Sunday, June 7, 2015

Fitting the new gearbox sprocket.

I got around to doing the "15T mod" as it's known to other RFVC owners. We're a very cool bunch.

Just to recap, I'm replacing the standard 14-tooth sprocket with a 15-tooth one, to increase cruising top speed while keeping revs at a reasonable rate, at the expense of a little bit of acceleration. That said, with the upgrades I've already done, standard gearing feels too low, so this should turn out to be just right.

The sprocket is made by JT and the part number is JTF 308.15. Renthal also make sprockets and there should be one that fits the RFVC engine, (part number 281-520-15) but  I don't know for sure, this time I went for this one.

Using the clever frame design, I put the bike on the jack for easier access to the plastic sprocket cover.

Off come the two bolts, these were a bit stubborn and took some penetrating fluid and a few gentle taps with a hammer to get them to budge.

When it came to taking the chain off I realised that the split link was actually riveted, so I had no choice but to break the chain using my team of skilled negotiators as pictured below... As it turned out, it was really stiff and just about ready to be replaced anyway, so no harm done there. Good job I had bought one ahead of time.

There is a metal chain guide that comes off, and this is what needs a bit of work when putting it back with the larger sprocket.

There simply isn't enough room to fit the larger sprocket, so the chain guide has to be ground down a little bit. The proper way to do this would be to have it machined to suit, but I don't have a lathe and I wouldn't trust a machine shop to do this for me here. Instead, I slowly ground it down with an abrasive disc, checking fit repeatedly until I got it right.
There really isn't a lot of room for error here, and I would recommend being very careful to anyone considering doing this: the chain guide is no longer available as a spare part, so if you stuff it up, that's it.

Then it's on to the two retaining bolts for the sprocket, the very clever locking tab and the sprocket itself.
I wish they were all made like this, this is easy to fit and remove, it's safe and it is simple yet effective: basically once you fit the locking tab over the splines you have to rotate it to align the holes with the sprocket and the assembly cannot come off, because the tab rotates just enough that its teeth won't engage the splines. Brilliant.

Counter to what I know, the face of the sprocket that's marked with the part number and the company logo actually goes towards the engine, whereas I would have expected it to face out. No big deal.

This was also a good time to check the splines on the mainshaft. As it turned out, not too bad considering this engine has essentially done the equivalent of just over two round trips of Jupiter. I know it's a gas giant, I mean the distance...


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