Enlarging Rod Small End From 21Mm To 22Mm?

[240260280z]

Anyone know of this mod and its success/risk?

It is needed to do the 88mm VW piston upgrade.

[Captain Obvious]

Never heard anything about it, but wanted to ask... What's the swing of your new lathe? You might be able to do this yourself if you decide to go that route.

[240260280z]

Ahhh the master to the student Thanks again for the great lessons and delicious food!

The swing is 10" so it seems to be a bit too short. I'll leave it for the machine shop to cut then hone to final size.

My worry is that it will weaken the rod too much but, by intuitive inspection it looks like it may still work fine as the big force is compression.

[madkaw]

Your intuition might be right if you aren't turn high revs. I would think a machine shop would Reem that.

[Captain Obvious]

I believe the intuition is correct. The highest of the forces should be compression straight down the rod.

 

And after all, one mm off the diameter is only a half mm off the radius (about 20 thousandths for us US folks). I'm no engine builder or materials science guy, but my seat of the pants engineering says I wouldn't be concerned about taking off .020 of material.

 

Anyway, I know it's academic because you're not ready to try to tackle a job like this, but if you were, you juuuuuust might be able to get that on your lathe. The FSM says the rods are 5.13 inches "center distance", and I assume they're taking center to center between the two bearing surfaces. With the big end lower cap off, you just might be able to swing it. (Get it... Swing it?)

 

Haha! I kill me. 

[madkaw]

Blue,

were you able to source the low compression pistons with the right pin height?

[John Coffey]

Well, the highest forces are on the ignition stroke but most engine failures caused by issues on the small end of the rod are not from that.  If you have issues with piston pin retention in the small end you'll see the pin shift or the small end snap off near TDC on a non-compression or non-ignition stroke.  Late model BMW engines are famous for that occurring on a free revving engine under no load.

Edited March 16, 2015 by John Coffey

[240260280z]

Hi stephen, I'm going to try the 88mm VW type 1's hypers with a shave off the top.

I'm having a race engine machine shop do the work so it will be in good hands.

I was only able to find the 39.5mm compression height pistons. I could not find the "low compression variant".

Thanks John for the rod info!

Edited March 16, 2015 by Blue

[Captain Obvious]

John, Thanks from me too.

[240260280z]

For the archives, I found the low compression variant (thanks steve):

38.0mm compression height: http://www.geneberg.com/cat.php?cPath=1_2668

I just ordered the 39.5mm ones and will shave to fit... a new build begins.

[madkaw]

Did you order just pistons or did you have to buy the cylinders too? Was wondering if they would cut a break on just pistons

[madkaw]

I also found an older post in hybridz regarding the sizing on the rod end. Looks like you are okay- according to Tony D