I’m going to have the Nikasil cylinders on this bike replated and honed to match the new 9.5:1 pistons I’m using in this build. I’ve learned from a well versed airhead mechanic, Tom Cutter, at Rubber Chicken Racing Garage, that the connecting rods are subject to deformation over time such that the distance between the hole centers of the big end and little end of the rod becomes a bit longer than when new. It’s worth having them checked and the flat of the connecting rod cap machined to achieve the design distance between the hole centers. This is not very expensive and ought to reduce wear and tear on the wrist pin and crank shaft throw.
Here’s a picture of the connecting rod with the parts labeled.
I already removed the top end of the bike and I documented how I did that work here:
The connecting rods are exposed, so I can remove them easily at this point.
The connecting rod is secured to the crankshaft with two special bolts. They are designed to stretch when torqued, so they are “use once”, and they have a special head requiring a 10 mm “serrated wrench” I got from my local NAPA store for under $10.00.
I use a 13 mm socket and slip it over the head of the serrated wrench so I can use my breaker bar to loosen the rod bolts.
I use my long socket extension and a breaker bar to loosen the bolts. The long extension allows the breaker bar to clear the cylinder studs protruding from the engine block.
I made a short video showing the steps for removing the connecting rods.
VIDEO: 1983 BMW R100RS Remove Connecting Rods
Remove Connecting Rod Bolts
I keep the green garden wire through the little end to help prevent the rod from hitting the engine case or accidentally dropping on the floor.
I insert the serrated wrench end of the breaker bar into the head of the rod bolt and break each of them loose. I remove the bolts while keeping my other hand on the connecting rod cap so it doesn’t fall in the oil pan when I separate the rod from the cap.
However, on the left side, the rod doesn’t want to separate from the cap. I use a long handle screwdriver and put the tip on the exposed lip of the connecting rod cap and give it a couple gentle taps on the top and bottom of the cap.
That loosens if from the connecting rod and I can get the rod and end cap off the crankshaft. There are two pins in the connecting rod that fit into holes in the rod cap. Sometimes the pins don’t want to slide out so easy.
I now suspect the reason for the end cap not coming off easily is because that rod had a slight twist in it. That can happen and would misalign the pin with the hole enough to prevent it from separating easily.
Inspect Rod Bearing Shells
Before I do anything with the rods, I label the connecting rod and the connecting rod shell so I am sure they will go back in on the same side of the engine they came from.
Here is a view of the rod bearings. They come in halves that fit into the connecting rod and the connecting rod shell. They pop out easily with a small blade screw driver.
There are signs of some wear on part of a bearing shell on each of the rods. There are also a couple grooves in the bearing surface from something hard that passed through the bearings.
I later found grooves in the oil pump rotors which is consistent with hard metal being circulated in the oil. This is consistent with the failure of the oil filter high pressure by-pass valve I found when I first got the bike. With the valve open, dirty oil circulated through the engine for an unknown period of time.
Connecting Rod Journal Measurement
Here is a view of the connecting rod journal on the crankshaft.
I use a 1-to-2 inch micrometer to measure the diameter of the both connecting rod journals to see if they are overly worn. The BMW specification for the connecting rod journal (aka, “crankpin”) diameter is 1.8888 – 1.8894 inches. I found both journals were 1.8893 inches, so there is no appreciable wear on the journals after 83,000 miles. 🙂
2019-11-21 Edits and typos.