How 'tight' should a gudgeon pin be in the little end?

[handyman]

Hiya, 
Current project is a little 2020 Honda CB125F, after the previous owner slid it into a ditch and then ran it till it would run no more... (i.e. 'Oh, I didn't know you had to put oil in it' kind of deal)

Picked it up super cheap after the local bike shop quoted 1500GBP to fix it... 

The engine is currently in about a hundred bits on the bench, the cylinder is away to be over bored +0.25mm and whilst I'm waiting I've de-plasticed the bike and removed all the 'old' clunky lookin' lights etc. My aim is to make a little retro-ish looking street/scrambler-esc bike,  for local pootling around, down to the harbour and back, or picking up parts kind of thing. 

Now it's a testimony to Honda that all the bearings are okay, big end is okay, all gears etc. are good. All were full of tiny fragments of piston and I flushed gallons (well, one gallon, repeatedly filtered back into the sprayer!) of diesel and compressed air through them till they sang as free as a bird. Oil pump back plate was scored but rotors survived. Tried flushing without splitting the crankcase but I was uncomfortable with that and I was very glad I decided to do it properly. On disassembly, I found lots of little chunks of piston in several places as well as 'gritty' bearings and oilways. 

 

My big worry was the little end as the old pin was damaged. The new pin came today and after carefully cleaning out the little end with 2000g wet and dry and oil, the new pin goes in and rotates as solid as you like, zero detectable play. BUT, that's my question (eventually!), how tight should it be? I was expecting it to be a tad easier to turn than it is. And if it does need to be 'free', what's the best way to remove said miniscule amount of metal inside the little end? I don't have access to a lathe etc.  Should I just continue with the 2000g W&D wrapped around a 12mm drill bit or is that too crude? 

I really need to not screw the little end up, as the conrods are not available, you have to buy the whole crank assy. at 350GBP

Cheers and ta muchly... 

Edited March 25, 2022 by handyman
sp

[Mississippi Bullfrog]

Is it an interference fit pin? If so you don't want to be removing any material from the rod. 

[MikeHorton]

I'm no help in afraid but I hope you get it back to life. It's criminal that a great little Commuter has been abused in such a short space of time. We need a RSPM royal society for the protection of motorcycles! 

[billy sugger]

There should be no detectable play between little end and the pin, but it should be able to slide through the little end without dragging/catching on anything

[handyman]

2 hours ago, Mississippi Bullfrog said:

Is it an interference fit pin? If so you don't want to be removing any material from the rod. 

I guess that's what I need to find out. I'm thinking yes, and now I've had time to think about it, the pin doesn't need to rotate 'easily' does it? It will only be going through a few degrees as the piston goes up and down. Totally different to the big end which rotates fully. 

[handyman]

11 minutes ago, billy sugger said:

There should be no detectable play between little end and the pin, but it should be able to slide through the little end without dragging/catching on anything

Yes, I think it is okay, definitely no play. I think I was just confused for a minute, thinking it doesn't rotate very easily. But it doesn't have to, it pushes in quite easily by hand so... 

[handyman]

23 minutes ago, MikeHorton said:

I'm no help in afraid but I hope you get it back to life. It's criminal that a great little Commuter has been abused in such a short space of time. We need a RSPM royal society for the protection of motorcycles! 

Gosh yes, especially the CB125F which have legendary reliability status if kept full of oil.

I'm still gobsmacked when I look at A, how bad the oil looked and B, how very little of it there was and C, just how good it was inside considering A+B lol!  

Worst of all, this 17 year old kid had blown up this bike and his parents just bought him a new Yamaha MT125 to replace it... shocking. I know what I'd have got if I'd blown up anything as a kid, let alone a brand new bike... 

Edited March 25, 2022 by handyman

[MikeHorton]

Throw away society sometimes. My 1st new bike was an xv250 virago in 1995, my dad took me to the bank to help sort my loan but it was my debt my parents taught me to budget and stand on my own 2 feet. I was so proud to have it and it was my pride and joy. On £75 a week as a YT I soon realised the value of money. 

If I had the money and room for  2nd bike I really would like a new supercub bizarre I know

[handyman]

9 minutes ago, MikeHorton said:

Throw away society sometimes. My 1st new bike was an xv250 virago in 1995, my dad took me to the bank to help sort my loan but it was my debt my parents taught me to budget and stand on my own 2 feet. I was so proud to have it and it was my pride and joy. On £75 a week as a YT I soon realised the value of money. 

If I had the money and room for  2nd bike I really would like a new supercub bizarre I know

Supercubs are an icon!  I know I'm getting older, but I reached a point where I just wanted a little bike to run around on, errands and the like. Something light, and very easy to ride. This 125 came up cheap and I thought why not? I was looking for a 250, sadly not many around these days since nearly everyone went horsepower crazy. I don't know about the roads where you are but here, speeding is getting pretty close to drink driving in the "public enemy No. 1" stakes around here... 

[MikeHorton]

1 minute ago, handyman said:

Supercubs are an icon!  I know I'm getting older, but I reached a point where I just wanted a little bike to run around on, errands and the like. Something light, and very easy to ride. This 125 came up cheap and I thought why not? I was looking for a 250, sadly not many around these days since nearly everyone went horsepower crazy. I don't know about the roads where you are but here, speeding is getting pretty close to drink driving in the "public enemy No. 1" stakes around here... 

Indeed its more fun to ride a slow bike fast than a fast bike slow

[handyman]

Well, I just did a bit more reading around and it seems that the gudgeon pin or wrist pin as some call it, should be "thumb tight". Obviously a very technical term lol!  
Reminds me of my dad, he used to say it was a 'feel', you just knew when it was right, as an engineer. So I'll have another look tomorrow, but this has set my mind at rest as I'm fairly confident it is okay. Also read that the clearance will increase as the piston gets hot, which is something I hadn't thought about. 
Cheers guys, was good to write about this to get the grey matter working  

Have a great weekend

Edited March 25, 2022 by handyman

[Bianco2564]

Are there circlips holding the pin in the piston?

If so it won't be an interference fit, unlike my 1275cc Mini has no circlips and you have to heat the rod up and use a press to push the pin into the piston and rod assembly.

 

[handyman]

2 minutes ago, Bianco2564 said:

Are there circlips holding the pin in the piston?

If so it won't be an interference fit, unlike my 1275cc Mini has no circlips and you have to heat the rod up and use a press to push the pin into the piston and rod assembly.

 

Yes, a pair of circlips. It pushes in manually with my thumb in the cold garage, so I think it's okay...  

I've seen Allen Millyard do that, heating up the pistons in his trusty BBQ before fitting the cold pins lol! 

Edited March 25, 2022 by handyman
add info

[skyrider]

22 hours ago, handyman said:

Gosh yes, especially the CB125F which have legendary reliability status if kept full of oil.

I'm still gobsmacked when I look at A, how bad the oil looked and B, how very little of it there was and C, just how good it was inside considering A+B lol!  

Worst of all, this 17 year old kid had blown up this bike and his parents just bought him a new Yamaha MT125 to replace it... shocking. I know what I'd have got if I'd blown up anything as a kid, let alone a brand new bike... 

i would have got walloped 

[Tiggie]

A girl at works boyfriend wanted a bike. I gave him my recommendations of a second hand Honda/Yam etc.  Despite them having little money he insisted on having new so got a chinese bike of some description on finance.

 

He wasn't happy with it and was always complaining something wasn't right. I'd go look and apart from some loose screws on the fairing and the headlight connector melting it ran fine. One day he told her it was broken and wasn't working. She asked me to have a look and though he wouldn't admit it I could tell he'd let all the oil out and rode it until it seized.

 

2 weeks later he's riding around on a brand new Yamaha YZF-R125 also on finance 

[bud]

They are great little bikes. My mate did 24,000 miles in a year and a half on his. Only replacing the chain and sprockets, and setting the valves. 

[Grumpy Old Git]

On 25/03/2022 at 20:12, handyman said:

Yes, I think it is okay, definitely no play. I think I was just confused for a minute, thinking it doesn't rotate very easily. But it doesn't have to, it pushes in quite easily by hand so... 

Exactly as it should be.