Piston to Block clearance has been determined!
05-05-2003, 10:55 AM
#1
Three Wheelin'
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Piston to Block clearance has been determined!
After some investigation and discussion, Millennium and I have agreed on the clearance between the JE pistons and new Nikasil surface.
Since these pistons are forged, and the engine is turbocharged, the clearance run will be right in the neighborhood of 23 tenths, or 0.0023". This is looser than the factory clearance of about 0.0016", but as the engine warms up, these pistons will expand more than the silicon alloy cast pistons.
I'd rather have a bit more room to breathe than risk scuffing of the piston crown. Those people running cast iron liners will have to run even looser than this because of the huge difference in thermal expansion rates for aluminum and cast iron. I'm very glad I ****-canned the sleeving option.
Factory clearances are close! I mean we are only talking about an additional 0.00035" per side here. That is a tiny, tiny number. There are three factory tolerance groups and each one is very close to the next. That is what makes these engines last so long...close tolerances and attention to detail.
I am feeling a lot more confident in this Nikasil, as well as the route I chose. Millennuim has done a lot of Ferrari blocks, as well as P-51 Mustang aircraft work. I'd assume they were a very good chioce for me.
I should have that golden-bore block back in my possession by the end of the week...
Let the games begin!!!
Since these pistons are forged, and the engine is turbocharged, the clearance run will be right in the neighborhood of 23 tenths, or 0.0023". This is looser than the factory clearance of about 0.0016", but as the engine warms up, these pistons will expand more than the silicon alloy cast pistons.
I'd rather have a bit more room to breathe than risk scuffing of the piston crown. Those people running cast iron liners will have to run even looser than this because of the huge difference in thermal expansion rates for aluminum and cast iron. I'm very glad I ****-canned the sleeving option.
Factory clearances are close! I mean we are only talking about an additional 0.00035" per side here. That is a tiny, tiny number. There are three factory tolerance groups and each one is very close to the next. That is what makes these engines last so long...close tolerances and attention to detail.
I am feeling a lot more confident in this Nikasil, as well as the route I chose. Millennuim has done a lot of Ferrari blocks, as well as P-51 Mustang aircraft work. I'd assume they were a very good chioce for me.
I should have that golden-bore block back in my possession by the end of the week...
Let the games begin!!!
05-13-2004, 05:38 AM
#2
Drifting
does anyone have more info on tolerance groups? i had a look in the workshop manual and became confused as to whether they apply only to the pistons or to the bores as well.
if swapping pistons from a 951 into the block, what are the numbers to be matched?
if swapping pistons from a 951 into the block, what are the numbers to be matched?
05-13-2004, 01:15 PM
#3
Rennlist Member
John, how much did it cost to get your block done. I am working on a mercedes motor, and am at the same crossroads, sleave vs nikasil. I would most definately prefer the nikasil. PM me off line if you prefer, and i do need to stop in and see your project. I will probably be heading down to Cinci again next weekend to check on my baby.
Thanks
Hans
Thanks
Hans
05-13-2004, 02:19 PM
#4
Instructor
John,
You're on the right track regarding expansion rates, but you have made a slight error. The expansion rate for aluminum is significantly higher than either cast iron or steel- not the other way around. Consider a single bore and remember that the volumetric expansion occurs in all directions (ie- for an aluminum torus shaped body the outer diameter would increase while the inner diameter decreased). This means that your aluminum block and aluminum piston engine combination will require a sufficient bore/piston diameter ratio to insure proper lubrication film thickness and piston/block expansion clearance. A ferrous liner would require less clearance as the decrease in bore diameter would be less versus the aluminum bore. Porsche is able to maintain tighter clearances as the Reynolds 390/Alusil block casting and Mahle/Kolbenschmidt piston material properties are very similar- as you mention. I would also confer with your piston manufacturer to ensure that your clearances are on target- everything changes when you increase the engines Mean Effective Pressure (MEP) with the utilization of any power adder (turbos, blowers, or nitrous). As a final thought, consider that a liner needn't always be iron or steel. KS manufactures liners specifically for the 928- http://www.msi-motor-service.com/dow...en/alu_en.pdf. Good luck with your project and keep everyone up to date with your progress. Of course, it goes without saying- PLEASE post pics once you have everything running.
You're on the right track regarding expansion rates, but you have made a slight error. The expansion rate for aluminum is significantly higher than either cast iron or steel- not the other way around. Consider a single bore and remember that the volumetric expansion occurs in all directions (ie- for an aluminum torus shaped body the outer diameter would increase while the inner diameter decreased). This means that your aluminum block and aluminum piston engine combination will require a sufficient bore/piston diameter ratio to insure proper lubrication film thickness and piston/block expansion clearance. A ferrous liner would require less clearance as the decrease in bore diameter would be less versus the aluminum bore. Porsche is able to maintain tighter clearances as the Reynolds 390/Alusil block casting and Mahle/Kolbenschmidt piston material properties are very similar- as you mention. I would also confer with your piston manufacturer to ensure that your clearances are on target- everything changes when you increase the engines Mean Effective Pressure (MEP) with the utilization of any power adder (turbos, blowers, or nitrous). As a final thought, consider that a liner needn't always be iron or steel. KS manufactures liners specifically for the 928- http://www.msi-motor-service.com/dow...en/alu_en.pdf. Good luck with your project and keep everyone up to date with your progress. Of course, it goes without saying- PLEASE post pics once you have everything running.
05-13-2004, 02:25 PM
#5
Instructor
Hmmm, not sure why Microsoft decided to abbreviate the link in the above post. this should work better- http://www.msi-motor-service.com/dow...ren/alu_en.pdf
