Connecting Rod Bushings

[Matt Wilson]

Anyone know the correct interference (press) fit between the connecting rod and piston pin bushing? This is for the 265 flathead I'm building, but I think it would be about the same for all the flathead, as I believe they all use the same size bushings.

Some bushings were bronze (or brass), while others are bronze/brass over a steel backing, so that could make some difference in the fit.

Also, should the bushings be burnished then honed to final size? My manuals say nothing about burnishing them, but I see folks online saying burnishing is really the best way, though they are not necessarily talking about our flatheads.

Thanks.

[Matt Wilson]

Got an update. I called a couple of machinists and they couldn't tell me how much interference there should be. I even found a copy of AERA connecting rod manual, and while it provides this kind of info for some engines, it doesn't for our flatheads. I then called Sealed Power yesterday and they told me the bushings need to be installed with about 0.004 - 0.0045" of press fit. I thought the rep didn't know what he was talking about and so did my machinist. That sounds like a ludicrous amount of interference. So I dropped into another machine shop - a big outfit - and talked to their head machinist, who has been doing engine machine work for 40 years, and he said that was about right for bronze bushings, which is what these are. I thought they were going to be steel-backed bronze, based on the description on NAPA's website, but they are bronze all the way through. I know this because I got a hold of three different parts (three different part numbers - one standard outside diameter and two different oversizes) and looked at the parts myself, and they are bronze. Anyway, the machinist said if the interference is only 0.0015" or so, which is what I was thinking, then the bushings will "fall" out. He also said burnishing them to expand them out against the rod bore is the best way to do it, but not many shops have the setup to do this, but he does. He also said such a large press fit is not needed for steel bushings, nor is burnishing. I got the impression that steel bushings only need about 0.002" interference, bit I'm not sure how accurate that is.

Further confirmation of the tight press fit for bronze bushings is the fact that when I measured the three different bushings I got a hold of, the ODs were all 0.004 - 0.005" larger than advertised online (again by NAPA as well as other sources). I guess when the OD is stated, they mean the final installed OD, which is essentially the size of the rod ID, but that was not clear from the specs they provided. I think I'm starting to understand how this works, though.

Since the machinist I spoke to this morning really seemed to know what he was talking about, I went and picked up my rods from my previous machinist and am planning to take them to this new guy.

I'm posting this in hopes it will help someone else at some point, maybe even me.