Moss Engineering

33 Kings Lane
South Croxton
Leics. LE7 3RE

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General Note

The tolerances quoted are those we believe are necessary to give optimum results and are based upon more than 25 years racing and developing Scotts. We accept that racing subjects all engine and drive train components to much greater stresses than normal use. These greater loads cause otherwise moderate problems, to assume critical proportions. These problems then require investigation and solutions.

The Scott engine will function with errors greater than those quoted.

If mileages are limited and higher revs avoided, no great problems should be experienced with errors that exceed those quoted by up to 50% or in some cases significantly more.

The Scott Motorcycle Company were proud of the accuracy they maintained in the production of their machines and demonstrated their commitment by using the “Made to limit gauge” logo. To achieve or better the original accuracy, we use toolroom methods and manufacturing equipment. We have comprehensive measuring facilities to ensure quality.

Crankcase cups

The main bearing “cups” in the crankcase were specially designed by Scott as no standard bearing existed at that time which fulfilled their requirements. These special components were fitted to a warm crankcase and then a heated steel ring was dropped into a face groove above the main bearing cup location diameter. This technique is similar to that traditionally used to fit steel bands to wooden cart wheels. In this case, the steel ring restricts the aluminium housing for the cup from growing with temperature at the higher rate of aluminium compared to steel.. The housing does not grow with heat to the degree that would allow the cup to come loose.

The Scott works then finally ground the internal diameter and gland thrust face of the cup in situ, on a special grinding machine. The internal bores of the cups are thus made in line with each other, but not necessarily concentric with their mounting bores or outside diameter. If a cup has been removed from a crankcase and it is measured, it will usually be found that the bore and outside diameter are eccentric to each other.

Many years have passed from the date of the manufacture of these crankcases with no facilities available to regrind cups in situ. This has forced some rebuilders in the past, to use good cups from a crankcase that maybe was severely damaged by a crank breakage and refit them into other crankcases. In such cases, the cups can be significantly out of correct alignment and with locations that come loose when warm. While such examples may be the exception rather than the norm, it is better to be aware of this possibility. The cups should be carefully scrutinised to see that the bearing rollers have been running evenly and that the spring loaded gland has left an even wear pattern on the thrust face.

The best answer is to have the cups inspected for alignment on a measuring machine, but this is not available to all.

For those who wish to take extra care that nothing is amiss, it is suggested that the case is warmed to a temperature that is the limit that can be tolerated with the bare hands, and the cups checked to ensure that they have not become loose.

For those interested in having the main bearings of crankcases renewed, please see our product list.

For measurement purposes, the thrust faces of cups should be in plane within 0.002”

The clearance for the total roller assembly should be min. 0.0005” max. 0.0015”

Ovality and taper to be within 0.001”

Note. Where cups are in good condition but oversize, special oversize inner rings can be supplied to match.

Where cups are good but unevenly worn, they can be re ground to restore truth and special inner rings made to suit.

Our usual practice if cups have been incorrectly replaced is to fit a new housing to accommodate standard ball races and synthetic seals.

Flywheel runout

The flywheel taper is 1.750” per foot and it is essential that the cranks are knocked up correctly as per our procedure.

If this is not done correctly, then there is a danger that the cranks will become loose and damage will be caused to the flywheel tapers. The flywheel effect dampens the rocking couple of the cranks, as this design is damped not balanced.

The more true the flywheel, the more smooth will be the engine, providing the engine is secure in the frame. It the engine is not secure in the frame, it will vibrate as a unit. See our assembly notes.

Sideways runout at rim to be within 0.005” Radial runout on periphery to be within 0.003”

Observe flywheel rim when engine is running. If it becomes very blurred when the engine is revved, it indicates that any existing eccentricity is becoming worse with increase in engine speed and thus that some items, perhaps the cups, are not stable and are moving under load.

Crank assembly end float 0.010” – 0.012”

If the engine has been assembled out of the bike, then this figure must be re measured when the crankcase or complete engine is installed in the bike. The tightening of the engine bolts, especially the internally unsupported front bolt, can distort the crankcase and alter this measurement. Do not over tighten front bolt. See notes re Vibration.

See our crank assembly notes.

Fit of big end inner bearing bush on crankpin

bush to be smaller than crankpin by 0.0005” – 0.00075”

Ovality and taper to be within 0.0003” fitted

Fit of fitted big end bearing bush on crankpin to Con Rod big end inner bearing ring

Clearance min 0.0005” max 0.0012” Rollers should not be nipped by bearing rings.

Fit of gudgeon pin in con rod bronze “little end” bearing.

Clearance Min. 0.0012” Max. 0.0022” Taper and ovality within 0.0005”

Note. If this fit is too tight or close, then the gudgeon pin tends to be held in the rod LE bearing and rotate in the piston.

This can cause accelerated wear in these piston gudgeon pin bores, especially where Silk pistons are used, as these are less durable in this area.

Squareness and twist of gudgeon pin bore in con rod to the axis of the big end bearing

Error to not exceed 0.001” in every 1” ie. If you insert a gudgeon pin in the con rod and measure each side of the rod little end, the measuring points being approximately 2” apart, then the deviation must not exceed 0.002”.

Note. It is usual to arrange that the con rod big end inner ring protrudes slightly clear of the side of the rod.

The bearing ring is ground in situ in the rod whilst being clamped to a true seating face. The side of the ring is thus used as the datum face.

The big end bearing arrangement on a Scott has insufficient width to fully control any significant tendency of the rod to wag sideways. Rod wag is caused by any significant inaccuracy in the Squareness and twist of the little end in relation to the big end. Hence the importance of this feature.

The traditional method is to insert new little end bushes, ream to size, then to bend the rod until it appears to be in alignment using a test bar through the little end.

First this method of measuring is not sufficiently accurate. Second, any steel item that is bent will retain internal stresses which tend to unwind when the engine is in use and performs the same duty as a commercial vibratory stress relieving machine. What went into the engine seemingly correct, can within a few moments of running, have become incorrect. Just measure the deviation of some used rods and wonder how long the engine worked with them in this condition.

This is not meant to be a horror story, but to draw attention to the importance of accuracy when dealing with con rods.

Our method is to grind the big end and then jig bore the little end bush in a special fixture to ensure alignment.

Fit of big end bearing ring in con rod

Fitting new rings and regrinding con rod big ends is not easy if taper and ovality are to be avoided. As this is a specialised subject that would require lengthy description, we will not set out the details of this operation.

If anyone wishes to try this challenging operation, we will be happy to supply basic process planning on request.

Ovality should not exceed 0.0006”
Taper should not exceed 0.00035”

Piston to head clearance

The flexure and stretch on a Scott crank, con rod, piston assembly is approx 0.027” at 5000 rpm.

We accept that most engines will not be run above 4300 rpm, which is wise if one is to avoid crank failures.

We suggest that an overtravel clearance of not less than 0.035” is used for general brisk use.

Bore tolerances

Reboring deviations should not exceed:

Ovality max 0.0015”
Taper max 0.001”

Piston to bore tolerances

For Scott pistons in an iron bore for normal road work, the clearances are

0.009” at ring land and 0.004” at skirt For fast sustained road work give 0.010” at ring land

For racing 0.0125” at ring land and 0.0045” at skirt

Tolerances plus 0.001” minus 0.000” ie allow more clearance rather than less

Note: Silk pistons have incorrect taper for use in iron barrels and must be reground for this use. As they are lighter they are good pistons to use where higher revs are envisaged. © 2003

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