Angus242164 I was taking pics of some parts in the shed last night and decided to take some pics of a B23E piston/rod ("M" rods) next to one from a '93 B230FX ("13mm" rods), to show the differences:
GpAvolvo Really important thing to know is that B21/B23 rods have about 6 thou lateral clearance in the pistons at the gudgeon pins, whereas B230 rods have about 4mm - yes 160 thou! - the B230 rods float on the wrist pins! This is fine when you have standard Volvo pistons which stay upright in the bore with very tight piston-bore clearance. When I first built a B230 race motor with forged pistons and 8 thou piston-bore clearance I wondered why the bores kept splitting, and rods poking out of the block! Lesson - if you want to run 6-8 thou piston-bore clearance pistons, get custom rods with 6 thou side clearance at the gudgeon pins in the piston.
GT_Paul The M rod looks a lot more substantial..interestingly enough i thought the M rod was the so called 13mm rod
Angus242164 The 13mm B230 rods are on the right in all pics. Yep the B230's use floating pins, B23 uses pins that are a press fit in the rods. I'm no expert but I think this is the difference between "crank steered" and "piston steered" rods, terms that the yanks use a lot. I'm actually not sure on the "M rod" name now, I saw "M" forged into the B23 rods and assumed they were the ones people call M rods, but maybe not? The B23 rods are certainly a lot more substantial around the entire big end area, and slightly bigger on the shaft area. Unfortunately I don't have any early B230 (9mm) rods lying around, it'd be interesting to compare them to the others.
GT_Paul Hmm i have never had to press fit the pins on any of the b21 /23's, They just push in and appear to be floating
Angus242164 I had to press that one out, when I removed the piston not long after I took that pic. It wasn't super tight though, I could have tapped it out, and there was a fair bit of gunk on the pin, so maybe one from a running engine in good condition would slide straight out.
