Wrist pin bushing reworked

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Like lots of us, it didn't take long to blow the wrist pin bushing on my "80CC" HT.:rolleyes:

I emailed the vendor that sold it to me, as well as several others, and none had replacement bushings available.

So I found some bronze stock and turned a new one.

While I was at it, I made it 16mm wide instead of the orighional 10....more bearing area seemed like a good idea.

The wrist pin was scored too, so I made one of those from O1 drill rod, and quinched and tempered it. It warped a couple of thou in the heat treatment, but I figured it would be OK..

I also made a bushing puller/pusher tool, and a couple of reamers...one to clean up the rough drilled (~14mm) bore in the con rod, and another to ream the bushing after pressing it in.

After ~400 miles the engine started getting just a little noisy (not like when the bushing went, though) and as the cylinder base gasket was weeping drops of hot oil onto my leg I decided I'd best open 'er up and see what was going on.

Well, the new bushing was fine, but remember that warped wrist pin I mentioned above? Turns out a couple thou of warpage is too much. It wallered (technical term) out the piston boss, but only on one side. (????)

Sooo how to fix that?

I thought about sleeving the piston's wrist pin boss, but I don't have a good way to cut a new groove for the snap ring to hold the wrist pin in place.

And as long as I was going to the trouble of sleeving the wrist pin holes in the piston, why not put bronze sleeves in there and TRIPLE the bearing area over stock.....yeah, that's the ticket: see attached photo.

There are oil holes on the underside of the bosses...right where the glare in the photo hides them. You can sort of see the hole on the back bushing in the photo if you are looking for it. Note that when the bushing is in the connecting rod, the bottom side takes the loads, but when in the piston the situation is reversed, so it is correct to have the oil holes on the bottom...thank goodness, as it would be a royal PITA to have to drill them from the top! Note also this puts the oil holes right in the path of oil slung from the flywheels.

I was worried that if I made the bushings too tight a press fit, they would distort the piston and cause trouble. If too loose they might come out under heat. Sooooo....I made them a bit loose, knurled the outside, roughed up the bores in the piston, and glued them in with JB Weld. I made the rough bore in the bushings .375 (3/8") and stuffed one drill bit through the pair to keep them aligned while the glue set up overnight.

I made another wrist pin, and tempered it a little softer, so I could finish turning it AFTER heat treating with a carbide tool. It came out about two tenths (.0002") out of round...that should be OK (crosses fingers). I forget how long the stock wrist pin is, but this one is quite a bit longer at around 44mm. As I explain below, no circlips are used, so bearing area is maximised.

I made a steel bushing for the connecting rod...press fit into the connecting rod AND on to the wrist pin...so the wrist pin rotates along with the connecting rod. The wrist pin is retained only by these press fits. This connecting rod bushing is wide enough that the wrist pin can't contact either piston wall...the faces of the bronze bushings act as thrust bearings if this tries to happen.

I made the new wrist pin .389 instead of 10mm so I could ream the piston bushings with a .390 drill bit and not have to make yet another reamer long enough to do both bushings in one pass (to make sure they are coaxial). I step drilled these from 3/8" with letter drills, so the last pass only took off the last ~.002" and they came out a nice slip but almost no play fit on the wrist pin.

So how'd it run?.....great! It was very quiet, and pulled 27mph up my usual gentle test grade....best I had seen before was 25!

Well, it ran real nice for ten miles or so ....then it siezed solid. :censored: Thank heavens my clutch doesn't grip too well or I would have crashed.

I let it cool for a half hour or so, and was eventually able to turn it around with a screw driver on the crankshaft (the small gear retaining screw).

It made some odd noises for a few seconds then ran great again :D

Except now it makes bird chirp noises every time I start it for the first 100 feet or so. It may be that the piston runs hotter than the 500 degree service rating of JB Weld....or something else might have happened. I haven't looked into this because:

About then the FedEx guy showed up with the real solution to the wrist pin bushing issue: A DAX roller bearing motor!

New out of the box, this had as much power as the modified motor I discribe here. I have about 150 miles on it, and it has gotten noticeably stronger. So I'm posting this for info, and to get others thinking. I am keeping the bushed motor for a spare...and in the unlikely event I have trouble with the DAX engine, maybe I'll update this thread with more info.


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Hi, kevbo
Wow, that was interesting! Thanks for the great information.
As I was reading your post, I was thinking, "Why not just make your own ENGINE?"
I also couldn't help but think about the movie "World's Fastest Indian" as I was reading. If you haven't seen it, I'm pretty sure you'd enjoy it. It's a true story about a guy who starts with a 1929 Indian motorcycle and, through sheer grit, determination and home brew skills, makes it the fastest bike in the world. I think his name was Burt Munro (I'm not sure about the spelling)
You could easily be that guy!

Thanks again for the great information!
Steve G.
Grants Pass, Oregon
I had the same problem. What I did was I reamed my rod with a .680 reamer and used a bluebird wrist pin bearing with the same wrist pin and the motor had no problems after that. It actually ran better.
If you are bored,want to get away from your better half,with time on your hands, have nothing better to do and are patient and well equipped too boot,by all means get one of these HT (Hobby Time) marvels,you will not be disappointed.Plenty of opportunity too for posting hard luck stories etc.
duivendyk: HT keeps me out of the bars, so better half sees more of me!

seabillco: I've never seen the movie, but I have the book. ("Burt Munro Indian Legend of Speed" by George Begg). Being at a point where building an engine from the ground up is a reality is one of my longer term goals. Until I can do my own castings, there is way too much metal to remove before I find the engine hidden inside the stock. PS: Munro referred to the bike as a 1920 Scout...not that there was much of it that came from the factory or was made in 1920 by the time he was done with it!

Jv52: Bluebird???? of course you can't mean a part for a school bus..I'll have to google and see what you meant.
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I hadn't thought of that,might get one too then,cheaper all around!.I 'm not all that skilled in metal crafting but very persistent.There is a French saying: n'est besoin d'esperer pour entreprendre,ni de reusir pour perseverer,which roughly translated says the following :It is not necessary to be hopeful in order to undertake something,nor to succeed in order to persevere.HT work might fall in that category.I had a rather stern task mastress, French Grandmother,who would have thought such activities appropriate in order to to stamp out frivolous inclinations&steel the moral fibre.
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