Dumb question time, of wrist pins & modifying pistons

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Rogue-AE95
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Dumb question time, of wrist pins & modifying pistons

Postby Rogue-AE95 » Wed Sep 23, 2020 7:22 pm

This is probably more of a general engine question, although the 4A-GE uses both types of wrist pin attachment. From looking at parts diagrams, it appears that the early 18mm press fit wrist pins are pressed directly into the steel of the con rods. The later 20mm floating wrist pins ride inside of a bushing (aluminum-bronze?) that is itself pressed into the rods.

The question is. Should press fit wrist pins only be pressed into the steel of a rod?

If you could have custom aluminum-bronze bushings made, would it be a bad idea to press wrist pins into those? From some reading around the internet, it seems like that is, in fact, a bad idea. Would it be possible instead to have steel bushings made, in order to use a press fit wrist pin in a rod that was meant to accept a larger pin? Or to modify the small end in some other way?

And about pistons. Is it possible to have cast aluminum pistons, previously only intended for a press fit rod, machined in order to fit them with wrist pin locks (and use them on "floating" rods)? From one page I was reading, it sounds as though at least one of the Honda guys out there had this done for a set of his otherwise-stock (press fit) pistons.
'88 Corolla All-Trac

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jondee86
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Re: Dumb question time, of wrist pins & modifying pistons

Postby jondee86 » Thu Oct 08, 2020 11:45 pm

If a question makes people think, then it is a good question :)

The general consensus seems to be that floating pins are better for high performance engines
and that they are easier to work with for assembly and maintenance. Pressed pins are a lower
manufacturing cost option for standard performance engines.

A successful press fit requires the rod to be capable of holding the tensile force and the pin the
compressive force generated by the press fit. A steel pin in a steel rod is the best option. For a
floating pin steel on steel is not suitable as one surface must be softer than the other to avoid
galling. Therefore the bush is made from a softer material and is a sacrificial component.

As you have discovered, using a softer material to bush down a rod to take a press fit pin is not
a good engineering solution. However, using a steel bush would be possible providing you don't
mind a bit of trial and error in getting the bore size for the bush correct. A 1mm thick bush
will compress when pushed into the rod and the pin will need to expand it into the rod to obtain
the necessary contact pressure for a correct fit.

Unless a "top hat" bush is used, there may also be a problem in preventing the bush from displacing
axially when the pin is introduced. The pin will have to expand the bush into the rod to get the
required contact pressure between the surfaces. Since the rod will not have been designed for the
stresses imposed by press fit I would not suggest using this solution for a high performance build.

I believe that there are companies that make rods with the 42mm journal and the 18mm pin. So
considering the amount of messing around and machine shop charges to modify used parts, the
viability of what you are exploring is questionable :)

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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Re: Dumb question time, of wrist pins & modifying pistons

Postby Rogue-AE95 » Fri Oct 09, 2020 4:14 am

Thanks Jondee. The rods I'm thinking about this for are Honda, which all use press fit wrist pins. There's a little bit of old school upgrading for their D16 engines (very skinny rod) using a B18A/B / B20 con rod (more substantial rod).

Some material is shaved down on the big end to make it narrower, and in the small end, a bushing is pressed in as the pin sizes are 19 mm (D16) and 21 mm (B18). From my research it seems that people doing this are using aftermarket "floating" wrist pin pistons and a set of custom bronze bushings. I'd be using a set of stock pistons, which require press fit. Unless the pistons can be modified to accept a clip / snap ring, then they could be floating... which is another thing I found when reading.

This mod has been done successfully in the past, but again, with bronze bushings and aftermarket floating wrist pin pistons. Unfortunately most D16 aftermarket rods require grinding of the bottom end in order for the rods to clear (don't have to using the B18 rods). I don't like the idea, or having to buy aftermarket rods when I can have a widely available & affordable set of stock rods modified instead.
'88 Corolla All-Trac

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Re: Dumb question time, of wrist pins & modifying pistons

Postby jondee86 » Fri Oct 09, 2020 1:30 pm

Have you considered using buttons ?

Image

If buttons allow you to use the standard pistons with a full floating pin and
custom "bronze" bushings, then problem more or less solved :) You would need
to get the machine shop to drill an oil hole in the rods for the bearing when you
send the rods out for grinding, and clearance the buttons so they are not full
time rubbing on the cylinder walls.

Cheers... jondee86
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Re: Dumb question time, of wrist pins & modifying pistons

Postby Rogue-AE95 » Sat Oct 10, 2020 7:53 am

I hadn't heard of buttons, or I'd forgotten about them. It's an interesting idea, although from one article I just read, they sound like they're intended to be used for race engines (torn down often). It seems that they basically sit in the space where the pin enters the piston, and the cylinder wall keeps them in place?
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Re: Dumb question time, of wrist pins & modifying pistons

Postby jondee86 » Sat Oct 10, 2020 3:12 pm

There are a lot of different designs for buttons, but the one I had in mind was a simple
teflon (PTFE) button... a variation of this...

Image

The actual design and dimensions will be dictated by the piston and pin design. Buttons
are only suitable for full skirt pistons, not the slipper skirt type. The buttons shown in
the pic would be a push fit into the pin and a sliding fit in the piston for extra support.
If the pin is longer the "head" of the button will be much shorter.

The installed length of the pin plus buttons allows some clearance so that the button
does not rub continuously on the cylinder walls. The pin is free to float and rotate. None
of this is new technology and buttons have been around for decades. Like all pin retention
techniques there will be those who use buttons and love them, and those that don't use
them and hate on them :)

If using buttons solves a problem then they are worth considering.

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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Re: Dumb question time, of wrist pins & modifying pistons

Postby Rogue-AE95 » Sun Oct 11, 2020 7:02 am

Thanks Jon, for some reason in my searching I hadn't seen Teflon buttons. I'll have to call up some machine shops in my area (few that there are) and ask about this. I'm sure there won't be anything off the shelf. I think I'd rather have this set up than trying to have a steel bushing made for the rods (for keeping the wrist pins press fit).
'88 Corolla All-Trac

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Re: Dumb question time, of wrist pins & modifying pistons

Postby oldeskewltoy » Tue Oct 13, 2020 7:27 am

I wont go into a lecture.......

A long time ago (mid 1970s, when I worked for VW) I learned the difference between "it will fit", and, "designed to fit THIS application". I've found I NEVER get the results others seem to achieve by taking short cuts - I do still test this on occasion.
OST Cyl head porting, - viewtopic.php?f=22&t=300

Building a great engine takes knowing the end... before you begin :ugeek:

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