I'm trying to decide on rods for my 3.625" (Cola) crank. It's made for 6" rods. I was also toying around with using a 6.125 rod, but after looking at the numbers it looks like that would only leave me with 1.0875" for the piston (as opposed to 1.2125" with the 6" rod). Assuming standard deck height.
The motor will strictly be NA, but even so, my assumption is the 6.125 rod is just not going to leave enough room.
Thoughts?
Thanks!
You can get pistons with a pin height as short as 1.000 inch, however the wrist pin is well up into the oil ring groove at that point and you have to run oil ring support rails. I'm not saying that's a bad thing, just making the point. The pistons for my latest combo have a 1.180 inch pin height and the pin does not intersect the oil ring groove. Personally, I would go with the 6 inch rod in your case and don't get too hung up on making the rod as long as possible. I know of one engine builder in particular who has won the Engine Masters challenge more than once that is a fan of short rods, and I know of sprint car guys running 4 inch stroke cranks with 5.850 length rods.
Have fun!
Dan
You will find that most aftermarket (catalog) piston pin heights are designed to match up with a 9.000" deck height. Someone must have decided a 9.000" deck insures that a stock block can be cleaned up to get the decks square and parallel.
I'm trying to decide on rods for my 3.625" (Cola) crank. It's made for 6" rods. I was also toying around with using a 6.125 rod, but after looking at the numbers it looks like that would only leave me with 1.0875" for the piston (as opposed to 1.2125" with the 6" rod). Assuming standard deck height.
The motor will strictly be NA, but even so, my assumption is the 6.125 rod is just not going to leave enough room.
Thoughts?
Thanks!
I was just out on the Wiseco piston site and they have a 1.060 pin to top piston for 6.125 rod and 3.625 stroke.
All of my 6 inch rod 3.750, 3.875, and my 4 inch stroker motors have had the pins behind the oil scraper ring. I do not run anything behind the ring.
There is no problem with running the 1 inch pin height. We have built most of our engines with pins that are well into the oil groove and have never had a problem. We have had good results using the longer connecting rods and feel that this is the way to go. Just a note, the sprint car engines mentioned above that are running stroker cranks and 5.850 rods are probably doing so out of necessity due to deck height limitations. They simply can not fit anything else in a stock deck height block even with the 1 inch pin height; that doesn't mean that this the best way to go by any means! Sometimes you have to do what you have to do and in many cases a $5000 aluminum tall deck block is not an option for anyone other than a sponsored racer!
There is no problem with running the 1 inch pin height. We have built most of our engines with pins that are well into the oil groove and have never had a problem. We have had good results using the longer connecting rods and feel that this is the way to go. Just a note, the sprint car engines mentioned above that are running stroker cranks and 5.850 rods are probably doing so out of necessity due to deck height limitations. They simply can not fit anything else in a stock deck height block even with the 1 inch pin height; that doesn't mean that this the best way to go by any means! Sometimes you have to do what you have to do and in many cases a $5000 aluminum tall deck block is not an option for anyone other than a sponsored racer!
Thanks for the info, Deakins. Since my motor will be 1/2 street, 1/2 roadrace, I'm concerned about longevity / endurance of the bottom-end. Do you see any additional issues with long-term reliability or ring oiling? I'm not expecting to get 100K or anything like that out of it, but don't want to have to freshen it every year or two either.
Thanks!
Not at all! One thing to mention about this choice is that I am currently unaware of a 1 inch pin height piston that is anything but forged. Now for an engine that has a very high piston speeds or massive amounts of pressure and heat, the 500-1200$ spent here is necessary. With the shorter rod you may be able to find an off the shelf hypereutectic piston that will fit. This does two things; first it lowers the cost, but secondly (and more important) is that these pistons are fit up at a much tighter piston to wall clearance. This equates to less piston rock, no cold start knock or oil consumption, and just better overall street longevity. If your application does not need the durability offered by the forged piston you may think about running a set of hypereutectics. Most guys will argue that point saying they are a problem waiting to happen but we have yet to have a failure! With that said, the all aluminum full competition engine we run has and never will see a set of these; it all depends on the environment the engine will live in and the load that it imposes on it's parts.
Thanks again! To be honest, I assumed I was going to go with forged pistons to be safe (although, not absolutely necessary for my combo). I want to be able to run it up to maybe 7200 or so max, but normally under 7000. I just want a margin of safety when I'm in the middle of a turn and need to hold it in gear a little longer.
At least now I know I have some options to work with.
Thanks again, all for your help!
Wrong You may think about running a set of hypereutectics.
I had a set of KB hypereutectics have a melt down and smear the aluminum into the ring lands. This was with a 3.48 stroke and never really run over 7200 rpm.
I called KB up and ask what was up with their pistons. They explained to me that Hyper pistons have FPM limitations (About 7000 rpm with a 3.48 stroke) Never again because the motor was trashed.
Do you see any additional issues with long-term reliability or ring oiling? I'm not expecting to get 100K or anything like that out of it, but don't want to have to freshen it every year or two either.
If your planning 7000 rpm with 1/16th inch racing rings. Figure about 10,000 miles and your going to start seeing light blue smoke. Leak down tests will show ring leak. They really need a rebuild by 15K.
Last edited by gkull; 01-17-2007 at 03:37 PM.
Reason: corrections
You will find forged pistons made from two different alloys. One is 4032 and one is 2618. The 4032 alloy has a higher concentration of silicon and this will reduce the thermal expansion. 4032 forged pistons can use pretty tight fits and you can avoid the dreaded piston knock. A good example are the Mahle pistons, their gaps are approaching the hypereutectics.
You don't get something for nothing, the 4032 alloy is not as strong (ductile) as the 2618. So in all-out applications, 2618 may be a better choice.
Corvette Store
Max Rod length for 3.625 stroke?
