Checking Clearances With Plastigage
It's very important to check the fit of ALL your main and rod bearings prior to assembly. The purpose for checking is so that you know the bearings fit as they should so you have good oil pressure and a long lasting engine. But how can you check these tiny clearances without expensive, special tools?? The answer is easy and CHEAP!! . . . PLASTIGAGE!!
What is Plastigage?? Plastigage is a very thin wire-like piece of plastic. It comes in 3 different thick nesses for varying degrees of tolerance. For your air-cooled VW, the GREEN colored Plastigage (.001" - .003") is what you will need.
How does Plastigage work?? You simply place a strip if Plastigage across your bearings or the surfaces they ride on and torque to the proper specs.
CAUTION: Never touch ANY bearing wear surfaces. The oil from your hands will cause corrosion to begin.
NOTE: It's very important to NOT SPIN the bearings with the Plastigage installed. This will smear the Plastigage and give you a false reading. Spinning the surfaces will widen the Plastigage giving you a "tighter" reading than you actually have.
Once you have put the proper torque on the bearing surface, you simply disassemble the parts and measure how wide the Plastigage has been flattened. Below are some photos of Plastigage before and after the bearing check. You'll have to look fairly closely to see the flattened piece on the rod journal. Also note that I am using a piece of the wrapper to measure with (it comes with a series of stripes that represent different clearances). Once you are finished with the Plastigage, be sure to remove all residue left behind prior to final assembly.
The proper clearance for the mains is:
Bearing 1 & 3: .0016" - .004"
Bearing 2 : .001" - .0035"
Bearing 4: .002" - .004"
Personally, I would recommend shooting for the lower end of these tolerances. This will ensure the best oil pressure over the life of the engine.
The proper clearance for rod bearings is:
ALL rod bearings: .0008" - .0028"
Again, the lower end of these tolerances would be best for oil pressure over the life of the engine.
If any of your bearings are found to have too much clearance at final torque, you can "tighten up" loose bearings by carefully sanding the edges with some 400 grit or finer sand paper. You can do this by holding the paper on a flat surface (preferably a piece of glass) with one hand and using the other hand to draw the bearing in one direction. CAUTION: Only draw the bearing one or two times and check it with Plastigage again. You can make a bearing tighter, but NOT looser, so go slow and take your time.
One little strip should be more than enough to check ALL the bearings on your whole motor. At about $1.25 a strip, it's sure to be the cheapest and most reliable tool in your box!!
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Revised: August 29, 2003 .