Compression test on 70 hp Force

[alcoon61]

I have read on different post the kill switch will do the same thing as grounding out the spark plugs. Is this sufficient to protect the cdi vs rigging up some ground wires?

 

[alcoon61]

Re: Compression test on 70 hp Force

Compression test completed! 120 on 2 and 3. 0 on #1. Piston has a guage by the exhaust port. Minor nick in cylinder wall can barely feel or see it. Feel sure I can hone that out. Think the fun part is going to be finding a piston and rings. Anybody have any ideas where I can find one please let me know.

Thanks

 

[Frank Acampora]

Re: Compression test on 70 hp Force

Yes. The kill switch grounds the CD boxes so they do not send voltage to the coils--thus no stray internal voltages to damage any component.

Pull the kill switch and crank the engine with all plugs out to test compression.

 

[Frank Acampora]

Re: Compression test on 70 hp Force

It takes more than a nick or gauge in the cylinder wall to bring compression down to zero. More than likely the rings are broken and the piston is damaged. This will take more than just honing. You will again, most likely need to overbore that cylinder. Before ordering a piston bring the engine to a machine shop and let them tell you if it needs an overbore. If so, then you will need to take off the block.

I prefer Wiseco pistons for several reasons, even though they are more expensive than other brands. However, you should be able to get pistons from Vertex, Sierra, and your local Mercury dealer. Some dealers also carry a small selection of Wiseco.

 

[alcoon61]

Re: Compression test on 70 hp Force

Thanks for your replying Frank. I am going to try and attach a few pics so you can see what the piston looks like with the head off. Will I need to redo all 3 holes or just the damaged one. The scratch I feel in the cylinder wall is very very light. I can barely catch it with my fingernail. I have looked for the piston on several dealer websites. Most list it as obsolete. I have located one new piston and ring set but only one on ebay for $110.

 

[Frank Acampora]

Re: Compression test on 70 hp Force

Yep! That piston broke the rings. If the damage to the cylinder is a s light as you say, then yes, perhaps honing will be sufficient.

A new Wiseco piston (part 3151) is about $106 but you must also buy a bearing kit for about 15 dollars. The piston comes with rings and new wrist pin.

SEe my private message to you.

 

[crappie14]

Re: Compression test on 70 hp Force

I have a buddy with about a 90 model 70 hp Force with similar issues, zero compression on cyclinder 3. My question is about how much money would one expect to pay for parts to fix this problem? Is it advisable only to address the problem cyclinder or had one just as well rebuild all 3 while it is tore down. Needless to say I haven't ever rebuilt a boat motor however I have exposure to rebuilding old farm tractors. I would basically like to take this project on as a learning experience but don't want to end up with an endless money pit. Motor has a good lower unit and other than compression seems to be good low hour motor. I live in an area where there are still shade tree engine rebuilders so cost to rebore should not be excessive. What I am asking is do you ever only address the problem cyclinder or do you always repair all 3 and at what approximate costs of piston/rings/gaskets/bearings etc. Needless to say I don't want to end up with $1000 in parts while learning from my mistakes.

 

[Frank Acampora]

Re: Compression test on 70 hp Force

It is common courtesy on the forum not to hijack a thread and start your own, but I will answer anyway.

Pistons for that engine run anywhere from 80 or so dollars to about 120. Boring a cylinder costs from about 40 to 65 dollars.

The actual disassembly and reassembly is rather easy and you can do it yourself to save money and learn.

If only one cylinder is affected, only that one need be bored. However, it is good practice to hone the other two lightly before reassembling the block

So, repairing one cylinder will cost somewhere around a couple of hundred dollars.

I don't know about shade tree mechanics in your area, but I charge 450 dollars for my time in dissassembly and reassembly, the owner pays for boring and parts. Marinas will charge 100 bucks or more per hour and usually want to install a rebuilt block.

So, you can see that you can save significant dollars by doing it yourself.

 

[walk-on]

Re: Compression test on 70 hp Force

I lost the ring on the #2 piston of my '95 90HP and I recently finished the rebuild and started it for the first time yesterday. I had to have the scored cylinder bored over 0.0020 but I had the shop hone the other two good cylinders. I took the advice of this board and replaced the other two pistons with brand new Wiseco pistons, rings, and bearing kits. My machine work was about $80, pistons about $250, gasket kit about $70. Now if the Susquehanna would ever drop to reasonable levels, I could get this engine tuned up.

The edge of my piston looked just like the picture in this thread but the head of the piston had a bunch of dings in it where the ring fragments bounced off of it several hundred times. Like Frank stated, the rebuild job is not that hard and I'm glad I did it because I really understand this engine a lot better than before.

 

[alcoon61]

Re: Compression test on 70 hp Force

I did finally finish this repair job myself. Just having a day here and there and the 100+ degree heat in Louisiana it took me 4 months to do it all but I took my time and just did a little here and there. Replaced the bad piston with a used one. Replaced the water pump impellor while I was at. I had read on here somewhere you could unbolt and remove the piston through the reed valves. I managed to get it out that way but gave up on trying to get it back in that way and split the block. Cranks and runs fine and compression is good. Will lake test it as soon as this tropical storm gets out of here and the wind dies down. Thanks everybody for your help and input.

 

[whiskerchasin]

Re: Compression test on 70 hp Force

Thats good to hear alcoon61! congratulations on the rebuild. I think I'm about to the same thing to mine and this thread gives me hope.