Fast Fred's Secrets of the OMC MOD50's Thread #1

[David_L6]

What clearance do you recommend for the TRW "Mod 50" pistons?

[seaward]

ive just found x3 of these fred .. this one has had a BOOM at some stage looking at it

Hello 850 Would you be willing to sell one of the 3 powerheads. Hal

[sheuninck]

so ... a stock cylyder is .008 clearance for forged pistons
and a .040 over cylider is .009 clearance for forged pistons
Therefore a .040 over cylinder will expand .001 LESS when at running temperature.... correct?
I see alot of racers removing the thermostat, but I was once told to run the coldest plugs and 180 degree thermostat vs the stock 140 degree thermostat. OMC used the 140 degree themostat only to run cooler temperature to reduce salt buildup in the water jackets. True?

[Tim Kurcz]

Hey y'all,

It's IMPORTANT to remember piston to wall clearance is "taken" from the piston during design and development. This is the same for all manufacturers. For instance, the Mod-50 has a standard bore of 3.030". The piston will be smaller by the desired clearance. To go +.030", you would bore to a finish dimension of 3.060", etc.

Also CRITICAL to note is most 2-stroke pistons have been cam turned since the 1970's. They are not round and most all have taper built in. So the clearance is theoretical unless you know where to measure, and at what temperature! You're best to follow the piston manufacturers recommended bore size or take your chaces if you want to fit looser or tighter - $$$.

Finally, as mentioned, the eutectic properties of the aluminum grade has everything to do with thermal expansion rates. Unless you're on the engineering team that selected the material, conducted the testing, and specified the clearances, again you're better off with the manufacturers recommendations.

I hope this clears up the question.

Tim

[sheuninck]

Gentlemen,
I just wanted to know if a bored out cylinder expands more that a stock one.
The block is alumunum, the sleeve is metal. With a bored out sleeve there is less metal, so would it expand more or less at running temperature?

[Tim Kurcz]

Nope. Physics is physics. The expansion rate is fixed for the either metal regardless of bore size, unless you change the operating temperature.

[850cc racer]

Hello 850 Would you be willing to sell one of the 3 powerheads. Hal

well i dont have them but know where they are.. your looking at like 4grand per power head

[sheuninck]

Nope. Physics is physics. The expansion rate is fixed for the either metal regardless of bore size, unless you change the operating temperature.

true, but there is a length change also.


change in length = length*change in temp*coeff

aluminum expands twice that of metal
metal expansion coeff =.0000064
.040 overbore results in a "0mean sleeve dia" change of .020
.020*Pie= .0628319 = "mean length" change of sleeve.
change in temp (70F to 140F)= 70
Therefore .0628319*70*.0000064 = .0000281

OK so basically no change. Thanks

[Fast Fred]

So you go bore plus .009 with a forged piston

NO, this statement is wrong and or not true.
this is true, the forged piston Diameter + .008 = the Bore. which would BE a tight fit for said bullet.

the more bore that is removed from the cylinder the weaker it's ability to transfur heat, so at +.040 over it's common to give it .001 more.

you have to Measure the piston to know what size it is. no two ways about that.

piston first, then bore, Never bore first.

[Fast Fred]

What clearance do you recommend for the TRW "Mod 50" pistons?
__________________
David
L-6

i'm likin .008, i'd see what Dave at Sea Way had to say, then go from thare.