25 Mercury Mercasil Block rebore

[Jerry Williamson]

Looks like I need to rebuild a 25 Mercury 1998 with a Mercasil block. One machine shop says he cannot hone because it takes diamond hones. I have 90 pounds conpression even on both cylinders what do you think and has anyone had any experience rebuilding a 2 cylinder Mercury. Thanks

[jeff dunn]

Jerry I bore several of these motors a year, I bore them just like any other outboard, I use the wiseco piston they make for this Mercasil block. Just make sure to use the clearance specs sent with pistons. Most of the time it takes a .030 to .040 over size to clean the clyinders. Jeff

[Hector]

Hey Mr Jeff , I have a 99 merc 25 the top has 140 and the bottom has 40 . do you have a shop ? can you help me rebuild it . need the boring done and the parts . Thank you HR Snethen

[25XS]

Jeff and Hector: I know this thread is really old now, but I'm interested because I own several Mercasil block 20/25hp Mercury engines with bad bores... Three of them were previously bored to 0.030" oversize and lasted less than a few hours in service. As it turns out, traditional machining tools & techniques were used which does not work on Mercasil bores. Mercasil is a high silica aluminum bore (think of plaster mixed with sand then hardened). The bores need super hardened cutting tools and a diamond hone will polish exposed silica crystals flat for rings to ride on. The aluminum between the crystals needs to be "relieved" from the surface either by acid etching or soft tool honing with basically a felt stone loaded with a honing paste that abrades the aluminum below the surface of the silica crystals to hold oil.

Done "by the book" the bores can be just like new. Done with traditional machine shop tools and honing stones, it's a crap-shoot at best.

Here is the Mercury Marine engineer's own paper on the Mercasil block project: http://www.ncccoat.com/PDF/MercuryNC...icalpapers.pdf

Anyway, I'm interested if anyone who has bored & honed a Mercasil block can follow up here with verified info that a previously oversized block is still in service and what the compression numbers read after the rebuild and after several hours were logged.

Jeff, I "think?" I bought a brown metalflake Python Vector 14ft speedboat from you several years ago...? Was that you?

Thanks,

Tom

[Onetime]

Sounds very similar to the old Chevy Vega blocks. Would think the same procedures would need to be used for correct cylinder resizing.

[FMP]

ttt, XstreamVking

[XstreamVking]

This is some of what I read about the mercasil blocks. Good info and prob will swing the whole thing to a new block. Also have read that the mercasil is generally 40 thou thick and when you bore 30 thou it can cut thru in places causing failure. (bore out of round) etc. Thanks FMP...

[FMP]

Where you getting a new block?

[25XS]

Also have read that the mercasil is generally 40 thou thick and when you bore 30 thou it can cut thru in places causing failure. (bore out of round) etc. Thanks FMP...

Another misconception: Mercasil is not a coating or thin layer. Mercasil is every bit of the aluminum squeezed into the block mold. 100% high silica aluminum all the way through, inside and out, top to bottom, etc... The older Mercury and U.S.Chrome blocks with "Nikasil" and straight up Chrome were both coatings added to the aluminum block bore. The 20hp and 25hp blocks never got a steel sleeve. 1994-1/2 model year was the change from hard coated bores to solid Mercasil blocks for the 20/25hp engines. For these engines, the factory never honed them! The new piston assembly was placed directly into a finely finished bore. The referenced paper from Mercury Engineering tells this entire story.

[FMP]

Are both chrome type and later mercasil both exactly the same in porting specs and proper healthy psi? My older chrome 25 is strong, not sure the newer 2004,5+ version is better in that respect.
Glad to hear the later can be finished and Wisecoed

[Chaz]

Diamond stones need continuous / hi volume oil flow. 50~50 mix hyd oil / kerosene .
To get started :
Roughing set ... $450.00
Bottom pressure set ... $450.00
Finish set .... $450.00
Reverse rotation Osborne brush ... $60.00
+ shipping ...

BTW .. they break easier than you think, might as well buy two sets of each unless you don't mind getting stuck halfway thru a job ..

Chevy Vega's , Merc 25's .......... >
















And now we have come full circle ... Merc V-8's