Engine rebuild Mille GT (Small valve big-block)

I thought I’d share some of the experience of this with you all.

Firstly, I used the excellent instructions available from https://www.thisoldtractor.com/guzzitech.dk/gb_en_complex-technical_engine-strip-pictures.htm, along with an old Haynes manual (for the Le Mans, etc).

Dismantling wasn’t quite a breeze, with plenty of seized fasteners, but these old engines aren’t difficult to work on, particularly if you have it on the bench. So, fairly quickly, I had the heads and barrels off, heads dismantled, alternator off, and timing gear exposed. I didn’t remove the cranks, but I did dismantle the timing gear so that I could replace the old camchain tensioner with the spring-loaded one.
Removing the timing chain and sprockets is relatively simple – the sprockets need to come off the shafts all together, after you’ve removed the 3 retaining nuts. The top one needs a deep 27mm socket, and the middle one needs a special peg-nut socket that I got from Gutsibits. It also has a special tab washer that needs to be replaced. The bottom sprocket (oil pump drive) is straightforward but has a Woodruff key to effect the drive(actually, it’s not really a Woodruff key, as it doesn’t have a curved side, but it’s the same sort of thing). Putting them back with the new cam chain tensioner in place isn’t possible though (well, I couldn’t do it!). So, I disassembled the tensioner (the mounting has to be done with the cam chain removed) and then it was a bit fiddly, but do-able. There are nice clear timing marks, and I’m happy that with them all aligned, everything will be OK. It was easy to refit the tensioner and now I’m happy. Just waiting for the timing cover to come back from the vapour blaster, and then we can get the front of the engine back together.

At the rear of the engine, dismantling the clutch is straightforward – but the clutch centring tool that I bought from Gutsibits (who have been very helpful throughout) wasn’t well formed. It was too large in diameter to fit into the drive splines, so I had to spend a couple of ours carefully filing each spline down so that it fitted accurately (and, importantly, concentrically). It’s a shame that I had to work on it, as it’s quite an expensive tool. Clutch removal is simply a matter of removing the 8 bolts that secure it. Happily, the clutch has plenty of wear in it, so it’s simply been a matter of cleaning out the filth and reassembling. Again, the instructions were good. I did have a moment when the centring tool didn’t compress the clutch springs enough for reassembly, but I popped a spacer between the bolt head and clutch tool and then everything was fine. Probably, just shortening the bolt by 5mm would have sufficed.

The top end(s) have all been taken apart, and the bare heads and barrels are with the vapour blaster. I’ve removed the valves – and it’s as well I did – one of the inner valve springs is broken. I’m expecting that rebuilding these and lapping the valves ought to be fairly straightforward. One point from the valve removal – I used a cheap valve spring compressor and noted that the valve collets were tightly engaged with the grooves and valve caps. I found that the best thing to do to release them was to apply some pressure and then give a sharp tap to the compressor above the valve spring. This allowed the collets to release nicely. I suspect that if you just wound up the pressure on the compressor, it would probably bend. It’s not all that robust.

Right, that’s it for now. I’ll probably post up more notes as I complete the engine rebuild.



Just read this Nick very helpful, I have one of these bikes waiting in the wings for attention, but have done a similar job on the front of a T3. You identified the issue with the key that no one else did. I fitted a new pump new tensioner and pressure relief valve from HMB. With a new chain the sprockets have to go on together as you say trying to ensure the key is in place for the oil pump is tricky I used an method learned previously. Clean off and stick with super glue. The HMB tensioner is a far better design and doesn’t throw bits of ? In the sump

Regards Ratt