The Knowledge - Velocette Owners Club

www.croesus-consultants.co.uk/v-tec/ If anyone need dimensions of main bearings I have access to
some of the original works drawings including big end and outer race. As per 's post I suggest you read
"Tuning for Speed" on how to lap big ends.
L #2027 I've managed to find a pair of flywheel webs machined for the mainshafts but not for the
crankpin (venom). Could someone give me a measurement for the gap between flywheel halves
(conrod + .008" ?), and measure the distance from the flywheel boss to the end of each mainshaft (i.e.
how far does each mainshaft protrude from the crank web)
#2040 If it helps the shoulder to shoulder measurement of the pin is 0.8125 + 0.002. All flywheels I
have seen have the mainshafts flush with the inside of the flywheels.
L #2154 Anyone know what the crankshaft side float should be for an all ally engined MAC? Got a
feeling that I read `less that 1 thou' somewhere. This seems pretty tight to me.
#2157 According to the BMS generic Velo workshop manual, side (end) float is 0.0015" for all engines
except for the MSS which has tapered bearings. e
#2162 Thanks for the reply. 1.5 thou seems pretty tight to me. Isn't the MAC the only Velo single that
doesn't have taper roller mains?
#2163 The Velo singles that had taper roller mains were VM, VR, Alloy MSS, Late iron MSS. All the
others have ball or roller mains. If you set up the mains with 1.5 thou., i.e. nothing, there will be much
more at running temps. With the design of the timing gears etc this is the only correct clearance. It is
normal to set up the OHC Velo engines with no clearance at running temperature. This gives a slight
nip when cold which disappears when near running temp. After some running there is then a free but
not loose bottom end. The same applies to all 'non-taper roller' Velo bottom ends.
#2184 I've done it this way on a number of bikes with success (end play adjusted for the current victim
of course) so I am going to once again quote the BMS Velo manual:
"With main bearing outer rings correctly shimmed and in the crankcase, the timing side half can be
threaded over the shaft. Set the connecting rod in the cylinder opening and turn the half assembled job
over to fit the drive side crankcase. Check all crankcase oilways and freely lubricate all bearings. Fit the
bolts and tighten up the nuts gradually all the while checking the flywheel assembly for freedom of
rotation. At the first sign of tightness, stop work and measure the gap between faces with a feeler
gauge. If it is less in some places than others than the bolts have not been tightened evenly. Correct
and check again. MSS engines with taper-roller bearings must be shimmed so that all side play is
removed with the faces apart 0.005" and no more. The shimming is then correct and the drive side
case can now be again removed, and the face lightly smeared with "gasket goo" and reassembled. If
the crankcases meet, or in the case of the MSS engine, the faces come closer than 0.005" before the
bearings are nipped, extra shimming may be required. All engines except the taper rollered MSS
should have the flywheels perfectly free to revolve when the engine is cold with side float of not more
than 0.0015.
L #2385 I had a fairly dramatic failure (in terms of disconcerting noises) at about 80 mph last
week and, after stripping the engine, the drive side main bearing has broken through the end of the
bearing housing, the result being a thin disc of aluminium + shim that use to be attached to the
crankcase. Although relatively new to Velos, I understand why it has happened (I didn't build this
engine, so I don't know how well or badly the crank axial float was set up, nor whether the bearing
housing was damaged at some previous time in it's 40 year life). Anyway, the obvious option is to find
another crankcase half or pair and rebuild. However, I've spent the afternoon taking measurements of
the whole bearing housing and crankcase around that area and it seems to me that a repair in the form
of an aluminium "top hat" could be turned and tig welded to the crankcase. The crankcase walls are
around 0.4 in. thick in this region ...not exactly over generous, but sufficient to attach aluminium of ~ 1/4
- 5/16 in. thickness. The primary chain back piece of tinware will need a few mods, but that's trivial. Has
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