After my oil pressure disaster I did some reading in the service manual regarding bearing clearance of the main bearings and the big end bearings, and came to some strange conclusions.
Calculating the coupling clearance or the thickness of bearing shells is quite simple: size hole - size pin - clearance = 2 x thickness bearing(half)shell. Exactly measure the sizes is a little difficult because you need an accuracy of at least 0.01 mm, preferably 0,001 mm. That is why the factory already did the measuring and stamped the size class in the concerning parts. And that concerns my first observation of the service manual:
The service manual shows a picture of the right side of the crankshaft showing on the left the weight class (fe 1B, 2B, 3A) and on the right the size class (fe 3). I think it is the other way around. On the left side is the size class of the crank pins (1 and 2) and the conrod pin (3), on the right is the weight class. There is no mention of separate size class for each pin. I checked the measures on my crankshaft with a digital micrometer and that confirmed my opinion.
My second observation is even more surprisingly: The manual says that the coupling clearance of the main bearings should be 0.043 - 0.072 mm and the big ends 0.055 - 0.078 mm. That makes sense although a racing engine might have a little (0.02) more clearance. In the manual there are tables to determine which shells you have to use depending of the size class of the concerning parts. They have a color code and a corresponding thickness. Making the calculations of the thickness of the shells myself, I concluded that all the shells are to thick and that the difference in thickness to the next shell results in only 0.01 mm of clearance. It is hard to measure the thickness of a shell with an ordinary micrometer, so I checked the thickness of a new shell with a digital caliper and it was about 0.02 thinner than in the table. My conclusion is that the color codes of the shells are all right (otherwise there would be more broken engines) and that you can fine tune the shells you use according to the actual size of the hole and pin and the desired clearance.
Of course these are only my personal observations and conclusions. Please comment.
Calculating the coupling clearance or the thickness of bearing shells is quite simple: size hole - size pin - clearance = 2 x thickness bearing(half)shell. Exactly measure the sizes is a little difficult because you need an accuracy of at least 0.01 mm, preferably 0,001 mm. That is why the factory already did the measuring and stamped the size class in the concerning parts. And that concerns my first observation of the service manual:
The service manual shows a picture of the right side of the crankshaft showing on the left the weight class (fe 1B, 2B, 3A) and on the right the size class (fe 3). I think it is the other way around. On the left side is the size class of the crank pins (1 and 2) and the conrod pin (3), on the right is the weight class. There is no mention of separate size class for each pin. I checked the measures on my crankshaft with a digital micrometer and that confirmed my opinion.
My second observation is even more surprisingly: The manual says that the coupling clearance of the main bearings should be 0.043 - 0.072 mm and the big ends 0.055 - 0.078 mm. That makes sense although a racing engine might have a little (0.02) more clearance. In the manual there are tables to determine which shells you have to use depending of the size class of the concerning parts. They have a color code and a corresponding thickness. Making the calculations of the thickness of the shells myself, I concluded that all the shells are to thick and that the difference in thickness to the next shell results in only 0.01 mm of clearance. It is hard to measure the thickness of a shell with an ordinary micrometer, so I checked the thickness of a new shell with a digital caliper and it was about 0.02 thinner than in the table. My conclusion is that the color codes of the shells are all right (otherwise there would be more broken engines) and that you can fine tune the shells you use according to the actual size of the hole and pin and the desired clearance.
Of course these are only my personal observations and conclusions. Please comment.
