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Shortest rods?

Posted By: gregsdart

Shortest rods? - 12/22/21 11:41 PM

The only reason to consider this is to use some valuable parts in a new application.
I am going to use the crank and intake from the 550 megablock motor in a new BMP Aluminum block.
So what I want to do with the heads, valvetrain , pistons, rings and block left over from previous build is to put a stroker crank in it and lower the compression to a liveable level for the 440-1 heads. My Cp pistons have a pin height of 1.662.
To get compression down to 11/1 with all this I would need to run a 6.700 rod on a 4.5 crank a friend has for sale. That ends up at 1.4888 rod ratio. Not too terrible, but there is also a 4.75 crank available which would need a 6.535 rod, which is 1.37. since this motor will be aimed at a power band of under 6500 rpm, maybe even below 6,000 rpm side loading will be somewhat less than a 7500 rpm race motor.
So let's hear what you think. By being able to reuse the original heads, pistons, rings, and block the cost of this build would be pretty low. And the 4.75" crank would make good use of the fully ported -1 heads I have also, and at low rpm
Posted By: sr4440

Re: Shortest rods? - 12/23/21 12:31 AM

I think that you may have counter weight clearance issues with a 6.53 rod and a 4.75 crank.


Joe
Posted By: gregsdart

Re: Shortest rods? - 12/23/21 02:37 AM

Originally Posted by sr4440
I think that you may have counter weight clearance issues with a 6.53 rod and a 4.75 crank.


Joe
Maybe, Not sure how to do the math.
Posted By: AndyF

Re: Shortest rods? - 12/23/21 04:55 AM

Originally Posted by gregsdart
Originally Posted by sr4440
I think that you may have counter weight clearance issues with a 6.53 rod and a 4.75 crank.


Joe
Maybe, Not sure how to do the math.


You can't do the math unless you know the size of the counterweights as well as the shape of the piston skirts. Most likely you'll need to mock it up. I'm pretty sure it won't work but if you have the parts sitting on the bench it will only take a couple of minutes to swing a piston and rod on the crank to see for sure.
Posted By: gregsdart

Re: Shortest rods? - 12/23/21 05:15 AM

Looks like the longest stroke I could use would be a 4.5 with those pistoƱs. I figured longer might add value since the cranks would all cost the same. But after penciling it out adding stroke eats up clearance in a hurry.
Posted By: jwb123

Re: Shortest rods? - 12/23/21 02:56 PM

You mentioned rod stroke ratio so you understand the concept. I like the longest rod I can stuff in the engine, will not see a performance increase, but the longevity is what I am after. My last engine is a 4.25 stroke 4.5 bore with a 7 inch chevy rod, it makes tons of torque, and just over 800HP and revs to 7000 like a smallblock. very happy with the combination.
Posted By: 440Jim

Re: Shortest rods? - 12/23/21 06:36 PM

I made a quick change to an old calc I did years ago on a low deck idea. I haven't double checked it yet.
But is this what you are looking for?
The distance under the piston pin can be a tight spot hitting the crank counter weight.

Attached picture MaxStrokeClearance1.JPG
Posted By: gregsdart

Re: Shortest rods? - 12/23/21 07:59 PM

Jim, I wasn't far off on my assumptions. Rod c/cc - 1/2 stroke , - .495 (1/2 pin dia, - .190,- .060 clearance= Max radius of crank cw.
Posted By: 440Jim

Re: Shortest rods? - 12/23/21 09:52 PM

As Andy mentioned, if the piston skirt is not well designed for this application, it could hit the counterweight before the piston pin boss.
My new B1 pistons are 0.205" below the pin, maybe for more strength then those 0.190 ones I had back then. And these B1 pistons are designed for 7.100" rods, 4.500" stroke, so there is plenty of room.
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