I am putting a stroker kit in a vortec 8.1l and was hoping someone could check my math so I order the right parts the first time!
Deck height: 10.22"
Bore: 4.25"
Stroke: 4.5"

If I use a 6.635" rod (Eagle CRS66353D) and a piston with a 1.34" compression height(Wiseco PTS524AS) then I should end up just about right, right? These parts are off the shelf, otherwise I'm looking at custom pistons$$$.
The stock setup is a 4.370" stroke, 6.7" rod, and 1.34" piston. Any thoughts appreciated.

EDIT: I see a made a typo on the proposed rod length. The rod is 6.635" in length.

 

The rod is an extra 0.035" shorter than the increased stroke, so the piston will be in the hole by 0.035" extra, all things else the same. Might be fine, depends on what you're doing with it.
The bigger problem with strokers is you're pulling the piston closer to the crank with the shorter rod, *and* pulling it closer with the longer stroke. Will the piston skirts clear the crank counterweights? You need some good drawings of the crank and pistons to try to figure that out - or just assemble it.

The only way to order right the first time is to get a kit that someone assembled and stands behind.

FWIW, custom pistons are not necessarily all that expensive. You may need to order through someone that has a deal with one of the manufacturers, but it's a lot cheaper than messing up and having to order a second set of pistons!

 

I updated my post, I see that I typo'd the rod length. The rod has a 6.635" length. Wiseco quoted me a little over $1k for a set of pistons, Diamond was 1.5k. Thanks for the reminder on the skirt clearance, that would have been a wake up!

The piston compression height should be identical between the setups if I did my math right.
Stroker: 10.22 - (4.5/2 + 6.635) = 1.335
Stock: 10.22 - (4.37/2 + 6.7) = 1.335

The piston should sit in the same spot in the hole.

 

Ah. That makes a difference. So, yes, the wrist pin and top of the piston will be in the same place. So the only thing to try to check is skirt clearance for the crank.
Diamond and wiseco are expensive. I got a set of AutoTec for ~$880 shipped. I think that included the rings? But the problem is you need to work through someone with a dealer agreement to get decent prices. Joe off the street will always pay extra.

 

Wiseco makes an off the shelf forged piston for this engine that I found for $486 and I found the eagle rods for $433 and picked up a forged 4.5" crank for $500. Not including bearings/seals I'm at $1419, not too bad for an all forged rotating assembly for this odd engine. Wiseco told me that the boss underneath the pin is .20" and would be the measurement to use. So with my measurements on the counterweight, looks like I have a theoretical 0.14" clearance from the counterweight to the bottom of the piston at the closest point. Miles of room!

 

Heres the math;

4.500/2 =2.250

2.250+ 6.635+1.340=10.225

 

Perfect, thanks all for the confirmation.

 

What crankshaft did you get ? Make sure the counterweights are not going to be too big for the 6.635 rods.

 

Hi Bill,
I got Dart's 9-45645006700 crankshaft. When you mean the counterweights not being too big for the 6.635 rods, do you mean the clearance between the counterweight and the piston skirt? As in the shorter rod would draw the piston too close?

 

Correct,
That crankshaft is designed for a 6.700" rod and there is a very good chance the counterweights will hit the bottom of the pin bosses in the pistons. The specs say 6.700 minimum rod length. By the way the part number is 9-454... not 9-456....

 

Bill, You've got me worried on the clearance now... I did not see that where I ordered it. I wonder what piston specs they used to come up with the 6.7" rod. The rods and pistons are backordered till June, I have time to change. I measured the edge of the counterweights at their deepest point from the journal at 4.695".
rod length - pin boss - half the pin - counterweight depth - half the journal = total clearance, not including bearing and pin slop.

6.635 -.2 - (.990/2) - 4.695 - (2.2/2) = .145

I have sent a question to Dart to see what they can tell me. Can you check that part number again Bill? I see it listed as 9-45645006700 everywhere. This is the GENVII 496cid engine with the one piece rear main seal. You would be correct on that number if it was a MKIV.

 

Nate,
First of all I didnt know Dart even made or sold cranks. I dont see them on their web site ?? The part number looks like a Scat number and that is probably what it is. Either way the 6700 at the end of the part number always indicates a 6.700 rod. One thing you need to keep in mind is that the counterweight is not perfectly round and is usually not closest to the pin boss at the very bottom. I will do some more looking in the AM at the shop. If they are saying it is for a 6.700 minimum rod length you can probably bet on it.

 

JE is using 1.245 compression hgt piston for their combinations but they are basing it off a normal tall deck block with a 10.195 deck hgt. According to my info the 8.1 block is 10+.231-10.241 so you would need a piston somewhere around 1.280 with the 6.7 rod.

Have you actually measured your block's deck hgt ?

 

Bill, the block is in line at the machine shop now waited to get checked. I ordered the PTS524AS Wiseco piston which was designed for this engine with the stock stroke of 4.37", 6.7" rod, and compression height of 1.34". Using a 6.635" rod keeps the compression height the same with the 4.5" stroke but as you and oddball pointed out, the counterweight clearance is likely a problem. These items are so far out on back order, it's probably safer to just order the 6.7" rods and the custom pistons rather than take the chance. By the way, I really appreciate all the help.