393 PISTON

danny clemens · Feb 2, 2006

If I stroke a 69 351 W out to 393 using stock heads do I have to use a dished piston to keep the compression down? If so how much of a dish? I will be using pump gas. I also would rather err on lower compression ratings if there is any question? Are there special pistons for the early w's because of the lower deck height? Thanks for all replys.

rbohm · Feb 2, 2006

that depends on how large the chambers in the heads are, how thick a head gasket you are using, and how far down in the hole the top of the piston is.

67GTCOUPE · Feb 2, 2006

YES.

Flat top 302 pistons give 11-12:1 CR.........TOO much for most of us.

I chose Mahle pistons and am VERY impressed with them and the customer service they gave me!

I have Mahle # SBF 600000i26 they have 26cc dishes, give 9.2:1 CR with 64cc heads, 9.5:1 with 60cc heads and 10.1:1 with my 54cc heads.

I got the pistons, locks, pins, and rings all for under $500.00!

Another company is CHP/Probe and SRP, they all have decent stuff for 393w.

brianj5600 · Feb 2, 2006

Uh, aren't they just 302 pistons? Don't they also make short and tall 302 blocks? CR will depend on the cam/intake set up. You need to give more info on your combo, and it will likely need about 20cc dish. Give or take 5cc.

milner351 · Feb 3, 2006

You might also check with the guys at DSS, that's where I got my rods and pistons, since I have twisted wedge heads, my pistons have to be fly cut appropriately for the TW head valve positions.

67GTCOUPE · Feb 3, 2006

brianj5600 said:
Uh, aren't they just 302 pistons? Don't they also make short and tall 302 blocks? CR will depend on the cam/intake set up. You need to give more info on your combo, and it will likely need about 20cc dish. Give or take 5cc.

No it is not that simple!

Stroke, rod length, piston comp height, decking all effect the static CR. Cam will effect dynamic CR.

So, like I said..........flat tops will put the damn CR thru the roof! You need dishers........and yes at least 20cc will be the ticket.

Shop around, but I think Mahle is impossible to beat for the price.

brianj5600 · Feb 3, 2006

A 393 is designed to use a stock 351 rod with a stock 302 pistons. Compression hieght is around 1.6". It is not one compression ratio fits all. If you don't know what cam you are using, you don't know what CR you need. I did not think I was disagreeing with you 67GT, just stating that 302 and 393 pistons interchange, and the right cr cannot be decided w/o cam specs. Oh, and compression hieght varies depending on what year they are made for.

67GTCOUPE · Feb 3, 2006

brianj5600 said:
A 393 is designed to use a stock 351 rod with a stock 302 pistons. Compression hieght is around 1.6". It is not one compression ratio fits all. If you don't know what cam you are using, you don't know what CR you need. I did not think I was disagreeing with you 67GT, just stating that 302 and 393 pistons interchange, and the right cr cannot be decided w/o cam specs. Oh, and compression hieght varies depending on what year they are made for.

I'm not trying to give you a hard time here, it is just that I have gone through this all recently as I just built a 393w.

Example: 302 flattop gives 10:1 CR in a 302..........that very piston put in a 393 will give 11to 12:1 CR........here is why:

The swept volume of the cylinder indicates how much air the piston displaces as it moves from BDC to TDC. Increasing the cylinder volume without making any other changes will increase the compression ratio because it enlarges the cylinder volume without increasing the combustion chamber volume. In other words, the piston will have to cram more air into the same amount of space.

So, one must be very careful of what piston one runs. Also chamber size of heads, and yes.....head gasket thickness and deck height all effect CR.

The 393 and 302 use 1.6" compression heights on the piston.

danny clemens · Feb 3, 2006

The 26cc dish may be the way to go. I may put aftermarket heads on it down the road. The cam will not be anything radicle. Just something to have weekend fun with (not drag racing, well not too often.)

Thanks

D.Hearne · Feb 3, 2006

And not all 302 flat topped pistons have the same compression height. They come in anything from 1.580-1.610. Something you have to watch for the opposite effect when building a 289 or 302.

dennis112 · Feb 4, 2006

You need a low compression 302 piston in a street 393. Been there, done that. As an example, here are the Probe pistons that I used in my 393:

302 - 3.000 Stroke - Stock Rod - 1.600 C/H - .912 x 2.500 - 113 Gram Pin Dish Volume 15.0cc

Bore / Weight / 58cc / 61cc / 64cc / 68cc / Part # / Ring Set
4.000 471 8.13 7.89 7.66 7.38 10684-Std 315-0036 Ring Grooves 1/16 1/16 3/16
4.030 487 8.23 7.98 7.75 7.46 10684-030
4.040 516 8.26 8.01 7.78 7.49 10684-040
4.060 534 8.32 8.07 7.84 7.55 10684-060

Note the compression ratios shown are when the piston is installed in a 302. I used the 10684-std set for a new block and my final compression, in the 393, was 9.8-1 with, from what I remember, 60cc heads. A 15cc dish was used and the pistons are .010 in the hole, .034 thick gasket.

393 PISTON

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