Engine Specific Info and Questions > IDI Engine

Scratch my 'Brow a 'Lateral' on the Thou...

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Mark(The Miser)UK:
On the 1.6TD Is the Bentley book correct with the piston to bore gap being only 1 thou?
Is this diameter or radius?
If engine block is being overcooled say with a 1/2 size pulley on the water pump...Is there not a chance of the piston swelling up and catching up with the bore on a newly rebored engine? Maybe the oil squirters are enough...Maybe the expansion accounts for some piston side wear...maybe they end up being wear to fit?
Great site that I rarely feel adequate enough to post on...But do enjoy lurking in the background... :oops:

Mark(The Miser)UK:
Coefficient of linear expansion for Al is 1/0.0000208 per deg C
                                        Cast  Fe is 1/0.0000108 per deg C

piston crown gets to 500 deg C;      
so 500 x 0.0000208=0.0104 or 1%

piston diameter 76mm...
1% is 0.76mm
                         or 30 thou!!!!!

This is 30 times 1 'Loren'....(8oD)
(Sorry a joke for another site)

Miser Math is Good Math

VWRacer:
Mark's math is correct, but what he writes doesn't tell the whole story. First, the maximum operating temperature for cast piston crowns is considered to be 300 C, with about 260 C being a "normal" high. Even though the temperature of combustion is higher than that, the cooling effect of intake air, plus conduction to the cylinder walls and oil bath draws heat away from the crown very efficiently. 500 C is well into tensile failure territory.

Also, the crown is not the maximum diameter of the piston, which is barrel-shaped in profile. The thickest part is where the piston slides past the cylinder walls, and where conduction is highest (and the CTE is lowest)...hence the very tight piston to wall clearance of 1 thou. Finally, don't forget that the 1 thou is for a cold (room temperature) engine. The cast iron cylinder walls also expand with increasing temp, though at about half the rate of Al. So, if the piston grows 20-thou and the cylinder bore increases 10-thou, the net growth is perhaps 10-thou -- absent cooling effects on the piston which keep the piston snug, but not galling at normal temps.

The tight clearance helps stabilize the ring package to maintain good compression, but there is still plenty of room for slop in the system. When I build gasser racing engines, I aim for about 2.5 thou cold, albeit with a Total Seal-type ring package. With that in mind, I strongly suspect one could easily run a TD engine at 1.5-2.0 thou with a good ring package, and that's what I intend to do with my racing engine. That is a lot tighter than a pure racing engine, but I don't intend to turn it to 11,000 RPM, either! :D

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