converting pressed-in wrist pins to floating and then some?

[burdickjp]

I'm looking at taking a set of connecting rods with 21 mm pressed in wrist pins and installing bushings for floating wrist pins. That I understand the small ends are bored out to accept a bronze bushing. My curiosity is if these can be bored offset to provide a small length increase. At the same time I'm curious about reducing the wrist pin size to 20 mm or 19 mm.
I've seen a few manufacturers list off-the-shelf bushings. I haven't found any which provide a wall thickness which would allow me to both offset for length and decrease in diameter. Any suggestions?

[user-23911]

They don't need boring for bushes, just honing slightly to make the difference between press and floating.

[burdickjp]

What about using the bushing process to offset the pin location?

[DCal]

What about using the bushing process to offset the pin location?

The bushings are made thin so that the steel acts as a reinforcement preventing distortion. My advice, if you're set on this is to have steel bushings made with the offset and diameter you want and then purchase the 19mm pins DLC coated. You're already buying new pistons and it might be cheaper to get new rods also.

[Mark O'Neal]

Trend will make you a bushing in any size you want...and there is no reason to increase the rod ctc.

[modok]

Can you put in a bronze bushing?
If the wall thickness of the small end is adequate, sure.
That's really the only concern I'd have.

[burdickjp]

Thanks for pointing at trend. I'll see if they have an opinion on the matter.

I'm less concerned with arbitrarily increasing the rod length and more thinking about what aspect of the relationship between rod length, wrist pin diameter, and compression height is going to take priority and drive the design. If I'm installing a bushing anyway, can it be done in a way to gain length? If I'm installing a bushing anyway, can I size the bushing to reduce the pin diameter? Does the smaller pin diameter gain me anything in ring packaging? Does the smaller pin diameter gain me any advantage in weight?

I'm expecting trying for an offset bushing won't realistically allow me to decrease my pin size without going for an unnecessarily thick bushing and that my compression height won't require any unusual ring packaging. I'm also thinking decreasing the pin size won't save me much weight overall.

It's all a balance. I'm trying to find where to balance.

Thank you for your input!

[Dave Koehler]

Narrow your goals just a little bit.
If those are like any other rod there won't be enough real estate to move the CtoC much. Just arbitrarily moving them .010 or .020 gains nothing but the need for custom pistons.

Resize the big ends first if needed. Then you can get the CtoC corrected on the whole the set with the bushings.

[modok]

Don't make an eccentric bushing.
An eccentric bushing is by nature weaker and heavier. I would rather weld it and re-machine.
Wall thickness under 1mm, or steel backed bronze bushing.

[burdickjp]

Custom pistons are already involved.
I'm not talking about an eccentric bushing. I'm talking about offsetting the boring operation when boring for a bushing.

[modok]

Good.

[englertracing]

Whats the purpose of offsetting it.
If its r/s ratio, it may not be worth your time unless its a horribly low number.

Also why a smaller wrist pin?

[Mark O'Neal]

Thanks for pointing at trend. I'll see if they have an opinion on the matter.

I'm less concerned with arbitrarily increasing the rod length and more thinking about what aspect of the relationship between rod length, wrist pin diameter, and compression height is going to take priority and drive the design. If I'm installing a bushing anyway, can it be done in a way to gain length? If I'm installing a bushing anyway, can I size the bushing to reduce the pin diameter? Does the smaller pin diameter gain me anything in ring packaging? Does the smaller pin diameter gain me any advantage in weight?

I'm expecting trying for an offset bushing won't realistically allow me to decrease my pin size without going for an unnecessarily thick bushing and that my compression height won't require any unusual ring packaging. I'm also thinking decreasing the pin size won't save me much weight overall.

It's all a balance. I'm trying to find where to balance.

Thank you for your input!

They will. Everyone has an opinion, most people have several.

What motor is it, what stroke and rod length?

[burdickjp]

What I'm thinking about and why is that I'm going for custom pistons anyway. I'm refurbing connecting rods anyway. What can be accomplished within the context of those operations? I think that's the crux of the conversation, and I apologize that it took me so long to be able to say it that way. Sometimes I'm thinking and asking at the same time.

It's a Mazda 1.8L for a 2005 Miata.
The motor has an 85 mm stroke and the stock rods are 133 mm with 20 mm floating wrist pins. The pistons have a 30.5 mm compression height and sit about 0.5 mm down the bore.
I'm looking at switching to rods from a Honda B18C. The Honda rod lets me use high quality OEM Honda rod bearings and provides an opportunity for what we're discussing here. The big end diameter and width is the same. The wrist pin is a 21 mm press-fit. The center to center length is 138 mm. This would give me about 26 mm compression height for the piston.

So rod to stroke ratio:
133 / 85 = 1.565 for the OEM rods. Not terrible. Still worse than the Toyota 4A-GEs I'm used to or the B18C that we're borrowing rods from.
138 / 85 = 1.624 for the Honda rods at their stock center to center length. Better! Not amazing, but better.

It looks like I'd have an opportunity to bush for 20 mm pins with the stock center to center distance as long as it cleans up or I can offset the operation and bush for 21 mm pins with an absolutely marginal increase in length.

If I keep the 21 mm pins and leave them a the same center to center distance I'd have a cornucopia of aftermarket rods at my disposal without having to change pistons. That is probably the smartest of the options and likely what I'm going to go with. I just want to make sure I'm not leaving anything unconsidered.

[user-23911]

So you're thinking that changing the rod to stroke ratio will make a big difference to something?

I can see some benefit if you're stroking it at the same time but otherwise, lots of work, no gains.