Merc 2.5l restoration - ring choice

[Patkmg]

I have had to hone out #4 cylinder more than the others, can I use a standard 3.0L ring and file down to spec gap? I appreciate any input- thanks

 

[Baylinerchuck]

MOD EDIT - It's moved. Don't re-post.

You should repost this question in the Mercruiser section or have a moderator move it. jbcurt00

 

[GA_Boater]

A 2.5 has a 3 7/8" bore and a 3.0 has 4" bore. Lets just say it won't work.

Welcome aboard.

 

[Scott Danforth]

I have had to hone out #4 cylinder more than the others, can I use a standard 3.0L ring and file down to spec gap? I appreciate any input- thanks

No

and by reading this, you did not have the block taken in to have bored and honed. if you used a 3-jaw stone hone or a dingleberry hone, how did you straighten the cylinder prior to honing?

how much taper did you measure with the bore gauge? how much out-of roundness did you measure?

 

[Patkmg]

Gentlemen thank you for your input.. since this is my first effort it is learn on the go..I should have read the specifications in the mercruiser service manual, which I have now done. Cylinders 1-3 are pretty close to spec but unfortunately 4 is not. I am using a three arm home.. I am measuring out of round with digital diameter measurements and then comparing ring gap measurements on both crank and head ends. The bottom line is that there was so much corrosion in 4 that I am at a 3.959 in diameter. It has been an interesting experience..

 

[achris]

On 4 stroke engines, you must bore all cylinders the same size. If one is out of spec and needs to be 'enbiggened', then all 4 must be done. You can't do just one cylinder.

And when you buy a new set of (oversize) pistons, rings for that oversize come with them.

Reading between the lines, are you trying to re-use the original piston in an over-sized bore?

Chris.....

 

[Patkmg]

Chris, thanks for the quick reply. So when I bought this engine for 100$- to Reuse the original pistons was my intention - I had no idea that I would have to hone out this far to remove the corrosion pits. It looks like my only option is to get it bored out for a full 3.0L setup..

 

[Scott Danforth]

you cant bore a 2.5 liter block into a 3.0 liter. the walls are not thick enough to go more than 0.060" over-bore. not to mention the stroke is different between the two. the block is taller on the 3.0 for the increased stroke.

for the cost of just buyin gaskets and rings for your 2.5 (not even buying bearings, etc), you could have bought a running 1992 or newer 3.0 and outdrive in a rotten boat and simply scrapped the rotten boat.

 

[Patkmg]

My son wanted the boat.. so I took it on as a buddy project. Since I am an engineer I am not afraid of taking on a challenge.. this one I just did not get enough research done before I got further in on it.. thank you for the verification on the ability to bore out the 2.5.. much appreciated

 

[achris]

An engineer a mechanic does not make.

Just because you can design them, doesn't automatically mean you can fix them.

No disrespect intended, but some of the worst damage I have seen done during a repair was by an engineer who thought 'how hard can it be?'.

Unless you are an experienced mechanic, don't assume you know what you are doing.

Chris

 

[GSPLures]

The service manual should give you specs of how much you can bore the block (At least the mercruiser manual I have does). If you already went past the biggest oversize you can install the block is trashed. If the rest of the parts are in usable condition you can search for a short block for replacement although as others have stated might be cheaper to find a donor boat with a good motor and drive.

 

[Scott Danforth]

An engineer a mechanic does not make.

Just because you can design them, doesn't automatically mean you can fix them.

No disrespect intended, but some of the worst damage I have seen done during a repair was by an engineer who thought 'how hard can it be?'.

Unless you are an experienced mechanic, don't assume you know what you are doing.

Chris

i agree with this about most engineers, and I am one. however long before I was an engineer, I was a wrench. Engine rebuilding is 5% text book, 20% labor, 30% having the right tools and 45% hands-on training

However now, I am striving to become a chemical engineer....... getting really good at turning whiskey and beer into urine.

 

[Patkmg]

Enough engineer bashing.. this is not my first mechanical project- in my 60 years I have done plenty of other mechanically oriented efforts. I was an electrician in the navy and have plenty of experience getting dirty. At a minimum I am treating this as a classroom experiment.. i am always open to learning new things, but like to work with my hands as I don’t get a chance to do that with my day job. I have appreciated the posts that have been value added..

 

[GSPLures]

Im not sure what year your motor is. This is out of the merc #1 manual which covers the early 120 motors

If you mic'ed your cylinder correctly your too big for the biggest piston they show which looks to be .030 oversize. Which is not to say you cannot find a piston large enough, but there is probably not enough meat left on the walls to not cause catastrophic failure.

 

[Patkmg]

Thanks for the help.. yes I am reading these earlier manuals with more understanding as I am absorbing the info.. I have shifted gears on my effort as it looks like the 2.5L block is now scrap.. interestingly with the 2.5L a 120was a throw in to the deal, but after tearing it down it looks rebuildable.. minimal cylinder wall cleanup needed..

 

[GSPLures]

Tear down the motor and keep all the good parts for backup. Some of those parts you cannot get anymore

 

[Patkmg]

The 120 piston are 3.835 in dia and the 2.5L has 3.875 dia.. I assumed they were the same .. but no.. the starting diameter of the 120 cylinders are all 3.875 in .. something to work with.. yes I have two carbs one mercarb the other a Rochester ..

 

[Scott Danforth]

the 120 and the 2.5L are the same engine. (never to be confused with the pontiac 151 "iron duke)

they are the 153 cubic inch chevrolet 4-cylinder that originally debut in the 1962 Chevy II. that was the launch of the new chevrolet 151/194/230/250/292 series of in-line motors. they are the 230 cubic inch in-line motor with 2 cylinders removed. 3.875" bore, 3.25" stroke. the smaller 194 had 3.5625" bore and same 3.25" stroke

if you are measuring a piston at 0.040 undersized at 3.835", the piston collapsed. most likely from an overheat