I am in the planning stages of building a 406 for my '81 Corvette. I have read and heard several opinions about rod length and thought I would ask the forum members for their advice. I am trying to decide on whether to use the stock 400 rods(w/ARP bolts) or a set of stock 350(w/ARP bolts). I have heard that the shorter 400 rod has some "strange angles" and causes excessive cylinder wear. Is this really a huge problem? I know that it may not be ideal but, I am not building a 500hp engine nor do I expect it to live for 100k miles. Also, I have heard that the 5.7" rod may cause some clearance issues with the cam and possibly the block I guess Ideally I would use the 5.7" rod but, I already have the stock 400's.
I would really appreciate any advice or your experiences in this matter.

Thanks,

Mike

 

I believe it's advisable to run the longest rod that is practical and will fit in your budget. If your budget is tight, and you already have the 400 rods and 5.7's are out of the budget, I wouldn't lose any sleep over it. However, you can probably pick up a set of recondition 5.7" rods from any good machine shop for less than $150.

You will not find a significant difference in power or torque between longer and shorter rods. The benefit is in the reduced side thrust loading and cylinder/piston/ring wear, but I don't know if I call it 'excessive', and since you don't expect 100K miles, I wouldn't worry about it. I wouldn't fear running the oil ring over the piston pin either, there are OEM cars that do this. I've had 2 engines that used pistons with the oil ring over the pin and have never had any problems with this design.

Besides the reduced thrust loading of running a longer rod, this arrangement also produces a lighter piston, which is less prone to piston rock, since it's pivot point is higher. This is true even though the skirts are shorter.

I wouldn't worry about the rod clearance either, most street cams with mild durations will not have a problem, and if you do, you can either slightly mill the appropriate rod bolts, or run a small base circle cam. Either method is common and acceptable. If interference is a problem, and this is a low rpm, low spring pressure application, I prefer the small base circle option, but that requires a custom cam. Custom cams are the same price as a regular cam, you just have to order them from the cam compnay rather than through JEg's/Summit, if that's your source for parts. Comp for instance can make and ship a custom cam within 2-3 days.