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Interference fit? Measurements.
The question was again raised recently regarding 10v engines. The answer
is "definitely maybe" for the 2.2 litre turbos, and "probably not" for 2.1
litre turbos. Reasons for the conclusions follow:
Since I have engines all over the shop at the moment, I did a bit of
measuring. Checked an '85 turbo (2.1 litre) and an '87 turbo (2.2 litre). At
TDC the tops of the pistons in both engines were above the block by about
There were some differences in cam lobes. The lobes on the '85 cam
measured 1.735" at the tallest point. Lobes on the '87 cam measured 1.760" at
the highest point.
Head gasket thickness is supposed to be the same for both. Cam followers
are supposed to be the same for both. So the difference in cams would be the
only difference in these two engines.
I set the head with the center exhaust valve at full extension. Then set
No.3 cylinder slightly below TDC and placed a piece of rosin-core solder
directly under where the valve would go. Next, the head was tightened up (not
fully to normal torque, though.)
The next step was to turn the engine by hand and see what happened. As
you'd expect, there was some resistance. But I turned the engine slowly
through it, then "wiggled" the piston back and forth a few times.
I pulled the head and retrieved the solder. It had been thinned to about
0.040". So this would indicate that the engine, in theory, is not
interference fit. With the bigger cam lobes of the '87 engine, there should
still be about 0.015" clearance.
Now, this was done with the engine cold, and the cam follower was at
nominal pressure, turned by hand. So the later engine, at operating temp,
with the cam followers fully pumped, at any significant speed could take up
that fifteen thousandths fairly easily. So again, in theory, (with a later
engine) you might lose one or two valves if the the timing belt let go while
it was running.
Having done the exercise, I still don't understand why my '87 chose to
eat all the exhaust valves without jumping time or breaking a belt. Maybe one
day I will.
Imperial Potentate, Bucksnort Quattro Club (Un-Inc.)