I'm in the process of building my stroker right now. Before i had 64cc "camel hump" heads with .100 dome pistons. With the 3.75 inch stroke crank and 5.7 inch rods my pistons now hit the heads when i turn the crank over. Basically what i want to know is what is the biggest dome you can run on a stroker motor?

 

sounds like you have different problems. .100 dome shouldn't hit. when stroking a motor it shouldn't take the piston any farther up the bore. it should just pull it farther down. or the pistons are upside down. your most likely to have piston to valve clearence issues before they hit the head.

 

I just checked it, and it isnt even the dome thats hitting the head. The flat part of the piston is hitting the surface of the head

 

so the piston is coming out of the block? thats bad.

 

So I'm guessing that these are the same pistons that you had when it was a 350?

If so, then they're now coming 1/8" out the top of the block.

Has nothing to do with the dome. You could put dish 350 pistons in there, and with the wrong crank like that, they'll hit the heads too. For the same reason.

 

heres the problem
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So you're saying the crank is wrong?

 

yeah that sucks man. i feel for ya. you gotta have some new pistons. them won't work.

 

The crank i bought said it was a late model 383 stroker crank. Which it is, cause it is a one piece rear main seal. How could this be the wrong crank?

 

No, your pistons are wrong.

 

are the pistons for 5.7" or 6.0" rods? that could be your malfunction.

 

the postion of the wrist pin is up a little farther on the pistons you would need to use. mine are halfway in the oil ring. i have 6.0 inch rods.
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you need a compression distance of 1.425". the pistons you have are probably stock hight of 1.56"

 

The pistons are from summit # STL-H618CP30
which should be for 5.7 rods. #350
So i should get pistons with a 3.75 inch stroke, thats what i saw on summitracing

 

Yes, those are the right pistons for building a 350 with a 5.7" rod.

You're not building a 350 with a 5.7" rod. You're building a 383 with a 5.7" rod.

 

So like 7.0 said i need pistons with a compression distance of 1.425 is this correct?

 

the ones for a 3.75 stroke and a 5.7 rod should have that distance.

 

Or thereabouts.

Incidentally, how much compression were you planning on having with a domed piston in a 383 with a 64cc head?

 

I knew the compression was gonna be high probably around 11 or so. But since i have to buy new pistons, i'm open to suggestions on what you guys run.

 

It might be time to step back and do a little research before just diving in and putting parts together.

 

I had really no idea what compression distance was. I'm wanting around 10 to 10.5 compression

 

What apeiron is trying to say, is not only are you going to need new pistons meant for a 383 with 5.7" rods (roughly 1.425" pin height, but a piston spec'ed for this use will work right). But you'll also need a big dish if you want to run 64cc heads. On a 350 with flat tops you're at about 10:1, a 383 picks that up substantially.
If you don't want to be mixing your own race fuel, shoot for 9.5:1 CR or so. About an 18cc dish or something in that area.

 

speed pro flat tops with 3.75 stroke 5.7 rods 4.03 bore compression ratio is about 10.4

 

i see everyone has all the basic covered.. so ya.. wrong pistons

 

Thanks for everyone's help on this. I really appreciate it.

 

The way this works, is that the block is a certain height from the crank centerline to the deck.

The sum of ½ the stroke, plus the rod length center-to-center, plus the piston's "compression height" (distance from center of wrist pin to face of piston) must equal the height of the block, within a few .001"s.

What happened when you just stuck a crank with a longer stroke in your motor, without changing the pistons or shortening the rods, is that now that sum up there, is GREATER THAN the height of the block.

The magic number in the SBC is 9.000". A stock block has a "height" of 9.025", plus or minus a few; and a rotating assembly height of 9.000"; and a deck clearance (pistons miss coming all the way to the deck) of .025". So:

For a 350, ½(3.48") = 1.74"; rod length = 5.7"; compression height = 1.56".

1.740" + 5.700" + 1.560" = 9.000

Likewise, for a 400 (or a 383), ½(3.75") = 1.875"; rod length = 5.565"; and compression height = 1.56"

1.875" + 5.565" + 1.560" = 9.000"

Or, for a 5.7" rod 383:

1.875" + 5.7" + (comp height) = 9.000"; compression height must be 1.425".

Which is why your pistons stick out the top of your block.

You gotta either put the right crank back, or get the right rods for the new crank/piston combo (400 ones), or get the right pistons to match the new crank/rod combo (1.425" CH).

 

I've been on this Board quite awhile, I've seen this explained many times, but I believe this is the first time I've seen pictures of the results of using 350 rods and 350 pistons with a 3.75" stroke crank.

 

You need 383 pistons. A 383 piston has the wrist pin moved upwards so you dont have those problems. A 5.7 rod and a 6.0 rod doesnt affect the compression.

 

I got all this and thank you all for your help. Got some H860CP30's on order.