Filler block in Sportmaster LU???

**rujr** · 08-08-2015, 06:14 AM

This is a 1998 Mercury 2.5 280 Offshore mid/260 electronics and sportmaster LU. Can anyone tell me what the "filler block" is/does in the Sportmaster lower unit. It is an aluminum L-shaped part that sits aft of the water pump housing, separating the area of the water pump from a larger void aft (possibly an exhaust passage?). I am in the middle of servicing my lower and when I pulled it from the mid section, I noticed silicon caulk along the top edge of the filler block. I removed the caulk along this top edge because it did not look like it could possibly be doing anything worthwhile as the bead was irregular. During reassembly do I need to seal this top edge of the "filler block" to a surface of the midsection? In the mercury parts catalog it calls for 736 high temp RTV sealant on the filler block but is not specific on where to put it. Thanks in advance for the advice.

Also, why does my LU and mid section have a 1/2" water pump outlet? It is exactly like a stock low perf water pump housing and tube. I was surprised to see this when I opened it up. I assumed it should have a 3/4" water system for extra cooling capacity. Is this going to be a problem in the long run and should I worry about it/fix it?

See image from parts catalog: (filler block is part #7)

Click image for larger version.

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**s10gto** · 08-08-2015, 06:29 AM

As far as I know that block protects the water pump from the exhaust. Just run a bead of hightemp RTV across the top.

**rujr** · 08-08-2015, 06:40 AM

Thanks for the response. That is all I can imaging that block is for as well. And that would explain the high temp RTV. Any idea why my sportmaster has a 1/2" waterpump? It concerns me being this is on a 280 block. I thought they were all 3/4".

**Dave Strong** · 08-08-2015, 09:00 AM

Must have a 260 adapter plate, some were 1/2" some 3/4".

Dave

**rujr** · 08-08-2015, 10:05 AM

OK thanks for the help. Is there a problem with having a 260 adapter plate and a 280 block. Anything additional I should do to make sure cooling is adequate since it is not the 3/4"? Why would some have a 1/2 and some a 3/4 is what I am trying to figure out. I assume the 3/4 would have more cooling water flow at lower pressure whereas the 1/2 would have higher pressure but less flow/volume?

**Markus** · 08-09-2015, 01:45 AM

Only volume matters (as long as the pressure is high enough to get everywhere).

We measure pressure only as a proxy for volume on a given engine.

If half an inch is enough to cool a 260, it should be enough to cool a 280, too.

**Tom Foley** · 08-09-2015, 04:08 AM

260's and 280's uses the 3/4" water tube and pump top housing . I've never seen one with 1/2" tube , thought that was only the Pro Max that had that .

**BUZZIN' DOZEN** · 08-09-2015, 05:48 AM

Originally Posted by Tom Foley

260's and 280's uses the 3/4" water tube and pump top housing . I've never seen one with 1/2" tube , thought that was only the Pro Max that had that .

Nope , early 260's were 1/2" , the one's without the pipe plug in the side, just sold one to TUX for his Rally Sport project.

**tux974** · 08-09-2015, 05:54 AM

Jeff, you beat me to it.
Great info! The very easy way to tell from the outside is the pipe plug!!
The tuner was also different but works very well on PM's.

This one below has the bolts pattern for the newer tuner.

This is the newer 2.5 style tuner.