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STVMatt

decked block,?

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I had to get my block(s3000) decked to get it right again.

I can either run decked pistons or a thicker head gasket.

There are different opinions on this. Some say the decked

pistons trick the engine into thinking the port height is

different. Will either set up run worse or better than the

other?

Thanks for the input.

Matt

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Matt,

Before I answer this, I need more information.

HOW MUCH did the you deck the block?

Is the S3000 stock or have the ports been raised?

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Normally the S3000 pistons extend 0.004 - 0.006 out of the block.

The thickness of the gasket will determine squish clearance (how close the piston comes to the head). Squish clearance is CRITICAL. Too little clearance and the piston will contact the head at high rpm when the rod (and everything else) stretches.

Too much clearance, and the engine will be prone to detonation. If the piston does not come close enough to the head, the boundary layers of air along the surface of the head and the piston (about 0.020 inch thick of each) will not be disturbed. If these layers do not get disturbed, they do not get cooled by the fuel and air mixture, and WILL develop hot spots. If these hot spots get hot enough, they will ignite the fuel and air mixture before the spark occurs (pre-ignition). This is just like adding timing advance. Eventually the hot spots can cause the combustion to become erratic and uncontrolled. When the combustion is no longer occuring in a controlled manner, it happens too rapidly - like a small explosion - and cylinder pressures increase dramatically - this is detonation.

The squish clearance needs to be kept between 0.035 to 0.037 inches. Not only will this reduce the tendancy to detonate, but it also causes better turbulance in the chamber and makes better power - no matter what compression you are running.

"Decking" the piston is a lot like raising the port heights of all of the ports. This can be good and bad. The biggest reason not to do this is that when the piston is at bottom dead center, the piston will be lower than the bottom of the exhaust port. This is NOT good for air flow through the engine. Another reason not to do this is that the ratio of the transfer and boost port heights has a lot to do with making power - especially good power at a specific rpm point. Raising all of the ports by the same amount will primarily just raise the power band to a higher rpm point where the engine will make peak torque and peak power. If the port area is not increased accordingly, you will still run out of air flow at the same rpm point - all you have done is made the engine more "peaky".

Trust me when I say I learned all of this the hard way.

I hope this helps.

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you know i have been thinking about this post wayne.. when figuring out the squish, esp on a hi-performance or racing engine, would you take any high rpm rod stretch into the figuring??? would you take into consideration say, the 8118's stretching more then the 5250's and adjust for such?? - if they do stretch more that is, id think they do but i have no basis for that..

just wondering-- the gears are always turning in my head and id really like to take my own personal engine building to a new level and then one day be able to help others better then i can now..

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The best way to check piston to head clearance is with solder.

Get some .060" solder and cut off enough to make a ring that fits in the bore, about 3/8" smaller. Stick the solder ring to the piston with some grease.

Assemble the head with the gasket you want to use, preferrably a used gasket that has already been crushed.

Turn the crank with a big wrench, you will feel the solder flatten out moving the piston thru TDC.

Take the head off and check the thickness of the flattened solder with a caliper, micrometer etc, that is your squish clearance.

If you use the Merc 26cc Drag heads, you gotta get right out at the edge of the piston with the solder cause of the tapered quench.

I have run OMC's as tight as .035" and had no problems using this method to check.

I dont think either rod stretches enough to be a problem unless something else goes wrong. like blowing a gasket or loosing compression.

2 strokes keep the rod in compression as long as everything is working correct. They are either building compression or making power at TDC. Unless something goes wrong, the rod never has to pull the piston down from TDC.

ER ;)

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i got ya er.. yes, i wsnt thinking about the solder- i have used it checking clearancewhen cutting heads on certain engines i was not sure of, i wasnt even thinking of it for squish..

on another note, i am helping a friend build his 85hp johnnie crossflow.. any thoughts on getting it running a bit more efficiently for best performance?? we want some more compression for sure but i have not checked it out yet, id love to hear any thoughts you have.. this is on a light, flat bottomed in the stern, play boat that will have a few people in it sometimes... thanks again

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