Can One Use Standard 4.04" 340 TRW Pistons in a 4.00" + .040" 360 Cylinder Block?

[dibbons]

My local "Mr. Mopar" owns a '67 Belvedere, '69 Charger, '69 Road Runner, '71 Satellite Sebring Plus, '72 Barracuda. He is interested in purchasing a standard bore set of TRW forged pistons (4.04" bore) from me to use in a 360 motor (4.00" bore).

I see the piston pin compression height of the TRW pistons L2322F is most likely 1.84" (or 1.74"). The factory 340 piston weight is 720 grams (not sure of the TRW weight because they are in storage).The compression height of the factory 360 piston is 1.59" and factory 360 piston weight is 584 grams.

I can see over-boring the 360 by .040" would physically accommodate the 340 TRW piston. But then he would need to do a lot of checking: the longer stroke crankshaft may very well interfere with the cutout of the piston skirts, the stroke difference/compression height difference would change the deck height of the installed pistons, and the piston weight difference would require a re-balance. May be other problems I have not listed here. I don't remember if Mr. Mopar planned to mill some of the dome off the piston top or not (that would lighten the piston for sure).

Is Mr. Moper biting off more than he can chew with such a plan? Thank you.

 

[Oldmanmopar]

Don't waste your time with those old heavy pistons. They will not fit a 360 .I wouldn't use them on a 340, they are old and heavy.

 

[PROSTOCKTOM]

Just buy a set of ICON pistons for a 360 as it will make things a lot easier.

Tom

 

[Professor Fate]

Technically, yeah- you could make them work.
Basically what Tom Hoover did forty or so years ago, except in the Hoover motor he used factory '72-'73 low compression pistons (and still chopped a bunch off the top) and clearanced the skirts. But the only reason he did it that way is because at the time, there were NO options for a performance 360 piston- not the case nowadays, not by a long shot. Rebalancing is a given any time you swap pistons.

 

[dibbons]

Doing the math:

factory 340 = 6.123 + 1.84 + 1.65 (3.31/2) = 9.618
factory 360 = 6.123 + 1.59 + 1.79 (3.58/2) = 9.503
340 high compression piston/360 crank = 6.123 + 1.84 + 1.79 = 9.753
340 low compression piston/360 crank = 6.123 + 1.74 + 1.79 = 9.653

Assuming block deck height of 9.600 the 340/360 combo would be sticking up very high above the deck over and above the factory .018:
9.753 - 9.618 = .135 which is a little more than 1/8" (1/8 = .125")

The only way i can think of would be to usea .135 shim/spacer along with the standard head gasket. I don't know how much that would diminish compression ratio.

 

[NC Engine Builder]

Those pistons are heavy, and antiquated. They would go immediately into my scrap bin. And no, they wont come close to working.

 

[dibbons]

Those pistons are heavy, and antiquated

340 factory piston 720 grams
360 factory piston 584 grams
340 TRW L2322F domed piston unknown weight

 

[Dan the man]

In simple English, they will not work because of the longer stroke of the 360.

 

[Professor Fate]

In simple English, they will not work because of the longer stroke of the 360.

In simple English, it wasn't uncommon in the '70s to run the low-comp 340 pistons with about a .040 cut off the top and a little skirt clearancing.
With the OP's TRW pistons though, it just isn't feasible for the amount of work they'd take and the comparatively lower cost of actual 360 pistons that are available now.
They definitely won't work "as is".

 

[413]

360 has more stroke so the compression height is wrong.

 

[Dan the man]

In simple English, it wasn't uncommon in the '70s to run the low-comp 340 pistons with about a .040 cut off the top and a little skirt clearancing.
With the OP's TRW pistons though, it just isn't feasible for the amount of work they'd take and the comparatively lower cost of actual 360 pistons that are available now.
They definitely won't work "as is".

That's what I was meaning, glad you cleared that up

 

[Killer6]

In simple English, it wasn't uncommon in the '70s to run the low-comp 340 pistons with about a .040 cut off the top and a little skirt clearancing.
With the OP's TRW pistons though, it just isn't feasible for the amount of work they'd take and the comparatively lower cost of actual 360 pistons that are available now.
They definitely won't work "as is".

Just clamp 'em in a v-block & toss 'em on the ban-saw,.....You're

 

[AJ/FormS]

A friend of mine did that about 25 years ago.
He had a lathe....Worked out pretty good for him.

You know how much room is in an X-head? answer, plenty. the factory 340 pistons weren't up anywhere near high enough to fulfill their potential.
You can make a pretty good Quench chamber with those, but the penalty is balancing it when yur finished.
IIRC the 340 pistons were 719 grams, to the stock 360s of 545? I forget. Whereas KB107s are 502grams, and so you still have to balance it. Adding weight to the 360 crank will not be cheap. Whereas taking some off the flywheel cost me nearly nothing.

360 rods were press-fit.
In mine, I used 318/340 rods, which had bushings.

I wouldda tried it too, but I just wasn't that poor, and 502 grams was pretty attractive to me; man my 360 revs pretty nice. Even with the heavy factory flywheel and CenterForce clutch..

Can it be done ?
It already has.
Is it worth it?
Idk, it kindof depends on what it's gunna cost you? Even with free 340 pistons, you still gotta fit them in the chambers, making sure the valves don't crash, and the rings don't get too close to the top, and clearancing the skirts, and then,
the balancing is gonna eat that savings right up.
And the bottom line is this, the ringlands are still used, and may not give you a good seal.

I think it might make a good winter project for an old retired guy, with a machineshop. Maybe.....

 

[RustyRatRod]

Kinda like bonin your neighbor's wife. Just cause you can, doesn't mean you should.

 

[jimjimjimmy]

I weighed one of those TRW high compression pistons but 0.030 over years ago and if i remember correctly they were 986 grams or so each .

 

[dibbons]

I weighed one of those TRW high compression pistons but 0.030 over years ago and if i remember correctly they were 986 grams or so each

I had the NOS .030 over TRW L2322F pistons shipped directly to my engine builder in Salinas, CA. so I did not weigh them personally. According to the builder, they weigh 680 grams. I have another set of standard L2322F pistons in storage at a relative's house that some day I can weigh myself. The builder also weighed the piston pins at 154 grams. I pray the builder's data is correct because that is what was used for the bobweight used when the crankshaft manufacturer (Bryant) cut my custom 2.96" stroke billet crankshaft.