2.5 ?

[mercrazy]

Can someone explain how to identify these?
big bearing block vs small
4 bolt main vs 2 bolt or what engine is a 4 bolt
difference in 2.4 and 2.5 crank and bearings

[25two.stroke]

Big bearing vs small bearing is the upper main. Only thing this mattered on is a 260...all factory 280 were big bearing, as were the Drags (so block stamps QQ, SS, S and R).
All promax are small, all fishing motors are small, all 2.4L are small (but also varied some), all carb motors (like the 2.5 245) were small. That is; if noone has bored them out. You can bore the upper main area to fit a big bearing on almost anything.
No way to tell on a 260 from the outside. In general most PP 260 are big bearing, and most LL are small. But I have seen variations to this in both directions. Every late 260 that has the alternator bosses of a 280 cast but not tapped [that I have seen] is a big bearing PP 260.

4 bolt main is the perimeter bolts, not the true 8 "main bolts."
4 bolt main have only 3 bolts for each side on the outer crankcase flange connection. These were typically a 1/2" head and 5/16-18 thread.
8 bolt main have 5 bolts for each side which were typically a 12 point 3/8" head and a 3/8-16 thread (the same as the 8 center bolts)

Someone pull up the crank thread from the archives...I don't feel like typing it all out again.

[tlwjkw]

"big bearing" is normally referred to tha top main bearing.. sometime mid 90's it was replaced with a large ball bearing similar ta tha bottom main..

tha 2.4 center mains were steel caged used with a non shouldered crank (production 2.4 motors only) tha hi performance 2.4 used a plastic caged with a shouldered crank.. this crank can be used in tha 2.5 block...

[mercrazy]

Is the top crank journal the same size, just the bearing is larger?
are there pictures somewhere so I can see the difference in a shoulder vs non shoulder crank?

[tlwjkw]

Mercury Tech 2.4 crank in a 2.5?

but understand tha tha only 2.4 crank with tha shoulders is tha HIGH PERFORMANCE 2.4.. production will not work...

[25two.stroke]

Is the top crank journal the same size, just the bearing is larger?
are there pictures somewhere so I can see the difference in a shoulder vs non shoulder crank?

No, the crank top main journal is slightly different on a big bearing. The big upper bearing is a caged ball bearing like the lower main...a small bearing is a needle bearing that rides directly on the crank journal.

[LakeFever]

I’m surprised no one has machined up something to use a small bearing crank in a big bearing motor. Can’t be that hard

[tlwjkw]

I’m surprised no one has machined up something to use a small bearing crank in a big bearing motor. Can’t be that hard

me also.... i could go with that before i could trust tha other way...... jus was never in favor of removing material thats not supposed ta be removed with some of these stem winders!... ie: 2.4 front half ona 2.5 block..

[25two.stroke]

me also.... i could go with that before i could trust tha other way...... jus was never in favor of removing material thats not supposed ta be removed with some of these stem winders!

Mercury did make this adapter and Jay Smith has mentioned it on here multiple times...but I think he is the only one who still has a couple on the shelf. I do not think it was ever in a production motor, it was an accessory available to race teams only.

[tlwjkw]

all these years and thats first i've heard of it!

[25two.stroke]

1159-832012t2

[tlwjkw]

good ta know for some that still believe in tha "little" motors though...................

[Stoker boy]

Mercury did make this adapter and Jay Smith has mentioned it on here multiple times...but I think he is the only one who still has a couple on the shelf. I do not think it was ever in a production motor, it was an accessory available to race teams only.

I have a 2.5 block that was made for the big bearing. I installed a small crank and did use one of those adapters.

[25two.stroke]

I have a 2.5 block that was made for the big bearing. I installed a small crank and did use one of those adapters.

Did it just look like a standard small upper main cap but wider to fit the big bearing bore? I have never even seen a picture of one.

[Stoker boy]

Did it just look like a standard small upper main cap but wider to fit the big bearing bore? I have never even seen a picture of one.

Everything‘s identical for outside diameter set up for large bearing crank only difference the inside diameter accommodates the smaller crank. As you know, of course, the top of the block halves are larger diameter for the large crank.