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Idler gear - Setting end float
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A fairly crucial part of how the idler gear operates is its tolerances and running clearances. End float is a continual problem as folk either ignore it through ignorance or lack of accessible information on how to do it, or belief special tools are needed. Also, later factory assembled engine units (from about 1992 onwards) were built up using whatever shims and thrust washers were available, since Rover were not making regular orders for all shim/washer sizes due to the forthcoming end of production. Consequently, many units left Rover with incorrect (usually too big) clearances. The idler gear was no exception.
Too tight a clearance and the idler gear will either seize solid when it gets hot, or destroy the thrust washer thrust faces in the comparably soft aluminium gearbox and transfer gear housing. The result is the same - possible idler gear bearing failure, but definitely tooth extraction on the idler, and usually both the primary and input gears. A very expensive exercise since these components are not cheap, and the fact that some of the metal debris is almost guaranteed to find it's way into the oil system which will cause considerable damage to oil pump, crankshaft bearings and so on.
Too loose a clearance and the idler gear will be allowed to twist too far out of alignment in the case of helical gears, or wander about in the case of straight cut gears. Neither is good. The result is usually collapsed bearings leading to the gear destruction described above. Not to mention the clattering racket at idle this excessive clearance makes - usually silenced by depressing the clutch pedal.
If some form of failure is experienced, with the loss of a couple of teeth on the idler only, don't be tempted to just replace the idler gear. It is highly likely both primary and input gear teeth were stressed when this occurred. Using them again is likely to result in them throwing teeth - and the mayhem above takes place once more. I know folks out there will say differently, claiming they only replaced the damaged gear. They have 'gotten away with' - and I really dislike that phrase. Be sure and confident in your engine assembly - not nervous and apprehensive. If the gears have seen a failure bin them to prevent them from causing another - if not to you, then possibly to someone else.
Also - the debris from such a catastrophe may not ruin the gearbox casing, but it will find it's way into the bottom of it. There is absolutely no way of flushing all potentially damaging fragments out of a completely assembled gearbox. You MUST strip it down, flush it out thoroughly then re-build it. It's the only way to ensure future reliability.
The best way to set the clearance is by using a DTI, but it is possible to do it using feeler blade gauges if a little care is exercised.
Before any tolerance measurements can get under way, you need to make sure the transfer gear casing fits easily onto the gearbox. Don't worry about including the gasket yet, that shouldn't make any difference. For pre-A+ and some A+ types, with all studs fitted, locating dowels in place and input gear pilot bearing outer race fitted trial fit the casing. The case should slide on easily, maybe with a little 'jiggle' but certainly with no real obstruction. Where late A+ fixings are used, just trial fit case with locating dowels and pilot bearing in place then fit several retaining bolts inside the casing around the idler gear bearing housing area by a few threads. The casing should move easily back and forth on the bolts. In either case, if the casing hangs up severely, investigate why. In most cases damaged or misaligned dowels or incorrect pilot bearing race causes this. Sometimes by a bent stud - but this should be obvious. Check out the threaded parts for aluminium graunched out of the casing.
If it's the dowels, remove one, check condition, clean up any protruding lumps/burrs, check casing fit, then re-fit that one and remove the other, check condition as before then check casing fit again to identify the problem dowel. Do not be tempted to bin the problem dowel and only run with one! They are there for a reason.
If it's the pilot bearing outer race, it is either the wrong one for the bearing. They are size-matched so the likelihood of getting two unmatched components to fit together properly is very, very low. When swapping casings about, it is essential to change the pilot bearing on the first motion/input shaft nose fit its matched outer race into the casing you are going to use.
Once the above is established, thoroughly wash of the idler gear and existing or chosen thrust washers. Fit a thrust washer to either side of the idler gear and fit into gearbox-mounted bearing. Fit the gasket, then the housing. Fit the five retaining nuts/bolts located inside the casing and torque to 18lb ft/25Nm. Yeas, that's right, 18lb ft/25Nm - a mere 'nip' to most of you. It is essential this setting is maintained and repeated with each clearance check otherwise there is no consistency.
This is best achieved before the engine is bolted to the block. Once bolted to the block, access is very much limited making accurate and proper setting a chore. But you must persevere.
You are going to need a ferrous metal plinth for a magnetic DTI stand to stick to. I use a sturdy piece of mild steel plate bolted to a couple of the gearbox to block bolt holes near the idler gear. Alternatively make a bracket for a bolt-on DTI stand/attachment.
Place the DTI pushrod against the vertical face of the gear then pull the gear towards the transfer gear casing with just one finger through the primary gear hole in the casing using a very small amount of pressure. Zero the DTI gauge then push the gear towards the gearbox casing with similar pressure and record reading indicated on DTI.
Feeler blade gauge method
Using two sets of feeler blades will make this method more accurate. Simply select a pair of relevantly size-matched blades, one down either side of the idler gear between gearbox casing and thrust washer. A slightly resistive slide fit is what you are after - like when setting valve clearances. Using a single feeler blade, you must be wary of forcing too thick a feeler blade into the gap. There will be some tilt/twist caused by the support bearing running clearances. Best way to do this is to push the gear up against the transfer gear casing with light pressure from one finger the use the other hand to carefully insert the feeler blade down between the thrust washer and gearbox casing, looking for that slightly resistive fit. In either case, make note of the clearance according to the feeler blades.
The clearance is stated in the official manuals as 0.003"-0.006". Personally I find 0.006" too big in any situation. The nature of helical cut gear teeth is to apply twist/side-loads so keeping the running clearance to the best working minimum is essential for longevity. For non-performance street cars, I use no more than 0.003". For performance street cars and weekend-warrior racers I use 0.004". There is some train of thought that suggests the clearance should err on the big side when straight cut drop gears are used in racing to allow for expansion when everything gets hot. I've never followed this line, as it seems somewhat mis-founded to me. There is next to no side thrust from a straight cut gear, so standard clearances should be more than up to the job and that's what I use as detailed above. If running these clearances causes problems, it's likely something else is causing a problem.
Alteration of the clearance can only be made by changing the thrust washers - either one or both of them dependent on what the reading you have taken is and the thrust washer thicknesses you have. When re-checking new clearance after replacing whichever thrust washers, be absolute sure to maintain those torque settings - and use the gasket you have been using to set them in your final build.
Useful part numbers:
Terminology:DTI - Dial Test Indicator