'87 Johnson 70HP Piston Question

[ddcarter3]

I went to the NAPA Machine Shop today to check on pricing for boring the block. The machinist asked if all pistons needed to be replaced? I told him one maybe 2. He said if I was replacing the pistons with OMC pistons I could replace just the bad ones. He said if I was planning on using aftermarket pistons I would have to replace them in a set. He said this was because of weight balancing. Is this true? Please explain. Thanks in advance.

 

[R.Johnson]

Re: '87 Johnson 70HP Piston Question

Does the NAPA store have experience with boring two cycle engine's? They are different than boring 4 stroke's.

 

[ddcarter3]

Re: '87 Johnson 70HP Piston Question

Yes. The machinist really seemed to know his stuff. Understanding that I only have automotive experience. He had the outboard board book for parts at arms length. I was really suprised he had it so easily accessible. This NAPA is really close to Lake Conroe, TX. I plan on asking the local Johnson Serive Dealer also.
Thanks,
David

 

[emdsapmgr]

Re: '87 Johnson 70HP Piston Question

OMC keeps their pistons at the same weight. Whether standard of oversize, they should weight about the same. That is why you can replace only one OMC with an oversize OMC. If you go to another piston manufacturer, it may have a different weight than the stock pistons in the engine. Also, other manufacturers pistons may expand at a different rate and may need different clearances when bored. I personally would not mix pistons from different mfr's in one engine.

 

[ddcarter3]

Re: '87 Johnson 70HP Piston Question

Just ordered a complete rebuild kit with Wiseco Pistons. Gonna have all three cylinders bored .030 over. Just looked like the best solution. Thanks for the info.

 

[mikesea]

Re: '87 Johnson 70HP Piston Question

dd,you on the right path,do all 3 new.Remember what I mentioned about the idleing when your finished. If you have problems you may need to change some jets. I had the problem with omc 3cyl. a few times.While you have the carbs off clean them.Also,is that a VRO motor?Down here in South Fl.We like to disconnect the VRO system and opt to mix our gas .You can use the VRO pump hust as a gas pump with oil disconnected.