Crankshaft spec 180 1000cc

Just curious. What would make a rod want to move sideways and beat itself to death against the crank web? There shouldn't be any axial forces on a rod at all.
Seems like it would take some creative engineering to make it go sideways. Maybe an off-centre bearing, misaligned rollers in their cage, non parallel (tapered) big end pin, bent rod, big end eye not honed true ... ?

Cam
 
Cam,

The crank pins are very rarely exactly cylindrical, usually a small taper somewhere to be found, around 0.005mm is quite common.  If the bearing is too tight a fit, the slightest taper will cause considerable side thrust.  Of course, bent rods will also create issues, as would out-of-kilter big end bores.

Had a '74 750 SFC web repaired yesterday, big end bearing had gone and ruined the crank pin in the process.  Ground undersize at the bearing surface, filled with weld and reground.  Grinder measured a 0.008mm taper on the unworn portion alone, came out good at the end though.  Nice, round and straight now... 

piet
 
If you get a proper engineer they pay attention because that's what we do, the problems start when the have a go's have a go. Just pressed a Ducati 900ss crank apart this afternoon, the drive side flywheel is fucked because the have a go pressed the crankpin in a bit pissed and just kept going ....


    Keith.
 
well ,My Crank jig material is ordered and ready for delivery,,, ;)  so on Friday i can get my hands on a couple of  pounds of STEEL and spend the better part of the  weekend in the shed ,in front of the "mill" ....
hrm... I do need to get hold of a reamer for 25 mm+ (H8)...
stay tuned..
 
The clearance of .0012" to .0015" for the rollers seems a bit excessive. Although I have not ever had to overhaul a Laverda crank I have done many other bike big end bearings (BSA, Ariel, Velocette, AJS..........etc) and I try to have the clearance at .0005" Now granted a Laverda is spinning at a much higher RPM so maybe that's the reason for the bigger clearance. On many bikes .0015" is considered clapped out.

Now for the other end of clearance, a very old HD shop I use to hone rods for bikes I am working on set up HD's at a slight preload. To me that sounds bad but he swears that's they best way to do it.

Bruce
 
1200ts said:
The clearance of .0012" to .0015" for the rollers seems a bit excessive. Although I have not ever had to overhaul a Laverda crank I have done many other bike big end bearings (BSA, Ariel, Velocette, AJS..........etc) and I try to have the clearance at .0005" Now granted a Laverda is spinning at a much higher RPM so maybe that's the reason for the bigger clearance. On many bikes .0015" is considered clapped out.

Now for the other end of clearance, a very old HD shop I use to hone rods for bikes I am working on set up HD's at a slight preload. To me that sounds bad but he swears that's they best way to do it.

Bruce
Click to expand...

Sounds strange to me as well.

When racing 883 Sportsters, we'd use well-used rods and hone them round again to fit over-size rollers, preventing further distortion.  Used 0.015mm clearance iIrc, equates to 0.00075".  Was good again with new rollers at mid-season overhaul, competitors changed their cranks every event... :D

piet
 
sfcpiet said:
Used 0.015mm clearance iIrc, equates to 0.00075"
Click to expand...

0.015mm = 0.00059"
0.00075" = 0.019mm

Piet - not a huge difference between 0.015 and 0.019mm, but which one did you mean?

I don't mean to be picky, but this is a critical discussion and I'm following all of these recommended clearances with interest.

Chris
 
beauchemin said:
0.015mm = 0.00059"
0.00075" = 0.019mm

Piet - not a huge difference between 0.015 and 0.019mm, but which one did you mean?

I don't mean to be picky, but this is a critical discussion and I'm following all of these recommended clearances with interest.

Chris
Click to expand...

When we're getting into 0.001mm, it is quite difficult for all to achieve exact fugures.  Differences in ambient temperatures can play havoc with these figures, as can the use of different measuring instruments.  Thes figures can only be a guideline.  The hundreths are what count, what happens between them is often luck... :D

No need to be too picky with the Lav crank tolerances.  Follow Keiths' advice and all will be good, on the tight side for a road crank, looser for a race crank.

If you want to be really picky, 0.00059" = 0.0149mm. :D

Surprising, isn't it, Bazzee? :D  Although the technology may be seriously dated, the manufacture and materials used are top-notch!

piet
 
I think I'm getting somewhere.

I just got a call from crank builder #2.  He found crank builder #1's error (crank assembled with ZERO clearance at the big-ends!!!  This is after I had been assured there was 0.02mm clearance!)

He also found a slight taper to the crankpins (@ 0.005mm) which Piet indicates is not unusual.  Crank will be going back together tomorrow, with the rods honed correctly.

QUESTION:
Assuming a tapered crankpin (0.005mm diameter difference at the ends), do I specify the big-end clearance (let's say 0.012mm) from the center of the crankpins, or the bigger or smaller ends?
 
at least machinist #2 can measure, you are indeed getting somewhere now
is #1 paying for #2 ?

I would use 0.012mm as the maximum clearance, not the minimum

HTH
Red


 
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