[Fot] Never be beaten by equipment

[yellow04 at tr4racer.com]

Scuffing is caused by piston expansion. Piston expansion is caused by heat. 
If you are running adequate piston to liner clearance and you are still 
getting excessive scuffing only on #4, perhaps the back of your block is not 
getting adequate flow of coolant. If the coolant at the back of the block 
does not circulate adequately, hot spots form, allowing the coolant around 
#4 will reach it's boiling point. Once the liner creates enough heat for the 
coolant to start vaporizing (boiling), a jacket of air bubbles is formed 
around the liner, further impeding the coolant from doing it's job. That jug 
gets heated more that the others, your piston expands more, and scuffing 
occurs.

Next time you are prepping a block for a build, and are at the step of 
rinsing the block with copious amounts of water, try this. Slide all four 
liners in the block, and set in place whatever head gasket you are using, a 
used head gasket is fine for this purpose. You are going to want to install 
a few head studs and jury-rig something to hold the head gasket in place. 
Then take your garden hose and stuff it into the water inlet hole in the 
front of the block. Turn on the water.

You will note the lions share of the water is flowing out the front of the 
block, and around #4 the water is barely overflowing. That's whats happening 
to your coolant when your engine is running. Your 'cold' coolant from the 
radiator enters the block at the front,  but most of it flows around 1 and 2 
and goes up to the head and exits the thermostat housing. The coolantin the 
back half of the engine is pretty stagnant, allowing hot spots to be formed.

For the most part, the holes in the head gasket are the what meters the 
water flow from the block up through the head. On the stock head gasket 
those holes allow too much flow at the front. Look at Mordy's copper 
headgaskets for this engine, you will see he has made the coolant passage 
holes quite small at the front.

To increase circulation at the back of the engine, you need to impede the 
coolant flow from the block to the head in at least the front half of the 
engine. I install small freeze plugs in the appropriate coolant passages in 
the head, and then drill small holes in the freeze plugs to meter the flow. 
This stopped all my issues with  excessive scuffing on the #4 piston.

But what do I know, I blew up this year as well!

Henry
yellow04 at tr4racer.com

----- Original Message ----- 
From: "Robert Blake"
Subject: [Fot] Never be beaten by equipment


> So other than lack of piston to liner clearance,
> what causes pistons skirts to scuff on the liner?  Can a lean mixture on 
> the
> rear carb cause this without showing up as a hot running condition?