Wrist pin stuff...

Going through the new version of my Turbo B20. So far, sourcing off the shelf forged pistons for 24mm wrist pins is proving to be not so easy (on the cheap anyway).

I can get my hands on a set of 22mm pinned pistons easily, and I have a set of 24mm bushed rods. So I started thinking about things.

The pistons are stock replacement, so crank steered rods, but pin bosses are internally machined parallel. New rods are piston steered... so I was going to need to machine up some spacers aiming for all right piston>rod side clearance. This got me thinking. I already have to make spacers, so why not make some wrist pin bushings... or better yet some bushings with flanges that are the spacers... So thoughts are buy a stick of Ampco 18 Aluminum Bronze bar or tube stock. lathe the ID to just undersize, run the od to .002-.003 press fit into the rod with a flange that would allow for proper side clearance. Press them in from either side, then hone/ream/bore or whatever to get them set for floating pins.

What say ye t-bricks machinists/engine builder peoples? Sound like it wont scatter on start up?

Sounds like a solid-ish plan. The only issue I see is dealing with the sleeves butting up at the center of the rod. Probably best to have a slight gap there as an oil reservoir.
You'll want to bore or ream the bores after the sleeves are pressed in, then follow up with honing to the proper oil clearance.

I think we might have different equations for determining when to buy custom pistons vs a whole bunch of other work.

An interesting concept for sure!

Curious, I know you can drop in B21 pistons and rods on the metric B20's, keep the 24mm pins. I wouldn't think getting an undersize B21 piston set made would be an issue, cost shouldn't be much different for a custom diameter if they already have the specs in the system. Have you tried hitting up Yoshi, see if he might be able to get a set? I went thru him on the 230FT pistons, ran the usual 3-4 week delivery time and they were a custom setup.

Pistons are 380/set shipped shelf stock in the correct overbore and comp height for my build. So cheap for some forged slugs. A chunk of material would be less than $100 (haven’t checked the scrap box yet, I might even have it already). From there it’s just a couple minutes in cad and an hour or three in front of the lathe on my day off.

One thing to consider as well is the added weight from using bronze for the spacers.
Usually aluminum or teflon is used for rod spacers.

culberro said:

One thing to consider as well is the added weight from using bronze for the spacers.
Usually aluminum or teflon is used for rod spacers.

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Good point. I would do all my weight matches after pin bushing install. And dropping the pin od would somewhat zero out added weight of oversized bronze bushes?. Maybe. Probably not. But it?s the thought that counts, right.

That?s probably a better plan. I don?t really see revving this to 7500, probably won?t go past 7k, but it might be safer to roll with oversized bushes and an aluminum spacer.

The ots pistons are nice because the source is reliable and there?s 1 more oversize available, so if I break it again, cost for another build could be less expensive. And they leave enough for a transmission upgrade in the budget.

I was dumbfounded by the shear massiveness of the Mahle turbo pistons. A locomotive piston might be under built compared.

I think the assy weighed 750grams, a Diamond LS Turbo piston assy was 400 something grams!!!!

That would be a huge reduction in reciprocating mass!

That’s definitely a thought. At one point I was tossing around the idea of sleeving my spare b18 block then oringing it. But that’s a rainy day thought. I don’t think it’d be super difficult. I would just need to buy some time on the big 5 axis. Or wait until the schedule is open and do it when it’s free.