Rebuild Finished Mystery - Hesitation every 120 Degrees
 

Bruce, who is rebuilding the engine in my car, has finished with the build, but there's an issue that he hasn't seen before. When he rotates the flywheel, every 120 degrees, at top dead center, there is a hang up as the piston changes direction, requiring a little more pressure to get past TDC.

The engine specs are these:
Carrera engine
Carrera cylinders, bored out to 3.4
Mahle Max Moritz 3.4 cylinders
Web 20/21 Cam
Eibach valve springs and titanium retainers

We've checked to make sure there is plenty of clearance to the cam shaft - 4mm intake and 5mm exhaust as I remember. Also checked to make sure wrist pins on the cylinders are the correct 23mm.

What else could be making this happen?
Thanks,
-Tom

I am sure Bruce knows this...

Are the plugs in? The hesitation would then be rotation past the compression of the atmospheric air charge.

pull the plugs and see if it goes away.

Case straight? Crank binding?
Cams straight? Cams binding?
loosen rocker clearance and see if this helps. Would isolate bottom end vs top end.

Could be piston to head or valve interference

Please elaborate......
 

Tom,

Did you mean rotating the crankshaft pulley and after every 120 deg. the problem/s noticed or observed? Change direction? Or obstruction in movement? How did you observe this 'change in direction' with the engine assembled? Thanks.

Tony

When the 2 opposite pistons come to the top every 120 degrees it takes twice or more the force to move past the Pistons change of direction. Just the one degree on the crank pulley then everything is smooth. The plugs have not been installed.
Bruce

How much was taking off the heads?

I see you went to a 3.4. This means new 98mm PC sets. JEs? Any chance the piston skirt is bumping the case web on the new pistons? I have seen this on an early car.

Unknown obstruction........
 

Bruce,

I am not in position to give you advise about engine rebuilding because I am the one that's been calling you whenever I got into trouble. As a matter of fact, I am currently diagnosing a weird engine problem that I would be diagnosing today.

If this motor is mine, I would start removing parts after parts to determine the root cause of the obstruction. Removing the flywheel, cam towers, cylinder heads, etc. until the hindrance to the stroke of the piston/s is eliminated or discovered. My suspicion would be insufficient clearance between piston and cylinder head dome. Keep us posted.

Tony

Boy that would be a real bummer!

Another thought. Is the clutch/flywheel installed? I have seen them drag the case when the PP bolts are too long. It has happened to me when I was shipped the wrong length.

The pistons are Mahles but I suppose there's a chance of that.

Back the rocker arm adjusters off and see if it goes away. Have you used these cams before? Check with WEB and see what the profile is, high lift and fast ramps can feel wierd when turning them over.

I have setup heads with Eibach valve springs before and I remember using the high performance springs you have to use their lower spring retainers as a kit" which are much thinner or you have to machine the porsche retainers down or you will never be able to set the correct spring height which will cause spring bind? Just tossing this out?

Good call Craig,

Didn't think about coil bind on the valve springs. Sounds very plausible.

Titanium retainers.
The drag is only at the point of the piston changing directions, at just the mark for Z1 and the 120 degree marks. At that point all valves are closed except the one intake at 1.9mm.
Problem is not a binding but the extra effort to get past that point of the Pistons changing directions.
Thanks for all the thoughts
Bruce

So it feels different from other engines you've built? I would expect the torque required to fluctuate with the compression as well as when the piston changes direction since static friction is greater than sliding friction.

Just a thought: can you evenly warm up one bank of cylinders with a heat gun and the slight thermal expansion may provide you clues as to what may be happening here. Maybe you get to notice the change at every 120 degree throw......

Bruce started the engine on Friday. Didn't sound good. Pistons are making some kind of light contact. I believe Bruce is taking the engine back down to investigate further.

The top of the motor will be off tomorrow. When I turned it over without plugs, sounded good, no tapping, oil pressure came up. Plugs in, motor started, shut down immediately. Sounded like an army helicopter with the Gatling gun.
Bruce

hmmm this is odd, I did a set of heads for a guy and called me because the motor was making a racket when running then lost one cylinder, no compression, we were thinking sticking valve and it hit the piston but I have never had a problem on a fresh set of heads: Took the heads off and found a 6mm nut inside one cylinder: It happens:

Case
 

I'm down to the point that I'm sure it has to do with this or at least a combination of two things.
I took a running 3.2 apart. Replaced the Pistons with Mahle 98P11. The Pistons have a high surface to the left away from the plug. The case is stamped 010 looking more like the cylinder bases were cut on the case. The base gaskets on the 3.2 were the brass looking .25mm, that's what I replaced.
Now I'm going to have to compare cases to see if I fact it is cut on the base spigots reality says I'm going to have to stack base gaskets for cylinder height.

Bruce

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