Small bore vs Big bore

[pnwboat]

I have a 1988 125 HP engine with a possible cracked block. I found a 1995 120 HP bare block. Has any one tried to switch the guts (crank, rods, bearings, seals etc) from a small bore 3.312 (1988 125 HP) cylinder block to a big bore (1995 120 HP) 3.375 block? Other than the piston diameter, almost everything else looks the same. At least the part numbers seem to be the same. Looks like there are some differences in the cylinder drain/recirculitory system, but nothing major.

 

[Frank Acampora]

Re: Small bore vs Big bore

Everything is interchangeable except the pistons (obviously). Rods are the same, bearings are the same, wristpins are the same, and small end needles are the same.

HOWEVER--and here is where I am not sure because I don't know the 1995 crank stroke. USUALLY, the 125 had a 2.875 stroke and the 120 had a 2.80 stroke. Merc may have changed that in '95. The ports are higher on the long stroke engine to compensate for the extra stroke and still maintain port timing.
The cranks look exactly the same with the same numbers and I don't know how the factory differentiated them. It is, however, easy to measure down from the top of the block to the top of the exhaust ports. A long stroke engine will have the ports about 1/16 inch higher in the bore. You can NOT re-bore a 3.312 engine to 3.375. The 3.375 has a larger liner cast into it.

AGAIN, HOWEVER: I have swapped a short stroke crank into a long stroke block and it works fine except it produced less horsepower and I lost about 4-5 MPH.

It can be done without it BUT you really need a special tool to press out the wristpins without damaging the pistons or needle bearings. The other side of the tool has a press in section. If you try to press in the wrist pin without this tool, you will jam the needles between the wrist pin keeper collars and, of course, then they won't roll as the rod oscillates around the wrist pin. The tool is Chrysler part T-2990. It consists of a pillow block for the piston, a shim marked .290 at one end for pin removal and .310 at the other end for pin installation, and a wrist pin alignment and driver.

When you replace the big end of the rods on the crank, you MUST work slowly and carefully. You must align the ground portions of the rod big end so that a pointed tool slides SMOOTHLY over the break. Any deviation here and the service life will be shortened since the rollers will bounce over the mis-alignment while engine is running. Do not tighten any more than finger tight until alignment is perfect. This is "cracked cap" technology and each rod has a distinct matching cap that only fits it one way. If you tighten too much while mis-aligned, you will dent the mating surfaces of rod and cap. Then, the rod and cap assembly is ruined and useless since they will no longer mate properly.

Proper factory procedure calls for new rod bolts since they are stretched a bit when torqued to 180-190 inch pounds. I have re-used them with no problems. You need a 12 point 1/4 inch socket to fit the bolts.

 

[pnwboat]

Re: Small bore vs Big bore

Thanks for your thoughts Frank. I was wondering about the stroke. I'll compare the exhaust/intake port placement on the two blocks. I've re-built a late 80's 125 HP and an early 80's 70 HP motor. They're still running and that was 9 or 10 years ago. So I guess I did OK. I've never tried to swap the guts from a small bore to a big bore though. The '95 block doesn't have any pistons, so I was planning to get some new Weisco forged replacements with new wrist pins and wrist pin bearings.

I've seen the tool that you mention in the repair manuals. I made a piston pillow block out of a piece of oak that I've used in the past to press the wrist pins in and out. Worked great. The shim is something that I always wondered about. Sounds like the .290 and .310 are thickness measurements? Good info to know.

I'm familiar with the issue with big end cap alignment when re-assembling. Just have to be patient and pay close attention. I've got all new rod bolts. I figure it's cheap insurance.

Ed

 

[Frank Acampora]

Re: Small bore vs Big bore

I forgot to mention the really important thing: Late Force --I don't know the year--mounted with an adapter onto a Mercury midleg and lower unit have different splines on the drive shaft and in the crankshaft. So if your '95 block has them, you would need to modify the '88 driveshaft to work with that block if you wish to use the '95 crank

It is possible to use a long stroke crank in a short stroke block. The block is the same, only the ports are different. Matter of fact, Chrysler used a splotch of red paint underneath the bottom cylinder to designate the 140 (2.875 X 3.375) block as opposed to the 2.80 X 3.375 115 block. They made 4 versions of the same block: 2.80 X 3.312 99 cu inch (105, 120), 2.80 X 3.375 103 cu inch (115 and I think 135), 2.875 X 3.312 101 cu inch (125), and 2.875 x 3.375 104 cu inch (140 only).

If you leave the porting alone, you would PROBABLY get a little better low end torque and still sacrifice a bit of top speed. If you raise all the ports equally, you will essentially get the higher horsepower engine. HOWEVER:You can also port the bypass side for better airflow and more power (square and trumpet shape them), BUT DO NOT PORT THE EXHAUST. This engine has a common exhaust cavity and porting the exhaust makes it more efficient at blowing exhaust gases back into the cylinders--unless set up for racing. The engine will produce less horsepower.---At least 20 less.

 

[pnwboat]

Re: Small bore vs Big bore

Yeah I've seen the listings for the early 1990's crankshafts with the different splines. Pretty scary. I plan on using the '88 crank in the '95 block so I won't have the spline issue with the '88 lower end.

I like the idea of going from the small bore to the larger bore. That's the main reason for attempting to do this. I've experienced the effects of increasing engine displacement in an automotive four stroke engine. I was assuming that I would see some type of increase in performance in the two stoke also. Didn't think about the relationship in port location verses stroke though. Makes sense. I will definately pay very close attention to the ports. Thanks again for the tips Frank.

Ed

 

[Frank Acampora]

Re: Small bore vs Big bore

It's not like boring an auto engine. The most you will see is 3 cubic inches--that's only 3%. The engine won't even see it. Porting is much more important although a large bore engine versus a small bore engine of same displacement is more efficient at horsepower production. That's the reason almost all auto engines are oversquare.-- or is it undersquare?--I forget which term it is when bore is greater than stroke.

 

[pnwboat]

Re: Small bore vs Big bore

I saw your pictures of the bypass porting job over at the "Chrysler Crew" WEB site. Definately looks like it'll flow more air/fuel mixture. Looks like the square ports are slightly higher than the stock round ports. I assume this will changes the port timing and power band. My guess is that it'll probably moves the power band slightly higher?