1982 Johnson 150 piston diameter

[LDS2121]

I have a 1982 Johnson 150TLCNM. I am rebuilding motor. I think the carcon got to and I had some scuffing of the cylinder. Had to bore one cylinder .030 over. The piston and ring are very loose in the bored cylinder, but the box and the part number say it is .030 over fo my motor. The bore for the .030 over is actually 3.529 and the new piston diameter is 3.445. The other cylinders mic between 3.49 and 3.50 with a piston diameter ranging from 3.425 to 3.435. I was thinking the new piston would be at least 3.455. Anyway I put it all together and have low compression in the bored cylinder. I would appreciate any advice on the situation.
thanks in advance
Larry

 

[emdsapmgr]

Re: 1982 Johnson 150 piston diameter

The standard bore is 3.500. Years ago OMC made .020 and .030 oversize pistons. I believe the .020 versions have since been discontinued. You may have a .020 version. All current Bombardier .030 pistons should have .030 stamped on the top of the piston. Is this a Bombardier/OMC piston? Your clearance concern is probably the reason for the low compression. You may also hear some piston slap on this hole.

 

[LDS2121]

Re: 1982 Johnson 150 piston diameter

Yes I have piston slap. Thats why I did compression check . I could hear the knocking as soon as I started it up. The new piston is the weisco brand. Do you know what the diameter of the piston is supposed to be. All my cylinders mic out right , but I'm not sure what the piston diameter should be.
Thanks
Again
Larry

 

[emdsapmgr]

Re: 1982 Johnson 150 piston diameter

Someone on the board can tell you the Wiseco specs. If you find it is in fact a .020 piston, suggest you replace it with the correct one: I would make sure it is the same mfr as the other 3 in the engine. Matched sets tend to expand at similar rates during warm up. I personally would not mix OMC and Wiseco pistons in the same engine.

 

[emdsapmgr]

Re: 1982 Johnson 150 piston diameter

Sorry, my mind was not paying attention, should read as the other 5 pistons in the engine.

 

[wilde1j]

Re: 1982 Johnson 150 piston diameter

Who did the boring? Most OB machine shops want the pistons in their hands before boring. As much clearance as you have is major trouble, not just a little piston slap.

 

[LDS2121]

Re: 1982 Johnson 150 piston diameter

Thanks for the info. With a bore of 3.529, should I replace the weisco with a .030 over omc piston? I do not know what the diameter of the omc piston is. Does anyone here know where I can find the information.
thanks again for the help.
Larry

 

[LDS2121]

Re: 1982 Johnson 150 piston diameter

Also , What is the proper clearance for the piston to the cylinder walls?
larry

 

[Dhadley]

Re: 1982 Johnson 150 piston diameter

3.529 is too tight. You're heading for a disaster -- very fast.

 

[Dhadley]

Re: 1982 Johnson 150 piston diameter

The size you mention is what most automotive shops will bore to. I'd suggest you get the block to a marine machine shop before you go any further. The piston part number should be 3173P3. Make sure it's not a 3073 or a 3013 part number.

 

[LDS2121]

Re: 1982 Johnson 150 piston diameter

A marine machine shop did the bore and honing of all cylinders. He told me he did not need the piston , all he needed to know was how much over I needed it bored, I said 30 over. He said the pistons are held to a very tight tolerance. I got the piston # 3173P3 from outboardmotors.com. There is no .030 stamp in the top of my piston. I am confused, you say 3.529 is to tight, but the piston is very loose in the hole. I am thinking that I got the wrong piston in the right box. Is there a specific clerance between the cylinder walls and the piston?
Thanks again
Larry