Moss Engineering

33 Kings Lane
South Croxton
Leics. LE7 3RE

 Moss Engineering | About us | Whats New ! | Scott Motorcycles | Other Interests | Links |
 scott motorcycles | a short history of scott motorcycles | audio and video  |

   Crank Dismantling procedure.

Very Important Note.

Do not under any circumstances try to remove cranks by hitting the central bolt as this energy will react against the sealing glands, cups and crankcase walls of the crank chamber and will risk cracking or disturbing cups, or in extreme circumstances breaking sections out of alum crankcase.

The Book of the Scott is seriously wrong on this point !

This is NOT conjecture, I have seen all these tragedies personally.

The method described was told to me by Tom Ward, who had an unsurpassed knowledge of Scotts as he had worked personally with Mr Scott before the first world war and had been in business supporting Scotts himself for several decades. This method I have personally used for thirty five years and it has proved totally and consistently successful.

  1. Loosen central screw two turns
  2. Turn flywheel till crankpins are at half stroke
  3. Put brass or alum bar approx 2" x 3/8" x 9" down through case mouth on to crankpin bush.
  4. Select small lump hammer of approx 1kg (2.5lbs) witha short handle.
  5. Give bar single medium blow starting approx 8” away.
  6. Rotate flywheel until same pin is at half stroke on opposite side and repeat process
  7. Repeat process on other crank
  8. Repeat approx three times per crank, after which both cranks should be free it be removed.
  9. Where centre bolt has been excessively tightened, or the tapers are unusually well matched, then the process might have to be repeated a few more times, but I have never had this need.

  Crankshaft assembly

Assembly Procedure

  1. Check flywheel and crank tapers are completely clean, undamaged and free of burrs
  2. Build cranks with all main bearing and sealing devices into crankcase
  3. Tighten centre cap screw moderately tight only
  4. Select 20 / 25mm dia piece of aluminium bar x approx 150mm long.
  5. Hold bar against centre of crank end over central bolt fixing.
  6. Select small lump hammer of approx 1kg (2.5lbs) with a short handle.
  7. Get a friend to hold the handle of a 4lb lump hammer against centre of the opposite crank
  8. Give alum bar single medium blow starting approx 8” away. (it is the mass that is important)
  9. Tighten centre bolt moderately
  10. Repeat 4-7 on opposite crank
  11. Repeat to a total of three times per side.
  12. Tighten bolt quite firmly but not excessively
  13. The crank assembly should be perfectly correctly assembled for use.

General notes on assembly

  1. The drive is taken solely through the tapers and the key is solely for the purposes of ensuring correct radial positional location.
  2. There are many opinions regarding the correct end float setting for a Scott crankshaft assembly.
  3. However great the care is taken in assembling an engine, on the bench, the setting can be completely destroyed by the distortion of the crankcase when the front and top rear engine bolts are tightened in the frame. The best assembly is with the crankcase bolted in the frame.
  4. If it is preferred to build the engine of a duplex frame model on a bench, then insert the front frame engine bolt with spacers or a pack of washers to take the place of the frame lugs, and tighten to the degree you would expect to use for the final installation. Next put the aluminium spacer into the top rear engine mounting position and mark top position for rotation. Next shim any free space between the spacer and the crankcase top rear inner faces until you have a snug fit so that the crankcase will not move inwards when the mounting bolt is fitted and tightened. By this method, you will replicate most of the distortion that will occur in engine installation and your end float values will remain fairly consistent.
  5. For single down tube frames, it is only recommended that the crank assembly should be built in the frame.
  6. End float values. I would aim for 0.008” / 0.012” to give allowance for thermal growth of the crankcase. In very hot climates, or for racing, a further 0.002” / 0.003” would be advisable
  7. End float setting. Build crank assembly as above, without any gland end float packing washers and then measure end float with a dial indicator. Dismantle as procedure below and add shim washers equally to reduce the measured clearance to the desired figure and retain best average centrality of the crankpins relative to the bore centrelines. Re assemble and check.
  8. After assembly, the flywheel should run as true as possible on the side faces of the rim and if deviation exceeds 0.005” then the engine will vibrate progressively with the degree of runout. Top face should be within 0.002” of truth and the same comment applies.
  9. Where the tapers in a flywheel have been damaged so that the truth cannot be obtained, then the tapers should be re qualified by grinding out a minimum of metal to obtain a cleanup. This operation is definitely entrusted to a competent toolroom or precision engineering establishment. The taper is 1.75” per foot.
  10. Where the flywheel taper has some moderate damage but on assembly the flywheel is found to run acceptably true, then the following extra security measure can be implemented.
  11. The cranks can be assembled together with Loctite "Bearing fit" which is a gap filler and has no adhesive in it. You clean carefully with spirit solvent or if not washing powder solution and dry completely to remove any grease from all mating faces. Next put a thin smear of Loctite on the parts and assemble with the centre bolt tightened only moderately. Do not “knock up” cranks yet, but wait 24 hours before continuing with assembly stages 4 – 11 incl.
  12. If the engine has been assembled on a bench, then it is ABSOLUTELY ESSENTIAL that the end float values are rechecked when the motor is finally installed and bolted up in the frame. © 2003

email or