I'm working on a T20SM motor, basically a total rebuild. I have found that the crank bearing on the primary side slides on too easily and there is perceptible movement on the shaft. Not much but a slight tick can be felt. The bearing is new (and correct). I'm guessing that any movement of the shaft inside the inside bearing ring isn't acceptable. Am I stuck getting a new crankshaft assembly. Otherwise the crankshaft is good. A nice condition alpha rod and roller bearing that doesn't need replacing. Do bearings come a few thousands tighter, inner diameter? Lots of Locktite or a smear of JB weld? Any suggestions? Peter
Loctite do a product called 'Bearing Fit' which is designed to take up slack in loose fitting bearing/shaft issues as you have. If you look at the full crank drive will the nut on the end have its movement translated through the inner bearing ring and force it against the crank and hence lock it tight then this will help the situation but I would advise regular checking. Alternatively get the driveshaft plated on the loose journal with copper which you can then ease with emery cloth until the bearing fits properly. This would be my prefered option if the facilties are available.
#51062 - 01/16/0711:20 pmRe: Slightly loose bearing fit - what to do
Joined: Oct 2003 Posts: 11,110RF Whatley
BritBike Forum member
I turns out that I wasn't too far off talking about a smear of JB Weld or Loctite. Loctite has a few products for "retaining and securing cylindrical assemblies". I'll list some below. Loctite 660, a quick metal retaining compound, high strength (3335 psi) fills voids up to .02", temperature resistance 300 F, for repairing worn coaxial parts, tapers, bearing seats, key seats.
Loctite 609,general purpose retaining compound, medium strength (1870 psi), fills voids up to .005", temperature resistance 400 F, retention of bearings onto shafts and into housings.
Loctite 680, high strength (2800 psi), fills voids up to .015" temperature resistance 300 F, retaining compound slip fit, best resistanse to dynamic, axial and radial loads, for retaining shafts, gears, pulleys and similar cylindrical parts.
I measured the difference between my bearing and the shaft and I come up with .0007 or .0008. So what would be better fom my application. # 680 or # 609? Perhaps the 609 since it is medium strength and migh be easier to disassemble in the future? What temperature would the primary side crank bearing be exposed to? Peter
On A65's a lotta guys actually emery cloth the crank shaft so that the bearing inner race slides on and off easier. I have never been a fan of this, but once you tighten the crank nut up the race is pinched tight against the crankshaft cheek and apparently does not give problems. When I worked in industry (years ago) we would have expensive machine rolls with damaged shafts that we would heat and "spray up" with a metal compound while turning in a lathe and then resize for the bearing. An inexpensive repair for an expensive shaft.
#51067 - 01/19/071:46 pmRe: Slightly loose bearing fit - what to do
Joined: Apr 2005 Posts: 2,332Mark Parker
BritBike Forum member
If you loctite the bearing on it should come off quite easily with some heat, like from a LPgas torch and a split type puller or screw drivers putting some leverage on it. Not that everyone aproves, but I loctite the clutch taper on my A65 and its super easy to get that off when heated. It doesn't get hot enough in use to let go. Actually the drive side main bearing in it is loctited in the case because it was loose, and its staying put with 7-8 years running 8-12,000miles per year, with 883cc thumping at it. I warmed the case a little so as to put a bit off interfearence on it, but you shouldn't overdo that or it will close up the bearing clearance. I think it would be worth warming the bearing just a little before loctiting it on to a colder shaft.
#51068 - 01/20/0712:19 amRe: Slightly loose bearing fit - what to do
The most important 'thing' about the use of Loctite is the PREPERATION required prior to using it. Bit like decorating eyc eyc... forget the preperation get a crap looking job. The shaft and bearing MUST be totally free of any oil etc. Loctite supply cans of spray for this-Loctite 7063 cleaner and degreaser. Or nick the wifes nail cleaning acetone and cotton swabs? Roughen the bearing and shaft with some fine abrasive paper. Clean again with Loctite 7063....or the wifes acetone? Personally I apply a spray of what I call Loctite N now called Loctite 7649 to the surfaces prior toapplying the actual Loctite and then I leave it untoched for 12 hours.
As for changing BSa mainshafts so trhe bearing becomes a sliding fit on the shaft instead of an interference fit. Did not BSA specify that the bearings were C3 fit which have more internal clearance than the 'normal' CN fit bearings specifically to allow for the increase in diameter of the bearings inner raceway when interference fitted to the shaft??? By removing it then there will be too much internal clearance and who knows how that effects vibration ...