Asking for help from old school Mercury experts on motor identification

[RHafley]

Hello all, I’m new to the forum so thanks for having me. I recently bought a 1991 model bass boat with a 1991 Mercury XR4 on it. I’m thinking about pulling it down this winter for a refresh but as I’m hunting around for parts I’m beginning to find out this motor may be an oddball. It looks like a 2.5 but it’s a 2.4 from what I can gather. Apparently the head gaskets are unique, the pistons are unique etc. This is my first Mercury, I’ve always had Yamahas. What did I buy? Is it any good? Can it be pepped up easily? I’m trying to educate myself on these motors and I appreciate any help or advice y’all can give. Thanks.

[RHafley]

I realize a little more information is probably in order so here's what I've got so far

Serial number is 0D007895
393632393633393634393635

[mach351]

That one could go two ways....you really need to pull a head and measure the bore.

it could be a “Fat Block” 2.4 steel bore xr4. Essentially stuck in between when merc switches from the 2.4 to the 2.5. So it’s got 2.4 sleeves inside a 2.5 Block. 3.375” bore

It could also be a 2.5 with compression reliefs that someone put on an xr4 mid/cowl. 3.5” bore

[RHafley]

Isn't there a plug somewhere on the top of the block with the HP stamped on it?

[Capt.Insane-o]

They were a 2.5 block with 2.4 sleeves, and the most horrible block to sleeve finishing or lack there of. Couldn't stand to look at mine so it was bored and in went some 260 sleeves.

[tlwjkw]

0D007895..... '91 XR4 "fat block" but still 2.4 piston ported motor like all its predecessors.. pretty much like capt. says..

[Da Bull]

I would sell it and buy a 2.5 200hp long block. Same bolt up and all your parts will fit. Amazing difference in power.

[RHafley]

I would sell it and buy a 2.5 200hp long block. Same bolt up and all your parts will fit. Amazing difference in power.

What should a fella be looking to spend on a 200hp 2.5 long block? Also what about a 2.4 200 if I can find one? Are they any good? Thanks for the help everyone!

[Da Bull]

For a 2.4 200 I would think no more than 1K. A 2.5 200 will be more of course. Keep your eye peeled on this site as well as others and you`ll find what you`re looking for.

[Da Bull]

It would be best if you can find a complete drop on. Your intake is vertical reed set up while the 200`s are horizontal. Now if by chance you find a 2.4 175 go for it. It has vertical reeds to so your intake is a direct bolt on. The 175 is the same block as the 200 with the same porting. It`s a good power improvement over your 150.

[Capt.Insane-o]

The fat block 2.4's were horizontal reed. Everything is the same as the 2.5 but the pistons and sleeves. That motor had the loser water cooled voltage regulator too, but pro marine sells a conversion kit for that.

[Da Bull]

Good to know about the fat blocks. I was unaware of that. Never had one. This should make it easier for Mr RHafley. What year did they come out?

[tlwjkw]

everything vertical went away in '91 with tha "fat block" arrival................ you won't see a "vertical" on anything but older (pre '91) 2.4/2.0 style blocks......

[BVfast]

The 2.4 fat block does not have the behind the liner boost ports that go to the bottom of the bore. These blocks have the worst miss-matched finger & boost port casting that merc. ever produced. To make it a 2.5 behind the liner motor will take a lot more machine work than just boring it out to put 2.5 sleeves in it. It is just too much machine work & hand grinding to make this happen with all the 2.5 blocks out there that can be purchased for a very fair price. This is just my take on this, having been in drag & tunnel boat racing & outboard machine and engine building for 40 years.

[RHafley]

So if I understand everyone correctly...my motor is essentially a 2.5 on the outside. If that's the case then can I get a carbed 2.5 200 longblock and have all of my external essentials bolt up and operate?