves in them.
Usually only HP pistons have them or some factory Mopars had full floating pistons.

V;

It's just a lot of machine work what doesn't accomplish anything.

Except for that 'ballance' issue, there's no point to it.

The early 'jimmys' may have been a GM experiment. A lot of that was done back then.

Hank you are right Mine were aftermarket 307s 12-1
pop-ups .030. The 283 pistons i picked up a few
year back to are aftermarket Pop-ups they had
the rod setup to let the Pin float.Again No
Bushing. When i get a chance I'll digem up and try to get some pictures.

So your saying that the rod was set up to float the pin without a bushing,right?

Greetings . . .

Full floating wrist pins aren't a performance mod - they are a counter measure to prevent wear under high performance conditions (loading/RPM).

An engine with a very long stroke doesn't really need full floating pins because most of the bearing surfaces get worked due to the of size of the arc the big-end of the rod is traversing.

Compare that with an engine that has a very short stroke - now the arc traveled is so narrow that only a part of the bearing surface gets worked over and over again.

Take for example a Formula One engine (minimum stroke is 1.56"). Tach that baby to 20 thousand down the straight away . . .there's an application that requires floating wrist pins . . .

regards,
stock49

I believe the reason that Mopar had put full floating wrist pins is for performance reasons & other reasons.
Possibly to gain advantage over Fords & Chevy's?

Why else would a manufacture want to spend more money & the extra machining process & extra parts & not benifit from it HP wise & reliability.
I would think it lessons the frictional HP loss?
Another two cents :-)

Hi MbhdA.Hank . . .

Not sure I follow where these 'HP reducing frictional losses' might be. In a press fit set up the wrist pin is held fast in the small end of the rod with bearing surfaces only in the piston shoulders. When switching over to a free-float an additional bearing surface is created in the small end of the rod - either by machining the rod end to tolerance or pressing in an appropriate OD bushing that has an ID to tolerance allowing the wrist pin to float.

So where's the frictional loss-gain?

As for Mopar's reasoning behind floating wrist pins in the new SRT - I'd read that is was done to allow for "Higher Loading"

regards,
stock49

If you had a piston/rod w/a floating wrist pin,
the other w/a pressed pin in the rod in your hands.
You would see & feel the difference that the full floating pin the piston moves so smoothly & flops all around so easily, thus less sticking possibly?
Have you had the two types in your hands @ one time?
The pressed pin will not rotate as easily usually.


In other words if a piston moves more freely on a rod,,there if less friction,, correct?
If it does not move so easily it has more friction thus creating more HP to make the piston move to piston back & forth from the crankshaft turning.
The Mopars I was referring to were the 60's & 70's era V-8's
Maybe this makes sense?

Hi MbhdA.Hank . . .

Yeah, I have had both in my hands. I understand what you're saying about the 'easy' free play on the bench. It's just that none of the engineering I've read about wrist pins talks about recouping HP by reducing friction in this particular part of the engine.

regards,
stock49

I have noticed VW air-cooled's use them, all the big diesels use them (high compression, long stroke, low rpm)but(as Stock 49 says) formula one allso uses them. I would think they reduce both friction and wear, because they rotate while the engine runs and a pressfit is stuck to wear on the same spots. They also make changing pistons way easy so maybe that has someting to do with it.

The main purpose is so the wrist pin does not stay in one position which in turn could cause wrist pin failure from wearing in one particular spot.

Those SRT motors are something else.I see they've improved the caddy 4-6-8 of the early '80s.

On the vw that was the only way to get the piston on the rod.The rods go on the crank and the 2 halves of the block sandwitch the rod/crank combo.

Ford used full floaters on some motors also.

So if it reduces a little friction,does it increase mpg slightly?

Not sure you guys know this,, but Crower makes nice connecting rods that are stronger than stock & made for floating pins.
I have 6" Crower rods here for my new engine.

To run the 6" rod do you have to have pistons made or is there a off the shelf piston with the right pin hieght?

Correct. I have JE pistons ,they are custom made.
I got them through Sissell's.
They(Crower) also make stock length rods.

Thanks,is this going to be a turbo motor?

Yes, when ever I get the time to build it.
It will be a turbo engine.
Right now I am planning to put a turbo on my car now with the engine that is in the car.
So I can get used to the EFI.
Compression is 8.4 to 1
cam?,238 - 248 duration @ .050 lift is about .585
w/a 114 lobe center.
I had a Paxton supercharger on it(made only 8-9 psi boost pressure),took it off, now I will try a turbo.

Thats just too cool.Are you building another 250 or you going with the 292 this time?

I am building another 250.I never tried a 292 yet.
Just mostly have 250 parts.

Looking to make some serious HP & torque.
Old school power,w/modern technology!!

Sweet, I like it.

Go get'em.

You building another car or will this be going in the camaro?

It's going into my Camaro.

Never used locks on my free floating pins, always used buttons, they are getting hard to find so I make them myself.

Hank are those the 6in Steel HEAVY rods??

Larry,
They are more heavy than a stock rod, .300 longer.
I need to look @ the card that came w/my rod set to tell you the actual weight.
I am not building an all out drag racing engine, if I was I would use aluminum.

And this is NOT a v8 rod? IT is for a chevy L6 and
it has the right size ends/widths?

Larry,
that is correct.
It is for the 194-250 Chevy six cylinders.
It is in there catalogs.
Mike Kirby ordered them for me.