Differences in Formula F233 Twin 165s

[drock360]

Hey
So I am looking to purchase a '72 Formula F233 with twin Merc 165 Inline 6's. The current owner stated he believes one of the engines was rebuilt in the late '90s. I did a compression check and found two things:

1) One engine had Cylinder #1 and #6 @ 100psi and the others ~@120 psi. According to the Merc Service Bulletin shouldnt be an issue?

2) The other engine had 150 psi across the board (maybe the rebuilt engine?) Question is, the plugs were all a different size from the other engine. There were larger in diamater (needing a different hose for the gauge). Could this be a head replacement? Would it require different plug sizes or is this possibly a regular chevy nonmarinized head? No unusual rust or anything is visible.


And finally, neither trim tabs dont work....this a red flag or an "easy" fix? ( did not get a chance to check power with a meter)

Thanks

 

[Don S]

Re: Differences in Formula F233 Twin 165s

If you post your trim tab problem over in the Boat Topics and Questions (not engine topics) forum you will probably get some help from Tabman. This forums is for engine and drive problems only.

Can't help with your engine question, not that familiar with the different heads on the L6 engines
100-120 is minimum and the engine is tired. Usable, but needing work.

 

[Will Fish]

Re: Differences in Formula F233 Twin 165s

I believe GM made modification to the head in the early to mid 1970's, which changed the plug size. The earlier heads used a 13/16" size socket for removal of the spark plug to a 5/8" socket (I don't remember the thread size). The 13/16" plugs use a flat metal gasket on the end of the plug and the 5/8 plugs have a tapered design.

 

[drock360]

Re: Differences in Formula F233 Twin 165s

I believe GM made modification to the head in the early to mid 1970's, which changed the plug size. The earlier heads used a 13/16" size socket for removal of the spark plug to a 5/8" socket (I don't remember the thread size). The 13/16" plugs use a flat metal gasket on the end of the plug and the 5/8 plugs have a tapered design.

Great thanks for the info. So that would not necessarily be a potential issue.

 

[drock360]

Re: Differences in Formula F233 Twin 165s

If you post your trim tab problem over in the Boat Topics and Questions (not engine topics) forum you will probably get some help from Tabman. This forums is for engine and drive problems only.

Can't help with your engine question, not that familiar with the different heads on the L6 engines
100-120 is minimum and the engine is tired. Usable, but needing work.

Thanks, at least the strong one has the power steering pump in case the other one quits on me

 

[Don S]

Re: Differences in Formula F233 Twin 165s

Does either engine have a Mercruiser serial number tag back above and aft of the starter?

 

[drock360]

Re: Differences in Formula F233 Twin 165s

I meant to look and record the serial #s so I could get more info on the engines, but forgot with my attention to the compression testing. Like I said this is a boat I'm looking into purchasing so maybe I can contact the seller to ask him to locate them.