Morini 175 GT Top End Inspection

Moto Morini 175 GT Top End Inspection

This section will deal with the inspection of the cylinder head, cylinder and piston, and the camshaft, with tentative conclusions about what will need to be done.

The valve train seems quite typical, with the possible exception of the use of dual valve springs.

The outer springs are ~5mm longer than the inners but there is a step in the top collar that compensates. One set of springs (both inner and outer) is ~0.5mm shorter than the other. I don’t know if this will be considered a reason to replace them, but I’m hoping to get some specifications on installed spring height and tension so I can have them tested.

When I started cleaning up the top end parts and inspecting more closely, I had two surprises. First, this head has the larger valves of the Settebello and Tresette Sprint, 31 mm intake and 29 mm exhaust (stock GT is 28/26 mm). Nice! There is very little measurable wear on the stems (7.40-7.43 mm) but there is some scuffing or scoring, not enough to force replacement on that score alone. The exhaust valve (bottom) has lots of metal at the edge so it could be cleaned up, faced and reused, but the intake (top) has a bit of a knife edge, not much room for grinding, and it is a little dished. I plan to replace them both.

It was no surprise at all that the valve guides are worn. The intake was worse than the exhaust. There were no oil seals on the guides.

The second surprise was the sparkplug hole. It must have gotten really boogered at some point and someone drilled it out, tapped it and threaded in an insert. The problem with this solution is that the hex head of the insert moves the sparkplug out of the combustion chamber, leaving the spark in its own little hole (not nice!). I’m definitely going to have to do something about this.

The valve seats don’t look bad, but it’s not a difficult job (for my machinist) to replace them with new seats that will stand up better to modern gasoline, so that will be done when he installs the new valve guides and sets up the new valves.

The camshaft lobes are in pretty good condition. My machinist will polish them a little in his cam grinding machine.

The main question is, what cam grind is this?

Considering that this GT engine has been treated to a high compression piston and a big valve head, what are the chances that the stock camshaft was replaced with something hotter? It’s an easy job on this engine because you don’t need to split the cases. It’s all done under the primary side cover. From a casual visual inspection, Scott Potter thought this profile has a fairly long duration, as would be expected of a performance profile. The lift is 4.4 mm on both lobes, on a base diameter of 22.7 mm. When we get the cam on Steve’s cam grinder we can measure the profile accurately. In the meantime I’m hoping to find some specifications for the factory cams.

As mentioned elsewhere, the piston in this engine was an overbored (60.6 mm) aftermarket Settebello piston made by Sciam and marked “535B”. The wristpin is 16 mm diameter. When the bottom of the skirt is lined up as shown in the next photo the deck is at ~68mm and the top of the dome is at ~78mm.

This piston is in good condition and I’d be happy to reuse it except that the overbore did not completely clean up the corrosion in the cylinder bore. Unfortunately, Settebello pistons with 16mm wristpins are not available in sizes larger than 61.0 mm and I need 61.4 mm for this one, by the estimate of Steve the machinist. Tresette Sprint pistons (which are also high compression) are available in sizes up to 62.0 mm but they have 18 mm wristpins.

I was about to go with a Tresette Sprint piston and have bushes made to reduce the wristpin to 16 mm, when Paul Compton suggested that the piston of a Honda CB350 twin can be used. The Honda piston is 65 mm diameter, which bumps the capacity of the Morini up to 202 cc. There is plenty of metal in the liner (70 mm OD) for this. The Honda’s wristpin is 15 mm so Steve will need to make a new small end bush. I’m checking a few more dimensions (e.g., deck height) but I think this motorcycle is going to come out of the shop with a nice fresh Wiseco piston and 200 cc of big valve cheater engine (which way to the Moto Giro d’Italia!). The Wiseco pistons are supposed to have 10.5:1 CR in the Honda but we’ll have to measure it after installation in the Morini.