Maybe I'm paranoid...

[georgesgiralt]

Hello !
I'm starting to gather the parts for a complete renew of my 200 000 km+ R65 motor.
So I began to buy new cylinders in order to buy the correct fitting pistons.
Funny BMW sent a "A" and a "B" jug ! So I've to order one piston "A" and one "B".
But inspecting my bike I saw that it was fitted at the factory with 2 size "A" jugs.
Is it somewhat important or ?
Thanks for your help.

[Barry]

I can understand your concern and wonder why they couldn't be bothered to send you a matched pair but the nominal difference in bore size is only 0.01 mm or 0.0004" which  I doubt makes any practical difference.  I'd still want to check the piston weights and balance them.

[georgesgiralt]

Thank you Barry !
I'll watch the pistons when I get them. And I bet I'll have to grind one of them a little.
I just don't know which part to grind ...
But wait and see...

[Tony Smith]

And I bet I'll have to grind one of them a little.
I just don't know which part to grind ...
But wait and see...

For trivial adjustments take metal from the bottom of the skirt, for more serious amounts the underside of the pin boss.  With modern pressure cast pistons it is rare to see differences of more than a few thousandths of a gram. The last new ones I checked were accurate beyond my ability to measure the difference.

[georgesgiralt]

Got the pistons today. Also from Mahle.
Came with the wrist pin, the two clips and all the rings.
I'm happy, they bear the same weight marks !
But they're not the exact same design as the original. The ridges above the top ring are not here anymore. And the wrist pin is free to move in the piston. No need to heat the piston to move them.
I'll weight them tomorrow to see how large the tolerance is at Mahle factory...
Boy, they look gorgeous !

[Rob Valdez 79 R65]

Boy, they look gorgeous !

What!  No pictures?

[georgesgiralt]

OK, I take some picts when I weight them...
Don't know if I will have time today. The wife has plans ...
Here they are ...

[georgesgiralt]

Funny, the image is upside down in the Browser and correct in my computer  ?
Another one

[georgesgiralt]

Again

[georgesgiralt]

As you can see, they're gorgeous but different from the original ones.
The weight is about 2 grams apart. Should I grind some metal from the heaviest ? (they're the same weight class by BMW standards) And if yes, where should I grind to get 2 grams off ?
Have a nice day !

[mrclubike]

I tried running a pair of pistons that were 7 grams different
I could notice the vibration
So I pulled it apart and balanced them to.1 grams
To remove that much I had to remove a little from everywhere  inside the piston except for the lower  machined area of the piston skirt
(I assumed that is where they were they balanced them at the factory)
  I took as much as I could from the wrist pin boss
After 3000 miles it has not come apart
After doing it I wondered if it would have been better to take some out of the inside of the wrist been also
My 2 original piston were only .2 grams apart
I would definitely balance them even with only 2 grams difference

[georgesgiralt]

Hello Guys,
I need some help.
I've bought from BMW the gudgeon pin bushing for the connecting rod.
P/N 11 24 0 618 100 which are the correct P/N for my bike (they fit almost every engine BMW has produced and are less expensive for motorcycles than the car counterparts....)
When young I had been taught that the small end of a connecting rod has a bushing with a large interference fit. Once fitted, a hole is made for the oil passage and the bushing is machined to match the gudgeon pin properly. There is also another way which consist using a thin cylinder covered in non friction material. This cylinder is split and has a diameter bigger than the gudgeon pin. Once fitted to the rod, the inside diameter magically matches the gudgeon pin dia.
I've got a pair of the former. But they do not fit the pins. they are way too small so they won't properly adjust when inserted in the connecting rod....
I complained to the BMW dealer. He says this is the correct part because the correct P/N is printed on the satchel...
So I'm puzzled !
So could you please, if you can, post a picture of the P/N 11 24 0 618 100 if you have one ?
Many thanks in advance for your help.

[Tony Smith]

They are the correct part!

However they are not supplied "finished". You need to:-

1/. Buy the correct sized reamer and finish them yourself after pressing them into the rod (not recommended unless you have a lathe or mill as whilst bores can be hand reamed accurately, it takes time and practice, you will most likely not get it right the first time.

2/. Buy an adjustable reamer and "sneak up" on the correct size. This is actually a sensible suggestion as a fixed reamer is only good for one hole whereas an adjustable reamer can be used for many  sized holes within a (admittedly narrow) range. Again though, unless you have used one before, or have a lathe or mill you can put it all in to maintain accuracy - I would not recommend it. The good thing is that adjustable reamers can usually be found very cheaply at 2nd hand tooling shops because they simply are not used much anymore (because customers baulk at the hourly charge for skilled hand work).

3/. Take the connecting rod, gudgeon pin and new bush to a competent machine shop and say "Sir, please press the bus into the rod and then ream the bush to fit this pin. Oh, and bye the way Sir, it is going into an air-cooled motorcycle" The last bit of allowing the machinist to select the running clearance is because I failed to find the BMW specified fitted clearance in the brief look I had just now.

[Tony Smith]

I tried running a pair of pistons that were 7 grams different

That looks like borderline criminality on the part of the manufacturer, I am astounded that two modern pistons could differ so much in weight - one or both should have been condemned at the manufacture.

I would not have tried running them and would have returned them for refund or replacement. Idly I just started the force calculation based on 7g weight at approximate maximum piston speed and I did not like the guesstimate answer that popped up. I am not surprised that your could feel a vibration, I suspect it would have been VERY noticeable.

[wilcom]

However they are not supplied "finished". You need to:-

I took option #4

The first motorcycle engine I was ever in was my R50/2 after I had dropped a valve. Being a "car guy" at the time I had no idea BMW didn't give me a part that I couldn't use. I called around ( pre internet) and found that this wasn't exactly a 283 Chevrolet and yes I had to ream the parts to fit.

I took my Brake hone for wheel cylinders and reamed it with that.  Where in the Hell am I going to get a reamer the proper size on a Tuesday afternoon in 1974

Before the World had gotten so small with the Communications we have today,  you fixed it any way you could