Curious question about Johnson 150 pistons....

[FlaCowboy]

I recently bought a Johnson 150 for a parts motor. The young fella I bought it from did not know much about it. The motor was on a boat he had recently purchased. He told me the compression was bad. When I went to look at the motor he had pulled the heads off of it and the heads along with the motor were laying on the ground for what appeared to be a long time. However for the price he was asking and the motor being complete I bought it. I was looking at it a few days ago and noticed that the piston numbers were different. I went ahead and pulled a couple of the pistons just being the curious person that I am. I took my micrometer and measured the pistons and found this...3 pistons are #393929 and the other three are 393924. One #393929 measured 3.504 One of the piston #393924 measured 3.477.

My question....is this enough difference between the two different make (numbered) pistons to cause a difference in compression?

Thanks in advance for all replies.

 

[Joe Reeves]

Re: Curious question about Johnson 150 pistons....

Piston part #393924 is a standard size piston for a standard 3.500 bore cylinder.

Piston part #393929 supersedes up to #396585 and is a .030 oversize piston for a related bored cylinder.

If the cylinders were machined properly etc, there would be no difference in compression regardless if a piston were standard or oversize.

 

[FlaCowboy]

Re: Curious question about Johnson 150 pistons....

Thanks Joe