GT380 Engine and Neutral Brake

I’ve torn down the GT380 engine and split the case to take a closer look at the gear clusters and the crankshaft. This engine came to me loosely put together, and when I pulled the clutch cover there were indications of rust, so it seemed the prudent thing to do. I know most people first try to get an engine running before deciding whether to do any open heart surgery, but most of the bikes I work on haven’t run in decades so I normally skip that step and just start taking things apart.

In actual fact the engine was not as bad as I’d feared. I had a receipt in the box of bits that came with it for a set of new rings and this proved to be true. Sadly rather than honing the bores, they really should have been bored to the next over size as all three showed lots of scoring. While it should have run, the likelihood of the new rings ever seating properly was next to nil. I’ve ordered new first over pistons and rings and will get the cylinders bored locally here in London. The centre cylinder head was damaged, but not the two outside ones. What appear to be pry bar marks in several places come very close to the fire ring, but I should be able to fix these with J-B Weld.

There was corrosion on the clutch plates and likewise on the gear clusters, but no pitting. The very light surface rust cleaned up fine so other than changing a few of the bearings in the gear clusters they should be OK. Likewise the crankshaft seems fine. Unlike with the GT750’s, the crankshaft oil seals are quite small on these, and there is no play in any of them or the bearings, so I’m just going to change the outboard seals and one outboard bearing on the right hand drive end of the crank. If the seals do eventually fail, the engine is small enough that pulling it out again to have the crank redone will not be too big a deal.

So now I wait till parts arrive, and for a machine shop to re-open post Covid-19 so I can get the bores done. Hopefully that will happen in the next couple of months. Fingers crossed anyway. 😉

And who out there is familiar with the GT380 neutral brake ? I have been doing some research and this clever (?) device was fitted to transmissions in the GT250, T350 and GT380 plus a few other models. I don’t know whether other manufacturers also fitted them, or indeed still fit them as I’ve never worked on anything newer than 1977 !

What it does is stop the rotation of the gears as you shift through neutral, to lessen gear noise when engaging from second or neutral to first. Suzuki used (possibly still uses ?) a rocking lever that has one end under the shift drum riding on the neutral détente pin, and the other end riding against a spring loaded pin which presses the rocking lever up against the underside of a a shoulder on the first driven gear.

On the GT380 I’m working on, the pin at one end was jammed in the bore, caused by aluminium shavings (see photo) that had been pared off the sides of the pin bore by the steel pin and leaving a step in the bore (see photo). With the pin jammed, the transmission didn’t shift properly due to a lot of drag (it was always partially in the ‘brake’ position). I was initially undecided about how to fix this so it doesn’t happen again. After spending some time swapping parts in and out and making a few modifications, it now seems to work just fine.  😎

The main problem was that the pin was catching in the bore and stopping the pin that rides on the underside of the brake arm from smoothly moving up and down. I had considered sleeving the bore, but the wall thickness is only 2mm which really isn’t a lot to play with. What I ended up doing was chucking the pin in the lathe and making a slight taper on the end. You can see the difference in the photo (somewhere near here) with a stock pin on the right and the modified one on the left (both pins are upside down in the photo – the dome end actually goes to the top). I’ve also polished the bore that the pin rides in.

I found that when I filled the pin bore with oil, it took some pressure to displace the oil up the flat on the side of the pin, so I also drilled a small oil hole in the side of the casting into the bore that the pin rides in. This will ensure that there is no chance of hydraulic lock. When the gearbox is filled with oil, the level should be above the top of the pin so I don’t see how this can hurt anything.

The last photo shows the neutral brake installed and you can also see the small oil hole I’ve added. The transmission now checks out fine, and shifts properly so next on the agenda is to take it all apart again and change some of the bearings on the two gear cluster shafts. Once that is done, I can button up the engine cases and spend a bit of time polishing the engine shrouds and engine side covers before I put the engine back into the frame.

Getting closer ! 😎

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