Jaguar piston clearance problem

Jaguar piston clearance problem

Author
Discussion

camelotr

Original Poster:

570 posts

167 months

Thursday 10th October 2013
quotequote all
Greetings!

We have an xk150s engine under rebuild. We wanted to use Mahle pistons. Factory pistons-bore clearance was 0.025-0.04mms. Mahle suggests 0.1mms, I find it HUGE.

- pistons are cast
- bore diameter 83mms
- solid skirt
- 3 rings
- 9:1 cr

What do You think?

Gergo

DVandrews

1,315 posts

282 months

Thursday 10th October 2013
quotequote all
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

Edited by DVandrews on Thursday 10th October 09:00

PeterBurgess

775 posts

145 months

Thursday 10th October 2013
quotequote all
You need to go with Mahle recommendations as they are modern pistons and made by Mahle, not old factory ones. If you are still unsure contact Mahle for specific advice.

Peter

Evoluzione

10,345 posts

242 months

Thursday 10th October 2013
quotequote all
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.

Pumaracing

2,089 posts

206 months

Thursday 10th October 2013
quotequote all
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.

Edited by Pumaracing on Thursday 10th October 16:10

Pumaracing

2,089 posts

206 months

Thursday 10th October 2013
quotequote all
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.

camelotr

Original Poster:

570 posts

167 months

Thursday 10th October 2013
quotequote all
Hi David,

Thanks for You input. And also nice to hear from You.

I will take a few pics and measurements soo that we can decided what to do. Only going for 0.1mms if it is realy necessary.

Gergo

Pumaracing

2,089 posts

206 months

Thursday 10th October 2013
quotequote all
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!

camelotr

Original Poster:

570 posts

167 months

Thursday 10th October 2013
quotequote all
This requested clearance is stemped into the piston crown.

Pumaracing

2,089 posts

206 months

Thursday 10th October 2013
quotequote all
OK. I'll wait and see what your pictures reveal to me.

So have you finally found someone who can do boring and honing to an acceptable standard over there after your Mini problems?

camelotr

Original Poster:

570 posts

167 months

Friday 11th October 2013
quotequote all








Piston height: 97,3mm

Pin center-crown: 49,2mm

What else do You need?

camelotr

Original Poster:

570 posts

167 months

Friday 11th October 2013
quotequote all
I just chacked my gauge and it is out 0.05mms. Please subtract this amount from the total values.

DVandrews

1,315 posts

282 months

Friday 11th October 2013
quotequote all
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

Pumaracing

2,089 posts

206 months

Friday 11th October 2013
quotequote all
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.

camelotr

Original Poster:

570 posts

167 months

Friday 11th October 2013
quotequote all
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?

Edited by camelotr on Friday 11th October 22:41

Pumaracing

2,089 posts

206 months

Friday 11th October 2013
quotequote all
Could be. Your guess is as good as mine.

Boosted LS1

21,167 posts

259 months

Saturday 12th October 2013
quotequote all
Why do people like Ross specify such huge clearances if they're probably not needed?

delcbr

84 posts

178 months

Friday 18th October 2013
quotequote all
you mean the piston manufacturer ROSS?

delcbr

84 posts

178 months

Friday 18th October 2013
quotequote all
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?

Edited by camelotr on Friday 11th October 22:41
What was the outcome with contacting Mahle did they say it was ok to run a tighter clearance?

Boosted LS1

21,167 posts

259 months

Friday 18th October 2013
quotequote all
delcbr said:
you mean the piston manufacturer ROSS?
Yes.