oversized cylinder boring and ordering oversized pistons

[astampfel]

I have a 90 Hp Johnson with one cylinder that has been scored by a broken piston ring. I'm going to bore all 4 cylinders larger, but I don't know what size to go to. I was looking through my OMC service manual, and they list the sizes for cylinder and pistons from 88 Hp through 175 Hp. I noticed that the 175 Hp engine has larger diamater cylinders and pistons, but the same stroke and crankshaft dimensions. I was wondering if I can bore my 90 Hp to the 175 Hp cylinder size and just use 175 Hp pistons and rings? It seems like it would work to me? Any advice and tips?

I haven't checked the piston open market, but are there all sizes of pistons our there, so many, that it wouldn't be a problem for me to bore my cylinder the minimal amount to clean it up?

 

[oldcatamount]

Re: oversized cylinder boring and ordering oversized pistons

You might want to check the prices before you go ahead and bore all four! Happened to me once! Cost me a ton when all I had to do is hone one cylinder!

 

[daselbee]

Re: oversized cylinder boring and ordering oversized pistons

NO...the 90 is a four cyl, the 175 is a six.

You have to get it measured by a qualified shop to see the oversize needed.

If you can haul it to Mims, I can measure it for you, (No Charge).

 

[Joe Reeves]

Re: oversized cylinder boring and ordering oversized pistons

Don't waste your time and possibly destroy the engine by dreaming of souping it up. Rebore only the cylinder(s) that are damaged and simply have the other cylinders honed (deglazed)..... saving yourself hundreds of dollars.

As Daselbee states, have a reliable machine shop inspect the block, after which they can advise you of the oversize piston you need.

 

[astampfel]

Re: oversized cylinder boring and ordering oversized pistons

Thanks guys! I don't have the block out yet, but hopefully I'll start soon. Joe, I'll definitely think about only doing one cylinder; it might save me a bunch of money and time. I'm not trying to soup-it-up; I just want to get the thing working again. I saw that buying a rebuild kit is cheaper than individual pistons, and I would think that I would have to buy all the same size pistons if I got a rebuild kit. That's why I was thinking about doing all the cylinders. But, I will consider just getting one new piston. Daselbee, I'll keep you in mind for the measuring help.

 

[emdsapmgr]

Re: oversized cylinder boring and ordering oversized pistons

You need to consider the pistons you put back into your block. It is factory OK to reuse 3 pistons and overbore the 4th one. You don't need to rejet the carb. Keep in mind that you want to match the piston for weight and it's expansion rate as it heats up. Best to purchase a replacement factory oversize piston to match the other 3. The oversize weighs the same as a standard piston. That should minimize any problems. That oversize comes in both .020 and .030 over and comes with a ringset. Be sure to rering the other 3 pistons with new also.

 

[Dhadley]

Re: oversized cylinder boring and ordering oversized pistons

We'll assume your 90 is a crossflow since you are comparing it to other crossflows. You can bore it as far as .064" over but it probably won't do what you think it will power-wise. We've had customer who paid us to bore a V6 crossflow to +.064 expecting a big increase in power. Didn't happen. Bigger gains were made with set up.