Jaguar piston clearance problem
Discussion
Ring them and check, it sounds like you've been quoted forged piston clearances, although there maybe a good reason for it.
Search out some of Puma Racings posts, there was a conversation about it some while back and he said that the piston manufacturers cover their arses by quoting big clearances which could be reduced if you wanted.
Search out some of Puma Racings posts, there was a conversation about it some while back and he said that the piston manufacturers cover their arses by quoting big clearances which could be reduced if you wanted.
Have a look here.
http://www.pistonheads.com/gassing/topic.asp?t=107...
This explains that it's piston shape and design that primarily determines the required bore clearance rather than whether it's a forged or cast piston. Without knowing whether these Mahle pistons have such things as expansion control struts cast in and the degree of flexibility in the boss to skirt region it's impossible to predict the required clearance. However 4 thou is very high regardless of any of these factors for an LM13 type alloy.
Some pictures, especially of the piston interior, would help.
Even if the piston is designed for strength rather than flexibility and low expansion I'd be surprised if you had any problems running it at 2 to 2.5 thou clearance rather than 4. If it's an OE copy then it should run OE clearances which are obviously even lower. I'd like to know why Mahle say 4 thou and suggest it's an error.
http://www.pistonheads.com/gassing/topic.asp?t=107...
This explains that it's piston shape and design that primarily determines the required bore clearance rather than whether it's a forged or cast piston. Without knowing whether these Mahle pistons have such things as expansion control struts cast in and the degree of flexibility in the boss to skirt region it's impossible to predict the required clearance. However 4 thou is very high regardless of any of these factors for an LM13 type alloy.
Some pictures, especially of the piston interior, would help.
Even if the piston is designed for strength rather than flexibility and low expansion I'd be surprised if you had any problems running it at 2 to 2.5 thou clearance rather than 4. If it's an OE copy then it should run OE clearances which are obviously even lower. I'd like to know why Mahle say 4 thou and suggest it's an error.
Edited by Pumaracing on Thursday 10th October 16:10
DVandrews said:
Rule of thumb for piston skirt clearance is 1 thou per inch. 83mm is approximately 3.3 inches. In mm 1/1000 would be .083mm. Forged pistons sometimes run with a little more clearance.
Dave
Hmmm. There really isn't any such thing as a rule of thumb for piston skirt clearance, especially with cast pistons and particularly those with expansion control struts and deformable skirts.Dave
Nice to hear from you too grasshopper. Hope you're well.
Where are you getting this 0.01mm clearance figure from? I hope it isn't just from the nominal skirt diameter in the Mahle catalogue which is listed as 82.99mm and is NOT meant to be either an exact size or an indication of the design clearance from an 83.0mm bore.
The first oversize piston is listed with a skirt diameter of 83.50mm but doesn't mean you run it at zero clearance!
Where are you getting this 0.01mm clearance figure from? I hope it isn't just from the nominal skirt diameter in the Mahle catalogue which is listed as 82.99mm and is NOT meant to be either an exact size or an indication of the design clearance from an 83.0mm bore.
The first oversize piston is listed with a skirt diameter of 83.50mm but doesn't mean you run it at zero clearance!
Pumaracing said:
Hmmm. There really isn't any such thing as a rule of thumb for piston skirt clearance, especially with cast pistons and particularly those with expansion control struts and deformable skirts.
A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. Compare this to heuristic, a similar concept used in mathematical discourse, psychology, and computer science, particularly in algorithm design.Dave
That's a pretty agricultural piston design with no modern features like expansion control struts and I suspect it's just a straight copy of the original. The thinning of the skirt on the thrust and anti-thrust sides is where the flexibility to accommodate tight bore clearances is built in so as the piston heats up and expands it flattens slightly on that axis against the bore and expands more across the pin axis which is oval turned slightly smaller in diameter to start with. So I'd expect it to be able to run fairly tight.
However Mahle is a major OE manufacturer and in general you'd expect to be able to trust their recommendations completely so their 0.1mm clearance puzzles me even more. I think you need to phone or email their technical department first and ask why it can't run stock clearances if it is indeed a copy of the original piston design.
Personally I'd expect it to be fine at 2 thou but I can't tell you to risk hundreds of pounds worth of components by going against their recommendation.
However Mahle is a major OE manufacturer and in general you'd expect to be able to trust their recommendations completely so their 0.1mm clearance puzzles me even more. I think you need to phone or email their technical department first and ask why it can't run stock clearances if it is indeed a copy of the original piston design.
Personally I'd expect it to be fine at 2 thou but I can't tell you to risk hundreds of pounds worth of components by going against their recommendation.
Same things I have on my mind. I wrote them an email, but no real answer yet (instead THEY asked me about MY bussiness activinty, which I find funny).
I have one source telling me that they run a set at 0.08mms, which is fine (even with occasional club level drag racing).
Two differences I could see in piston design copared to the originals were: first of all the Mahle is much-much lighter, with reduced material at the crown area and secondly the top has no strenghtening ribs. Unfortunatly the original piston was soo dably seized, that I had to destroy while taking it out, but I think the skirt is also thinner then the original.
I also have a suspect on my mind: theese cars are prone to overheating even with perfectly working cooling systems. Dont You think that it might be a prequotion to avoid unnecessary complains?
I have one source telling me that they run a set at 0.08mms, which is fine (even with occasional club level drag racing).
Two differences I could see in piston design copared to the originals were: first of all the Mahle is much-much lighter, with reduced material at the crown area and secondly the top has no strenghtening ribs. Unfortunatly the original piston was soo dably seized, that I had to destroy while taking it out, but I think the skirt is also thinner then the original.
I also have a suspect on my mind: theese cars are prone to overheating even with perfectly working cooling systems. Dont You think that it might be a prequotion to avoid unnecessary complains?
Edited by camelotr on Friday 11th October 22:41
camelotr said:
Same things I have on my mind. I wrote them an email, but no real answer yet (instead THEY asked me about MY bussiness activinty, which I find funny).
I have one source telling me that they run a set at 0.08mms, which is fine (even with occasional club level drag racing).
Two differences I could see in piston design copared to the originals were: first of all the Mahle is much-much lighter, with reduced material at the crown area and secondly the top has no strenghtening ribs. Unfortunatly the original piston was soo dably seized, that I had to destroy while taking it out, but I think the skirt is also thinner then the original.
I also have a suspect on my mind: theese cars are prone to overheating even with perfectly working cooling systems. Dont You think that it might be a prequotion to avoid unnecessary complains?
What was the outcome with contacting Mahle did they say it was ok to run a tighter clearance?I have one source telling me that they run a set at 0.08mms, which is fine (even with occasional club level drag racing).
Two differences I could see in piston design copared to the originals were: first of all the Mahle is much-much lighter, with reduced material at the crown area and secondly the top has no strenghtening ribs. Unfortunatly the original piston was soo dably seized, that I had to destroy while taking it out, but I think the skirt is also thinner then the original.
I also have a suspect on my mind: theese cars are prone to overheating even with perfectly working cooling systems. Dont You think that it might be a prequotion to avoid unnecessary complains?
Edited by camelotr on Friday 11th October 22:41
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