WISECO Clearance in OMC

**RH16** · 05-08-2006, 02:06 AM

I rebuilt my OMC with WISECO's and gave them an additional 2 thou clearance, I think it ended up about 6 thou in total. After I ran it in (about 10hrs) I did a comp check and leakdown. The comp is about 125 down the STBD bank and about 110 down the PORT bank. I then did a leakdown and got about 3% on all 6.

The funny thing was when I did the leakdown on the STBD side I rotated the flywheel clockwise until each piston was just before TDC, held it there and then did a leakdown. This worked fine for the STBD side but not the PORT side. Using this same method on the port side it was 10%+, however if I let the piston go just past tdc it was OK.

Has anyone had this before ?

**Jay Smith** · 05-08-2006, 06:54 AM

The issue with the height in the bore and the distance from TDC of the piston when leak down testing side to side has came up before. Balzy who makes the best leak down tester on the planet ( In MY opinion ) wrote a short post explaning why the piston can cock and give you a bogous reading then the piston be moved down a couple of degress and seal fine might try putting in a search for it , it was an interesting and educational post. 3 % is a great leak down. I like a little more clearance ( .008" ) with the production Weisco forged pistons in steel bores but if your completely through breakin and haven't "stuck" you should be home free! Also I haven't had good luck with the introduction of a thin 100 % syn oil to the combo of piston and bore you have until the motor is VERY broke in , stay with the stickier more load bearing petrolium oil , I've found the level of bore protection to be greater during the breaking in periods and a bit beyond........This is just from what I have found with the Mercury steel bored motors with the addition of forged pistons verses the stock cast OEM ones which have less expantion rate and less prone to gauld than forged when claearance and lubrication issues arent addressed ......

Good luck,

**Instigator** · 05-08-2006, 07:10 AM

I have built several Johnnys with Wiseco's with 100% success.
One of the biggest issues with them is the warm up period.
Don't run it w/o thermostats.
Most OMC/Wiseco failures I know of were "cold siezes" due to putting your foot in it before the pistons have grown to shape (round).
Let it warm a bit between trailer and WFO

Gary

**Lockjaw** · 05-08-2006, 12:53 PM

Letting it warm up first is a good idea for ANY engine.

My tundra upshifts at 2000 rpms when you first crank it. Its such a pita when you have to get out in traffic because that engine doesn't really come alive until about 3500.

**RH16** · 05-08-2006, 05:58 PM

I have left the thermo's in - I guess I'm just concerned that when the engine is running it's only getting 110lb of comp on the port bank - no real way of knowing I guess.

I will do a search for Balzy's post

**us1** · 05-08-2006, 07:17 PM

Originally Posted by RH16

I rebuilt my OMC with WISECO's and gave them an additional 2 thou clearance, I think it ended up about 6 thou in total. After I ran it in (about 10hrs) I did a comp check and leakdown. The comp is about 125 down the STBD bank and about 110 down the PORT bank. I then did a leakdown and got about 3% on all 6.

The funny thing was when I did the leakdown on the STBD side I rotated the flywheel clockwise until each piston was just before TDC, held it there and then did a leakdown. This worked fine for the STBD side but not the PORT side. Using this same method on the port side it was 10%+, however if I let the piston go just past tdc it was OK.

Has anyone had this before ?

If the lead down is all the same then there is a difference in the head, gasket of deck clearance from one side to the other.

**RH16** · 05-08-2006, 08:34 PM

John - thought that too. Deck clearence is pretty close to 40thou across the board. I measured this with solder on both the top and bottom of each piston

I'm wondering because of the direction of rotation if the PORT pistons are rocking in the bore a little causing the ring not to seal - and then for some reason they square up just after top centre.

The Port side wouldn't get the good l.down numbers until just after top centre, and the comp test reflects this.

I'm worried this problem is happening when the engine is running. Do you think the same problem would occur at 6000rpm as when turning on the starter.

**us1** · 05-08-2006, 09:01 PM

You can always start the motor with the compression gauge in one of the holes and shoot a rev. I do that test all the time.

**RH16** · 05-08-2006, 09:28 PM

I sort of thought that, but assumed there would be a good reason not too - I'll give it a go......

Do you tend to see much difference compared with cranking on the starter ?

**us1** · 05-08-2006, 11:40 PM

Only if it is a slow cranker. Thats why I do it. To get a true reading.

**Forkin' Crazy** · 05-09-2006, 03:24 PM

Originally Posted by Lockjaw

Letting it warm up first is a good idea for ANY engine.

The main reason for the warm up is forged pistons vrs cast. The forged piston is more dense, so it expands more. I have seen car engines that you could actually hear the pistons rattle until it got to temperature.

John, I never thought of running the engine with a compression tester in it...... Makes good sense though.

**Lockjaw** · 05-09-2006, 04:59 PM

You can run closer tolerances on forged, depending on the silicon content, the more silicon, the closer you can run the, but the more prone they are to cracking.

I have forged in my 280 ZX turbo engines, and they are really quiet. I put them in a 5 thousandths I believe.

If you were a really diligent engine builder, you could heat each piston and measure it, and adjust your tolerances accordingly.