Merc 2.5 FF block deck hight question

**Jay Smith** · 12-02-2018, 08:40 PM

I’m building a 2.5 225 PM this week that was decked .009” on both banks customer bought this motor from a buddy , I built the motor in 2012 , has .030” top pinned Vertex pistons installed , motor was initially built with 1.25 mm thick head gaskets and ran fine , the port side had some issues that I don’t know what , the original owner didn’t notice the .009” stamped VERY prominently on the side of the block next to the deck , he inadvertently installed a 1.00 mm head gasket popped #2 took out gasket as well as the parent metal around sleeve that will have to have a flanged sleeve installed to repair the issue. I’m going to install a new set of pistons and plan on fly cutting the new pistons BEFORE I have Marles top pin them , I’ve tried to fly cut a set of pistons years ago that was top pinned and wasn’t real successful that tooling cuts the aluminum piston like butter BUT when the tooling contacts the hardened pin sh** hits the fan , just food for thought ... If your in doubt take a piece of soft solder bend a crook and install in a plug hole to reach the squish band area , turn the motor through and slowly allow the piston to crush the solder remove and measure it , my danger distance is less than .044” , anything less created by a cut deck will make me go to a thicker head gasket ...Good luck !

Jay @ JSRE

**Jay Smith** · 12-02-2018, 08:50 PM

BTW a deck mike answers a magnitude of questions before you install heads about your blocks deck height !! Most older blocks if not has been decked will render a piston Max .004” petruding out of the deck or .004” below deck , unless you have one of the newer blocks that has the block casted to use a flat O ring head this one at TDC stroke on the crank the piston will come .044” from the top of the block , this creates a safe squish with the totally flat head with only a combustion machined but no squish area furnished ..
Good luck ,

Jay

**xcheesehd** · 12-04-2018, 07:41 PM

Jay, Not to steal this thread but I have a new to me 280 that has over .070 in the squish area. Knowing that your danger area is less than .044 but what is the # that is to much squish? Thanks

**Jay Smith** · 12-05-2018, 05:56 AM

O ringed heads indeed can have more squish nothing you can do , my comment was to warn about an absolute tight tolerance condition that at high rpms can cause contact piston to head causing ring land clearance to reduce preventing the rings from “breathing” causing engine failure and if enough contact can bend rods and could cause crank con rod bearing failure ... My suggestion is to ( if available ) use a deck mike to confirm deck / piston position, most garloc style gasket compression fire ring can be measured , with these two known dimensions the assembler can go together with certainty of a safe clearances..

Good luck,
Jay

**Chaz** · 12-05-2018, 03:41 PM

Originally Posted by xcheesehd

Jay, Not to steal this thread but I have a new to me 280 that has over .070 in the squish area. Knowing that your danger area is less than .044 but what is the # that is to much squish? Thanks

It's not one person's thread to steal , it belongs to everyone. And you proved my point about , maybe clipping the OP's block .014 might not be A BAD THING IF THAT'S WHAT ACTUALLY HAPPENED ! If he has the same deal as your's .070-.014= .056 still shy of making the cover your azz minimum of .044 ..

Keep in mind that when measuring 2.5's that they have top guided rods (most of em) and if they are being checked while upright and you are turning them over by hand, the rod will droop down to the crank cheek and cock the piston in the bore. Tight on the botton, loose at the top. Brendon Powers makes a slick deck bridge with two gauges for those who don't want to measure one side then the other and split the difference to establish true deck clearance.

**Dave S** · 12-05-2018, 05:42 PM

ChaZ... YOUR RITE ONN ROD DROP... tHE WAYYY THWEY THE BORE IS SLOPPY MORE CRANK WOES YOU WILL LIVE....IF IT IS TIGHT IF NOT BOTTOM GUILDED......I DID THIS 30 YEARS AGO......44 CID TOP G AND LET HER LOUSER/..... JUST SAYING....

**Dave S** · 12-05-2018, 05:49 PM

jUST TO SAY .... SOME THING HAS TO KEEP THE ROD STRAIGHT... CRANK CHEEK OR PISTON PIN.... ON OLD MERCS THE ROD SETST IT STRAIGHT..... NEWER 2.5 .... THE PISTION KEEEPS IT WITHIN RIGHTS....MY SAY.... IF SLOPY BORE WILL EAT ROD BEARINGS.... NEED STEEL CAGEES.. NUF SAID.....

**Jay Smith** · 12-05-2018, 09:16 PM

Thanks for the help with the parts Dave S...
Happy Holidays ,
Jay @ JSRE

**Dave S** · 12-06-2018, 11:04 PM

Thank YOU ... Jay S......Merry Christmas.... To all.

**baja200merk** · 12-06-2018, 11:17 PM

The newer O ring blocks are stamped EE last one I measured needed ~.040 off the deck to run gaskets

**Dave S** · 12-06-2018, 11:23 PM

Queens NY.....Remmber... Broad channel races?......Best of the brungem what yaa got funnnn....

**baja200merk** · 12-06-2018, 11:28 PM

I was too young for the good stuff but plenty of Gerritsen beach racing, top end passes by Jimmy D from Rockaway in his HST at 127 and been chased out of starrat city on dirtbike before the mall went up

**Dave S** · 12-07-2018, 12:14 AM

Starat.... RAT city....Dammm went thru that every day on the way to redhook bk....Worked on the 150 st bridge new overpass....JFK.... sucky days.... now in sunny Va.....But the Broad beach was the thing in the old days....