Can 1275S crank be used in 1071S block with custom pistons?

[Twincam]

Would fitting a 1275S crank to a 1071S block be a problem if custom pistons were used with higher wrist pin location to compensate for stroke increase?

[smithyrc30]

Never tried it but 0.25" is a lot to try and get out of pin height while still having enough crown thickness to survive, clear the small end and give you enough for some clearance volume.

You might run into issues with the rod hitting the bottom of the bore or a cam lobe.

If you already have a block, cam, crank and rods, dry build it and have a look.

[Twincam]

Good suggestions for problem areas.

Went out to garage and all I have are assembled pistons and rods. Cut off a small piece of scrap 1/4" aluminum plate I had hanging around and was able to pass it between the rod and underside of the piston with room to spare. Taped some washers to increase the thickness and got to about a .375" stack which seemed near the clearance limit. For reference, the pistons on the rods were AE (stamped 16632) with a 4 ring stack.

Looks like wrist pin would slightly interfere with oil control ring of 4 ring stack but I'm thinking custom pistons would allow the flexibility to shift a 3 ring stack slightly upwards to eliminate that interference. Will have to go further with assembly mockups to clarify other potential interferences mentioned. Thx.

[OzOAP]

1 inch crown height pistons are available for when using 6.25 length con rods.
Could also measure up a dished set of 1.250 height pistons, making them into flat top, depending on dish depth/crown thickness.
Pic from left to right
5.75 rod/1.480 piston
6.00 rod/1.250 piston
6.25 rod/1.000 piston

[Twincam]

That’s a pretty dramatic picture of wrist pin locations. Great visual! Thx.

[Vegard]

Never tried it but 0.25" is a lot to try and get out of pin height while still having enough crown thickness to survive, clear the small end and give you enough for some clearance volume.

You might run into issues with the rod hitting the bottom of the bore or a cam lobe.

The 1071 block is the same as the 1275 so this will not happen.
The only thing you need to consider is thrust bearing with They need to be custom. Cannot remember if you need thin or thick, but both are easy to achieve.

[smithyrc30]

Never tried it but 0.25" is a lot to try and get out of pin height while still having enough crown thickness to survive, clear the small end and give you enough for some clearance volume.

You might run into issues with the rod hitting the bottom of the bore or a cam lobe.

The 1071 block is the same as the 1275 so this will not happen.
The only thing you need to consider is thrust bearing with They need to be custom. Cannot remember if you need thin or thick, but both are easy to achieve.

Nope, the 1275 has a taller block with increased material to increase the deck height.

970 and 1071 have common deck height which I think is the same as the small bore engines, though not 100% sure.

As far as I am aware, the centre main bearing width is different between 1275 solid block and Cooper S block (all sizes), which means an S crank will not fit in a non S block without different thrust bearings and vice-versa.

[Twincam]

Never tried it but 0.25" is a lot to try and get out of pin height while still having enough crown thickness to survive, clear the small end and give you enough for some clearance volume.

You might run into issues with the rod hitting the bottom of the bore or a cam lobe.

The 1071 block is the same as the 1275 so this will not happen.
The only thing you need to consider is thrust bearing with They need to be custom. Cannot remember if you need thin or thick, but both are easy to achieve.

Nope, the 1275 has a taller block with increased material to increase the deck height.

970 and 1071 have common deck height which I think is the same as the small bore engines, though not 100% sure.

As far as I am aware, the centre main bearing width is different between 1275 solid block and Cooper S block (all sizes), which means an S crank will not fit in a non S block without different thrust bearings and vice-versa.

smithyrc30, I think Vegard dropped the “S” designations from both the 1071S and the 1275S since that was the headline of the topic. If his posts are read with the “S” inserted, they all work out. Everyone has given valuable insights. Thanks. -Gregg