S/B chevy 400 pistons in a 360?

[Deathtrap]

Has anyone ever heard of boring a 360 .045 over and using 400 S/B chevy pistons on stock 360 rods? If this can be done, I wonder if you can bore a 360 .085 over? I have a nice set of .040 over 400 pistons and would like to know if anyone else has ever done this.

Thanks, Ken :-|

[jeepsr4ever]

On early blocks you can over bore tremendously. You should get your block sonic checked to confirm. Also you should check that pin height..You can do it but you will need to bush your rods as the Chebby pin is smaller. I can remember the pin height issue,,, :-|

[Rogue Racer]

There are a couple of ways to do this, but neither is a slam dunk. It takes some fitting and planning to make it work satisfactorily. The Chevy pistons do not have as much compression distance as AMC defines. Depending on the Chevy piston, common numbers are 1.540" for cast rebuilder type pistons, and 1.560" for forged or performance type pistons. And those numbers are for pistons using stock Chevy 400 rods (5.56" c-to-c). If the pistons are for a 400 using 350 type rods (5.7" c-to-c), the compression distance is too short for stock AMC 360 rods. The nominal compression distance for the AMC 360 pistons is 5.81", so you can see that using the 400 Chevy pistons, the deck clearance will be increased (the piston will be down in the cylinder a bunch at TDC). There are some other things to consider, though. Stock AMC 360 pistons have a 26 cc dish; 400 Chevy pistons can be had with a 17 to 19 cc dish, flat top, and domes of various displacements. If you calculate carefully, a flat top 400 piston with the increased deck clearance will work out to a desirable 9.0 to 9.5 to 1 compression, even though you are giving up the efficiency of quench (though it is probably not any more inefficient than stock dished 360 pistons with little deck clearance). Because of the large choice of Chevy 400 pistons on the market and the possibility of fitting the 360 with SBC Chevy rods that are also available in a variety of lengths, there are lots of other possibilities if you figure the combinations.

Here are some numbers to start with. The nominal deck height of your 360 block is 9.208" from the center of the crank to the top of the deck. The crank will push the rod-piston 1.72" (half the 3.44" stroke) toward the top of the block at TDC. That leaves 7.488" of distance for a rod-piston combination. The stock 360 rod is 5.875" c-to-c, so the maximum compression distance for a piston without pushing it out of the cylinder is 1.613". If you use a 6" Chevy rod, the piston can have a compression distance of 1.488" An LS1 Chevy rod is 6.1" c-to-c, so that's another choice. Chevrolet manufactured a rod that was 5.94" c-to-c for their 265 cu in L99 engines in the mid-90s. That creates other possibilities. You just have to figure out what can be done and realize the options before you start.

Fitting the 400 piston to the AMC rod is really simple if you have a set of AMC pins. The Chevy uses a .927" wrist pin; AMC uses a .931 pin. A machine shop with a standard Sunnen rod hone can hone the pistons to fit the AMC pins without a lot of trouble. You just have to use lots of fluid and go slow without a lot of pressure. Using the stock pins means you can have a pressed fit pin in the stock AMC rod and avoid the expense of machining the rod.

I would recommend first, fitting the pistons to the rods. Second, get the block bored and honed to fit the pistons. Third, mock assemble the crank assembly to measure deck clearance. Fourth, measure the volume of the combustion chambers of the cylinder heads you are going to use, the volume of the head gasket you are going to use, and the volume of dish or dome of the pistons you are going to use. Fifth, determine what compression ratio you want and then calculate how much to machine the block to get what you want. Remember that if you machine more than .020" off the block, you will have difficulty getting the intake manifold to align without also machining it.

I built a 360 for a guy two years ago using a reversed dish 400 piston (-12cc) on stock AMC rods and calculated an honest 9.4 to 1 compression to run on pump gas. I had to cut the deck .045" to get the compression I wanted, and machine the intake to fit. He has logged over 14K miles in a Jeep with it without a hitch. So it can be done.

[jeepsr4ever]

Yep Thanks Rogue...you did the thorough explaining I dont have time for :t:

[KEV]

=D> Ok, so I got these teflon coated pistons for my Rotax....................

[Mudrat]

=D> Ok, so I got these teflon coated pistons for my Rotax....................
912, 914 or the 'S' version? Got an AWC (air worthiness cert) with it?

[KEV]

=D> Ok, so I got these teflon coated pistons for my Rotax....................
912, 914 or the 'S' version? Got an AWC (air worthiness cert) with it?

They are for the old 580 twin that I used in my watercross sled back in the day. The teflon coating cut down on the piston scoring from all the moisture in the air.

[Deathtrap]

Thanks Guys ! This is a great forum. I will sit down and do the calculations to see if it's going to be worth the trouble to use these pistons I got on a trade. They are ROSS forged racing pistons(.040 over) and are supposed to be "good stuff". This will be a race only engine(Mud Bog), so I want to go all out.
Thanks again for your time and trouble,
Ken

[bricklin]

you can also ofset grinda 390 crank and use a chev rod and the 400 piston for a poor mans stroker.