Cutting Heads, Keep Getting Same Compression

[DIAMOND]

Hello All!

2.5 mod fishin motor

The first cut, 35cc and I get a reading of 125# on all six. Second cut, 33cc and I get a reading of 125# on all six again! :confused:

Now either some wierd voodo stuff is going on or my compression tester is crap. Does anybody have some suggestions? If my compression tester is off now I probably have too much compression. I'm going to buy another compression gauge and redo the test. Who makes a reliable and accurate compression tester? My first one I bought from the local auto parts store for I think $35.

Thanks for your advice!

DIAMOND

[Jay Smith]

My guess is its your gauge BUT always remember that head gasket thickness will render a weird reading and also the condition of the ring itself will also sometimes determine wheather compression can be held by a worn out ring as it is the last determining factor wheather cutting heads to a "normal" nomanal amount will be successfull. I have cut heads from a stock 38 cc's to 34 cc's for folks and gained no compression and on the free trip back to be cut again cut as much as .070" off more and still gain nothing only to find out that the rings were leaking by at some goofy 20% or so if thats the case you will have to cut the head in half to get where you want to be and when the motor is freshened and leaking down at a reasonable 2 to 8 % the heads are rendered useless..........

just from experience.........

[DIAMOND]

Thanks Jay,


The motor has about 3 hrs on a fresh rebuild so I'm leaning toward the gauge being bad. If that is so you will be getting another set of heads from me to cut and a set of 33's will be for sale......LOL:( The lesson of the day is to make sure your test equipment is well calibrated before you base decisions from them. Thanks again.

[The Dark Allison]

Diamond ,
I am no expert by any means but you need to wait till the motor has some time on it before cutting anymore. I just went thru the same thing. The difference being mine was 150 with 3hrs on it and jumped to 170psi after numerous hours on it. So I ended up detuning to stay at a safe level for pump gas. You may also go to a .75 gasket instead of the 1.00 it maybe good for small gains.




Just My Opinion!




James

[fasteddie]

Correct me if I am wrong but I seem to remember that the production blocks have some sort of a compression relief, cut or machined into the block, that will only allow the cylinders to build a limited amount of cranking compression.

[The Dark Allison]

Fast Eddie,
I don't know on the Mercs but the OMC's did have compression relief in the 80's and early 90's. My last two Mercury Fishin motors a 1994 XRI 150 & 1996 225 Promax I was able to achieve 145-150 psi compression with 33-34cc heads.I had to put stock heads back on the promax beacause it went up to 160-165 after breakin.:cool:

[DIAMOND]

Thanks Guys!


I'm going to buy a new better gauge a recheck the compression. Between the two cuts, from 35cc to 33cc, I noticed the starter motor was working harder after the second cut (33cc). This leads me to believe my original gauge is junk. My motor does have the relief cuts above the exhaust ports. I'm told they are there to bleed a little compression off at Idle to help the motor run smoother at Idle speed with a stock motor. The original stock heads only produced around 95 lbs of compression. After the first cut (35cc) it raised to 125 lbs. Based on the same gauge. I'm starting to believe that either the gauge is maxing out at 125 lbs or like Dark Allison said the rings need more run time to finish seating. Thanks again! DIAMOND

[us1]

3 hour on the motor would explain why the rings are not holding, they haven’t seated yet.

[DIAMOND]

Thanks US1,

I was wondering that myself, if the rings have seated yet. How many hours should I put on the motor before it's safe to make a decision on the motors compression? 10+hrs?

Thanks again all!

[Techno]

You could also try putting a leeetle bit of oil in the cylinder to seal it.

[us1]

They actually seat in the second 10-hour period when the motor is running 32:1 and is able to develop some heat.

[Dave S]

How about when the mota is outa roun and the rings are old used peaces o **** ? When will I ever git commpression?:D

[Stv Euro]

Your motor does NOT have enough time on it. Run 32:1 oil NOT all synth. The rings need some minerals to seat well. After they seat get you some Penn 100% synth if you can find it and turn the rpms on up!! Have fun!

[DIAMOND]

Thanks Guys!


I can't wait to get this motor past the break-in period and let it rip. Jay Smith performed his magic on this motor. It idles soooo smooooth! It feels like a racing horse waiting for the stall door to open. One more question, who sells a decent compression gauge? I just don't trust the one I have.