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

[modok]

Can you get DLC coated pins?
Works great full floating, no bushing needed.

[burdickjp]

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.

I'm doing custom pistons and servicing the rods either way. I don't see it as much more work to try and get more length out of the rod.

[Dave Koehler]

Hey, go for it.
Bear in mind that everyone is telling you that if you think there is some HP with the longer rod you will be disappointed.
Understand that and you are golden.
However,
1: Getting the center to centers the same is a good thing.
2: If this can shorten the piston pin height you will gain a little more piston stability.

[burdickjp]

Hey, go for it.
Bear in mind that everyone is telling you that if you think there is some HP with the longer rod you will be disappointed.
Understand that and you are golden.
However,
1: Getting the center to centers the same is a good thing.
2: If this can shorten the piston pin height you will gain a little more piston stability.

No, I'm not expecting any difference in power. I'm hoping to beat the bearings up a little less, though.

[Dan Timberlake]

"Getting the center to centers the same is a good thing."

Also if using a setup that locates of the newly finished Big end bore ( like Tobin Arp pin boring machine ), the end result is a " straight" rod, with big end and pin bore parallel in 2 dimensions. Also a good thing.

Neither is necessarily the case when honing the pin end for steel-on-steel full float or to resize new bushings in bushed rods.

In the last millennium in some groups the question of whether to straighten con rods by bending or "bore them straight" was about halfway as controversial as religion, politics or oil.

Acceptable minimum steel wall thickness after boring for a bushing was also often debated.
I thought, and think, the rod eye geometry, writs pin thickness, and a bunch of other stuff probably enter into it too.

[clshore]

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.

Nominal 0.003" press fit on bushings.

Liquid nitrogen on the bushings, and 400F preheat on the conrod small ends yields a slip fit
(but you have to place the bushings quickly before they heat up and expand).

Another way is to hone the conrod ends out to a few thousandths clearance, then have them bronze electroplated
and then honed to finish size, it's a common industrial process.

[ProPower engines]

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.

Its a ton of drama to over come when aftermarket rods could be had to suit the project.
While there is about .200 difference in length between the Acura rod to the mazda rod so I see a benefit there but if the only reason is to get the longest rod you can in the engine just to close up the extra comp. height it seems more costly then just getting the rods you need along with the custom piston that puts the RL to SR closer to what you are shooting for.

Them little engines don't have much for meat at the small end on either engine make making a new rod the best choice to go with.

As far as bearings go there is several manufactures of race bearings that are way better then OEM honda in tri-metal designs and coated as well in a variety of sizes.
As far as widths go its as simple as using a rod to make a jig to machine the shells to suit the rods you want.
I do this all the time to use a better quality bearing where undersizes are not available or just not made because of how popular that particular engine is.

And the aftermarket rods can be had with your choice of finished bushing size to suit what ever pin size you need.

[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.

I'm doing custom pistons and servicing the rods either way. I don't see it as much more work to try and get more length out of the rod.

Suppose your rod is 6 inches or about 150 mm.
You move the centre of the bushing to give 1 extra mm.

That's about 0.6 %


Is it really going to make a difference?

[user-23911]

The only real difference it will make is boring the rod small end 3 mm over to fit a bushing. It'll weaken it too much.

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.

Honda rods on a Mazda?

B18C uses a 45 mm journal (clearance space taken out of the bearing)
Mazda uses a 44.956 journal (clearance taken from crank) with a 45 mm bearing.

One of the common traps?
So you'll end up with double the bearing clearance.


Is that what you want?
So you''ll need a custom crank grind down to the first undersize for Honda undersize bearings.


As a generalisation Mazda and Nissan usually finish the crank at XX.00 mm minus the bearing clearance so it's XX.956 mm dia.
Mitsubishi, Honda, Toyota , Subaru usually finish the crank at XX.00 mm and build the clearance into the bearing so the bearing ID is XX.05 ish mm.

Forget about using inches or any other imperial measurements on jap engines, ..........or anything else built metric, it'll only get you into trouble.They're not designed nor built using inches.

[modok]

Aren't you the same guy who said only yesterday, that's it's real handy to grind the crank to get the desired clearance?
Just kidding.
i still don't see an reason not to do it.
Downsides....none, so, why the heck not?? Just because you can pay somebody else to make a completely new rod? eh? honda makes a good rod. Your not going to improve on that so easily,.

[user-23911]

Aren't you the same guy who said only yesterday, that's it's real handy to grind the crank to get the desired clearance?

Yes, exactly

BUT

The OP isn't expecting to have to get the crank reground.
It adds to the price.

It's OK if you can do it yourself for free.


Lots of people come out with real hairbrained ideas without doing all the homework.
Some just go ahead and do it without talking about it.......but......if you've got to ask lots of questions.........it's too hard in the first place.

Why not just throw away the engine and put something better into it?

[burdickjp]

Its a ton of drama to over come when aftermarket rods could be had to suit the project.

They're around $1000.

As far as bearings go there is several manufactures of race bearings that are way better then OEM honda in tri-metal designs and coated as well in a variety of sizes.

OEM Honda bearings are trimetal construction manufactured by Daido. This is the same manufacturer and construction I got the last time I ordered Clevite bearings for a Toyota 4A-G.

Honda rods on a Mazda?

B18C uses a 45 mm journal (clearance space taken out of the bearing)
Mazda uses a 44.956 journal (clearance taken from crank) with a 45 mm bearing.

One of the common traps?
So you'll end up with double the bearing clearance.


Is that what you want?
So you''ll need a custom crank grind down to the first undersize for Honda undersize bearings.


As a generalisation Mazda and Nissan usually finish the crank at XX.00 mm minus the bearing clearance so it's XX.956 mm dia.
Mitsubishi, Honda, Toyota , Subaru usually finish the crank at XX.00 mm and build the clearance into the bearing so the bearing ID is XX.05 ish mm.

Forget about using inches or any other imperial measurements on jap engines, ..........or anything else built metric, it'll only get you into trouble.They're not designed nor built using inches.

Hondas bearing shell thicknesses run from 1.489 - 1.510 mm
Mazda says bearing thicknesses from 1.500 - 1.512 mm

The difficulty I'm going to run into is that I have two crankpins which are already under 44.938, the bottom of the Mazda spec.

[user-23911]

The difficulty is that even if all the parts were brand new your factory 0.044 ish mm big end clearance (roughly 1.6 thou inch ) will be double (3.2 ish thou inch) that figure if you use Honda bearings.
Add wear to the crank, it's even more.


How much clearance would you think is acceptable?


Bearing thickness, normally about 3 mm difference between journal diameter and tunnel diameter in the rod.The difference being whether the bearing clearance is taken up in the rod journal or by the bearing itsself.

It'll sink in soon?

[burdickjp]

The difference being whether the bearing clearance is taken up in the rod journal or by the bearing itsself.

It'll sink in soon?

Did you miss my reply? The OEM Honda bearing thickness range covers the majority of the OEM Mazda bearing thickness range. Also, my crankpins are under the Mazda minimum, so I'd be turning the crank down anyway.