Different JS550 heads?

[wrench]

I am rebuilding a '88 550 for a guy and needed to replace the head. Wanted to go with a stock, not milled. So I bought one off ebay.

Please see picture. The new head is on the left. There is a much deeper step between the head and dome, than on the head I took off the engine. But the original head does not look like it has been milled to me, and this guy has owned the ski for around 20 years and says the engine has never been touched.

Now I am worried that the engine will have too low of compression to run.

Anybody have any thoughts?

[bandit88]

Re: Different JS550 heads?

1986+ 550 heads have smaller domes than earlier heads. I am not sure if there is a way to identify which is which from the outside.

[wrench]

Re: Different JS550 heads?

So in theory, compression should be the same?

[mcn6]

Re: Different JS550 heads?

Like Bandit says there ore two different 550 PP heads, pre 1986 and 1986 and newer. One has 72mm diameter 29.9cc domes and the other has 75mm 28cc domes.

Both heads have cast domes and squish relief (undercut, step, whatever...) can vary from .050" to .078", sometimes on the same dome! Basically, if either head has less than .050" squish relief it has been modified.

[wrench]

Re: Different JS550 heads?

Thank you.

[wrench]

Re: Different JS550 heads?

I got motor put in the ski and everything hooked up. Fired it up and it ran for 30 seconds or so then I noticed fuel leaking out of the back side of the carb around those steel plugs. See picture. I am not sure what would cause it to leak gas there though. I just rebuilt the carb with a Mikuni kit.

Any thoughts?

[wrench]

Re: Different JS550 heads?

I also want to mention there was an extra gasket installed in the carb that didn’t get replaced with the mikuni kit. There was a #7 gasket on both sides of #6 diaphragm. As the picture shows, the mikuni kit only gave me 2 of these gaskets for both sides of the regulator body.

Could not having that 3rd gasket be causing the fuel leak? Pretty odd the oem rebuild kit did not replace that gasket.

[john zigler]

Re: Different JS550 heads?

It was common for the old BN carbs to leak at these plugs. I clean off all paint, hit with brake clean and coat these plugs with JB weld to seal.

[wrench]

Re: Different JS550 heads?

Thank you, the owner was a little annoyed with me....."it never leaked there before you rebuilt the carb".......