Hi I am new to the forum and new to mx5 ownership although my gf has a MK3 NC 
I recently bought a MX5 Mk2 NB 1999 1.8i 102k FSH. A couple of days in to ownership and it threw its alternator belt off and I noticed the crank wobbling. So my local saab private garage who are excellent in Swaffham took it apart and said the nose of the crank is fine but the woodruff key was slightly bent and the boss had a slight bit of wear. So new water pump, timing belt kit, seals, pulley boss, woodruff key, new bolts and harmonic balancer pulley. All fitted and drove the car home last night no problems. This morning in the daylight I can see the crank pulley still wobbling at idle, not as bad but definitely there. As soon as you hit 1000rpm or higher there is no wobble, the car only has this wobble at idle. I also noticed a slight whinewhirr from the cambelt between 1.5 and 2000 rpm when the revs drop. The mx5 is going back to hannants at swaffham on Tuesday to inspect again for overly tight cambelt. What could cause a wobble to still be there at idle and is it anything to be worried about?
Thanks Robbie
Hi Robbie
Cambelt tension is created by the spring that tensions the belt tensioner pulley. Difficult to understand how it would be possible to overtighten, partcularly when new parts fitted by a garage,
Sorry to say that the only reason I can suggest for the crank pulley to wobble is its fitment to the crankshaft. It does not sound like it has come loose so can only suggest that it was not fitted properly by the garage.
Taking it back to them to inspect and sort out is the best move in my opinion.
First, is the crank pulley itself distorted? If the pulley is true, then is the pulley actually wobbling side to side, or is it perhaps moving fore and aft? It can be difficult to work out the motion of a moving component which is why it may appear to be stable at higher engine speeds. If the pulley is true and solidly fixed on the crank nose, I’d suggest a check for excessive end float of the crankshaft. Worn crankshaft thrust bearings would be the cause of that.
JS
There was an issue with early MK2 cars crankshaft thrust washers which I thought would be passed history by now.
There was a company that did thrust washers for Triumph Sports cars of 1960’s vintage which was always an issue with them. There was a note on their website about MX5 thrust washers.
http://www.customthrustwashers.com/
Here is the Mazda technical note.
http://www.miata.net/garage/tsb/sb00-01-014r-808r2.pdf
Thank you all for replies. I thought my engine at 102k with full history was past the thrust bearing danger mileage of under 35K. The car has no loss of power or miss fires etc? All parts to do with the crankshaft apart the actual crank itself obviously and thrust bearing are new. Is the fact that a small increase in rpm removes the wobble not a good sign? Would a wobble at idle cause wear of the components involved?
I could be totally wrong here, but I seem to remember something about the woodruff key being tapered and needing to go in only one way round for correct seating, otherwise the wobble?
I’m dubious that reduced wobble with increased engine speed is actually the case. It’s virtually impossible to detect that visually. Thrust bearing wear can occur at any mileage, usually high. It was pretty common in the old BMC ‘A’ series engines. It will not result in any loss of power or misfiring. It’s easily measured with a dial gauge on the pulley and a lever to move the crank back and forth. Just lever will do if it’s really bad. Thanks to Drumtochty who has posted links here which very helpfully give the details. If the pulley is properly seated on the crankshaft nose, it should not, indeed, it can’t ‘wobble’. The front of the crankshaft itself can’t wobble laterally either, unless the front main bearing were extremely worn, which would be quite obvious. The function of the key is only to ensure the correct radial positioning of the pulley on the shaft, so things like timing marks are correctly aligned. The key is not there to transmit torque. That’s done by the fit of the pulley on the shaft and the clamping load of the fixing bolt. A damaged key is indicative of relative movement having occurred between the shaft and pulley, maybe because of an insufficiently tightened crankshaft bolt. Such movement could have worn the crankshaft nose, as noted in your original post, and noted by rhino, so a new pulley won’t seat properly.
JS
Thanks for replying. If it is the thrust bearing then is that a new engine or is it financially viable to fix?
Proba.bly a replacement engine not a new one
The old woodruff key was bent and there was some wear on the pulley boss, would a worn thrust bearing cause this to happen?
I would doubt that it’s a thrust bearing issue, those control axial movement of the crank and have no effect on radial movement. My guess would be that the pulley is either not seated correctly, is distorted, maybe from previous attempts to remove it or it is in fact not actually wobbling but it just looks like it is.
That sounds very like the pulley was loose (wobbling) on the crankshaft and had damaged the key. In those circumstances, wear could occur on both pulley and crankshaft. I only mentioned thrust bearing as a possible, because it’s extremely unlikely the crankshaft was moving laterally (thrust bearings only retain the crankshaft from movement along its axis), but from what you’re now saying it seems likely the pulley was moving because it was loose on the crankshaft - much more likely. Please don’t assume the reference to thrust bearings is a diagnosis, it’s was a suggested check that you could do. Has the worn pulley been replaced with a new component? If so does that fit properly on the crankshaft?
JS
EDIT. Robbie beat me to it.
Hi Robbie nice to see another local Robbie I am only 20 miles from Ely. The pulley and boss pulley are both new and the crank appears to be wobbling up-down, side to side motion and not in and out so you are right it does not seem to be a thrust bearing. Strange as the saab mechanics are very competent in Swaffham so struggle to believe they did anything wrong unless of course the new pulley itself is at fault.