This engine turned up with peculiar problems. We started with simple checks – does it turn over, has it got oil, that kind of stuff. Then securing the stop lever on the STOP stop (?) we cranked it over on the starter. You could feel air coming back out of the inlet manifold, one of the symptoms of a failed head gasket, so next step is head off.
And, here we have one of the pistons (both showed the same symptoms):
Notice the circle, top left – the same size as the inlet valve:
So, the valve had been hitting the top of the piston – not good. It was the inlet valve on the other piston too, very odd. Now, if the piston hits the inlet valve while it is opening, something has to give. The valve train should look something like this:
The inlet valve on the left, pushed down by the rocker at the top (which is pivotted in the middle), the push rod at the right going down to the cam follower.
Instead, one cylinder looked like this:
That pushrod is a bit bent
Comparing the two inlet valve pushrods to one of the straight exhaust valve ones:
We have a straight one, a bent one and oh dear…
Now, why isn’t the middle one as bent as the other? There was as much force going into the valve train from the piston. Oh, the rocker:
The pushrod has punched out the bearing cup on the rocker
What’s gone wrong? Well, first possibility was the hydraulic cam followers. Lots of dire warnings about those in the manual, and changing the oil, and using the right grade. I inspected the followers – nothing wrong. Also, why both the inlets, not one inlet or an inlet and an exhaust, or all the valves?
So, I took off the timing cover – no, all the timing marks aligned. As a last possibility (primed by Peter Thompson of Marine Engine Services), I did a rough check on the valve timing from scratch. First, I used DTi to set the piston at top dead centre (TDC), then assembled the head and valve train on the same cylinder and marked the point at which the exhaust valve is closeing and the inlet is opening. On most engines, this is at or near TDC. On this engine:
The valve timing is approximately 24 degrees advanced. This means that the inlet valve is opening too soon, and getting clobbered by the piston
But how? Well, it seems that on this engine the drive gear is a tight interference fit on the camshaft, there is no positive timing device like a key or dowel. This isn’t a problem under normal circumstances, so what went wrong with this engine? Stripping it right down revealed that the centre bearing of the camshaft was not good:
Not a good picture. The centre bearing has seized at some point in the past, you can see scales from the bearing material welded to the camshaft. The bore in the block confirms this:
There’s a very bad score in the block. Naturally, taking out a camshaft with extra lumps on it has caused other damage:
The lumps have damaged the shell bearing
So, now the block is off for line boring. We are going to machine out the centre bearing to fit a shell, same as the drive gear end. I say we, the excellent chaps at Coventry Classic Engines are going to do it with a jig borer