Crankshaft Assembly and Rod Clearance

With spring time here, and all the other projects around the house (plus getting other cars ready to the track), not a lot has been done to the 914 engine. But I have managed to put a few 15 - 30 minute sessions in over the last few weeks.

I did a dry assembly of the crankshaft, connecting rods and camshaft to see how everything would fit together and to check for clearance. The first step is to put the bearing, cam gear and distributor drive gear on the crank. Note to everyone: remember to put the bearing on first!! I confess - I had the bearing sitting right there on my bench next to the crank, I heated up the cam gear and was so excited how easily it slipped on once it had expanded, that I forgot to put on the bearing. Of course the gear had cooled enough to be a tight fit, so I had to pull the gear and start over.

Finally, after remembering to put on the bearing, I again assembled the gears on the crankshaft:

Animated gif showing the progression of the gears being assembled onto a Porsche 914 crankshaft


Due to the layout of the type 4 engine, when the cam's #2/#4 exhaust lobe is pointed up, the #2 connecting rod is in close proximity; and when the cam's #2/#4 intake lobe is pointed up the #4 connecting rod is in close proximity. These are the two areas where connecting rod clearances needs to be checked on any motor that is receiving non-stock internals. I found that the #2 connecting rod had just enough clearance with the exhaust lobe, however, I had a problem with the #4 connecting rod:

Close-up of camshaft/connecting rod interference in a Porsche 914 engine


After much head scratching and measuring parts, I found that my connecting rods all had slight variations from their manufacturing processes. When measuring from the center line of the bore to the edge of the rod (2 measurements per rod - 4 rods total) the minimum to maximum variation was 0.012". I then put the "narrowest" rod in that location and found minimal clearance:

Measuring clearance between camshaft and connecting rod in Porsche 914 engine


0.003", which is a little smaller than the diameter of the average human hair, is a little too close for my comfort. As the engine warms up in operation, parts will expand and a collision then would be most unfortunate. I have decided to have an extra 0.005" taken off each side of each connecting rod. This will give me adequate clearance, but it will require that the rods then be re-balanced end-to-end, and then re-matched to each other (to ensure the weight of each rod is within half a gram of its peers).

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