Hi MrAlex,
Let me be sure I'm understanding correctly...
You are seeing just the belt pulley wobble, right? Not the entire extension? Or the fan out on the end of the extension?
In any case, it's not likely you have a crankshaft problem.
If just the belt pulley is wobbling, well it's rather cheesy, made of two halves of pressed steel. Likely it got bent somehow. It also might just be loose, there are a series of bolts around that hold it to the extension hub. If loose, they can be tightened without much disassembly. They should have Nylock nuts to prevent loosening, though.
If the pulley is bent, it is probably best replaced since it can be hard to straighten accurately. The major vendors all supply the parts.
Unfortunately, you'll need to remove a number of things to bolt on a new pulley, since it attaches to the *back* of the flange on the extension. Start by removing the radiator shroud, then drain the coolant system, then remove the radiator. Next the large bolt in the end of the extension needs to come out, then the extension can be pulled forward and off the end of the crankshaft. Some folks drop the steering rack out of the way, but don't remove it entirely. I've been able to get by without doing that, but that's with the early type rack.
When you remove the extension by pulling it toward the front of the car, be careful of the Woodruff key between the end of the crankshaft and the extension. Try to avoid it dropping down into the front engine cover, which would probably need to be removed to retrieve it. Also watch out to not damage the lip seal in the front engine cover, that rides on the rear of the extension. Inspect the rear of the extension, for any gouging or damage that might effect that lip seal, too. (If rough, it might damage the seal... a Speedee Sleeve can be put on the extension, if necessary.)
These warnings may not be all that important if you are planning to remove the front engine cover and replace the chain tensioner, too. If you do that, try to get the silver replacement tensioner, not the black one. The black ones have been known to break pretty frequently. Many folks reuse the old, original tensioners without much concern, so it might not need replacing. But a good inspection might be a good idea, depending upon the history of the car. If the tensioner has little wear and isn't cracked or broken anywhere, it's likely safe to keep using. The original ones seem to last a long time and never lose their springiness. The spring tensioner often gouges the front plate of the engine, but that's a major job to remove and correct, requiring a lot more disassembly to get the plate out, have it brazed and then grind it back down smooth.
If you take the front cover off to replace the chain tensioner, you should plan to replace that lip seal in the front cover. It's just worth doing while it's apart, rather than redo the whole job later if a re-used seal starts to leak. You'll need a new gasket for the front cover, too, unless you make your own.
A couple things to watch if you remove the front cover. There are usually a few shims behind the extension, that serve to help line up the pulley with the ones on the water pump and the generator. There is also an oil spinner on there, to help prevent leaks at the lip seal. Just be sure to note how it's installed and put it back on the right way, if you remove it.
This can all be done without removing the fan from the extension. However, if you do want to remove it for any reason, watch the balance weights on it and mark their location. There should be a hole drilled through the weight marking it, about 1/16" diameter, but that's not always the case. Usually there is just one weight.
If the entire front hub/extension/fan assembly is wobbling, that's most likely just the large bolt at the end of the extension is loose. This might indicate an imbalance, though, that has caused vibration and loosened things over time. Again, check to see if there are any weights (sort of crescent shaped piece under one of the bolts) or if it might have been removed in the past. More rarely, I've also seen balance weights on the bolts at the pulley.
Unfortunately, no one seems to know how to re-balance this assembly. There are no instructions for balancing in the service manual.
If it's only the fan that is wobbling, Bob is correct and new bushings and bolts are a good idea. The Roadster Factory sells a complete kit with all the pieces included except for the balance weight. If the fan has a wobble, again it might just be loose, perhaps Nylock nuts weren't used (or were re-used, they should always be replaced with new). Again, a fan might gradually loosen due to vibration if balance weights were omitted. Another thing to check is if the blades are bent or cracked. The 4-blade, metal fans are notorious for this (similar fans put two out of three TR3s out of LeMans, the first time Triumph went racing there... which is especially silly since fans are useless at 114 mph). Could be time to install the yellow plastic fan that Bob mentions. As far as I know, no one ever uses balance weights with the plastic fans.
The fan bolts can be tightened or removed, and the fan can be replaced without removing the radiator, although clearances are tight. Just be careful about the radiator fins.
While in the area, check the pulley and extension clearance to the steering rack, frame cross-member, etc. If you have trouble getting the belt on and off the pulley, it's a sign that the two front engine mounts are getting old and the engine has settled a little bit under it's own weight. Jacking up under the oil pan (with a wide piece of wood to protect it) is a temporary solution to get the belt off. The engine mounts really should be replaced, if the engine has settled like this.
I recommend a little caution replacing the front extension. It also serves as a dampener for the crankshaft. This might not be an issue with a well-balanced engine or one that's not driven very hard, but harmonics can make a mess of the gearbox and/or the crankshaft, if not dampened. The TR3 extension Bob mentions seems okay, as far as I know. I've converted my TR4 to fanless (electric instead), an alternator and a thin belt, so am using a proper harmonic balancer/pulley.
Hope this helps!
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