QuestionI recently purchased this bike. All together but has set up for a while and not running. I did a compression check on the cold motor and both sides were at 120. The book calls for a hot motor to be 170. Will the motor hot make up this much or should I go a head and plan to rebuild the motor?
AnswerHi Mike,
Pistons are cast or forged in an oblong like shape. Cylinders are bored as round as humanly possible.
As the engine reaches operating temperatures, pistons change shape and grow more round like resulting with better sealing piston rings, valves, and cylinders.
When an engine is cool, the metals are contracted. Compression will differ from readings taken with a warm engine.
It is necessary to add about a tablespoon of oil to the cylinders when performing compression tests on a cool engine. The oil helps seal the combustion chambers resulting with more accurate readings.
The answer to your question is "Yes." The cylinders can produce 170 PSI readings and higher if the piston crowns and cylinder head squish areas are coated with carbon build-up.
Depending upon the engine hours and mileage, expect to have compression readings in the 150 range. The service manual provides high and low service limits for compression.
If compression is low or a difference of more than about 10% exists between cylinders, perform a leak-down test to help determine the worn/defective parts.
Respectfully,
Mark Shively
