Hi,
I am rebuilding a 3.4 150S engine with Mahle pistons.–
Ad Dei gloriam et ad gaudium nostrum
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In reply to a message from Camelot sent Tue 8 Oct 2013:
The factory manual suggest 0.025-0.04mms, which is on the
tight side for the bore size, but on the top of the piston
0.1mm is stamped in, which is HUGE for the size and road going
car.
???–
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In reply to a message from Camelot sent Tue 8 Oct 2013:
Hello Sir…(name inserted here),
The factory bore clearances were for hand graded, OEM split
skirt pistons made with period materials. They are no longer
relevant to the pistons sold today. Even new pistons from the
OEM mfg need wider clearances. You need to contact Mahle and
ask them for their recommendation. If they are forged
pistons, they will seize at the factory spec resulting in
sorrow in the kingdom.
Paul–
The original message included these comments:
In reply to a message from Camelot sent Tue 8 Oct 2013:
The factory manual suggest 0.025-0.04mms, which is on the
tight side for the bore size, but on the top of the piston
0.1mm is stamped in, which is HUGE for the size and road going
car.
–
PS
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In reply to a message from Camelot sent Tue 8 Oct 2013:
Clearance is marked right on top of the pistons. Factory
manual specs are only for the original factory/OEM
pistons. And yes it’s huge relative to what’s in the book.
I don’t know why but it is closer to what you’d use on a
forged piston than a cast part. But, Mahle is a well
respected company and I’ve never had a problem with their
parts in 30 years.–
John
Boston, MA, United States
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In reply to a message from PS sent Tue 8 Oct 2013:
‘‘Hello Sir…(name inserted here),’’
Erm sorry. Gergo.
Thanks all who responded. I understand Your points. Does any
of You tried theese pistons with tighter fit? Is soo, what
tolerances?–
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In reply to a message from Camelot sent Wed 9 Oct 2013:
Gergo, why would you deliberately install a piston too tight?
If you have already had the bores machined to install those pistons
at the clearance shown for the original pistons, I would remove
them and increase the bore diameter to Mahle’s clearance. Running
modern pistons at split-skirt clearances will almost certainly
result in partial or full seizure. Not much gaudium in that…
Pete–
The original message included these comments:
Thanks all who responded. I understand Your points. Does any
of You tried theese pistons with tighter fit? Is soo, what
tolerances?
–
1E75339 66 D, 885958 62 FHC,1R27190 70 FHC, 1R28009 70 FHC
Gaithersburg, Maryland, United States
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In reply to a message from Camelot sent Wed 9 Oct 2013:
I had a Coventry ‘‘specialist’’ fit these pistons to a 4.2 unit
which picked up on two pistons on the very first gradient on
the first road test. These were fitted with a clearance of
.0007, cast pistons by the way. I ordered some new ones and
took them to a real specialist who examined the old ones and
machined the new to give a clearance of .0035 thou…after
cleaning up my bores and having them measured accurately. I
used this engine in my car when using it to the limit on a
drag strip and getting best time from an all Jag line up run
by Jaguar world.I still have the pistons having built a new
engine up using EP forged pistons, the Mahles only have slight
scuff marks after ten years of good service…–
The original message included these comments:
In reply to a message from PS sent Tue 8 Oct 2013:
‘‘Hello Sir…(name inserted here),’’
Erm sorry. Gergo.
Thanks all who responded. I understand Your points. Does any
of You tried theese pistons with tighter fit? Is soo, what
tolerances?
–
Keith.P. Series2 Roadster
exmouth, United Kingdom
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In reply to a message from sozfingers sent Wed 9 Oct 2013:
35tou equals to 0.088mms, and You used it in a drag-race
car, where thermal load is a way higher than on a normal
road going car, an You were still 15% percent below factory
recommendations. Thats why I am lamenting.
Although it is a complex issue, dependant on piston
material, shape, power-volume ratio etc, but normaly with
cast pistons, and road applications we take 4-5 tenthousands
of the bore diameter, soo with a 83mm bore that should be
0.04-0.05mms. The piston skirt is long, so it puts a bit on,
maybe 0.06mms. For the roa. It is quite far from 0.1mms.
PS: 7 thou is deffiniatly not enough, it equals to 0.018mms,
which will fail.–
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In reply to a message from PeterCrespin sent Wed 9 Oct 2013:
‘‘Gergo, why would you deliberately install a piston too
tight?’’
Running a piston with huge clearance will result in a noisy
engine, higher oil consumption and shorter engine life.
The engine was not bored yet, I need to decide the
clearance.
The factory dta for ‘‘Brico’’ semi-split pistons: 0.04-
0.05mms, and for ‘‘Aerolite’’ Solid skirt 0.025-0.04mms. Just
for comparision, an air cooled Porsche 911 with 80mm bore
should be put to 0.11-0.12mms. And air cooled engines always
hae much higher clearances.–
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In reply to a message from Camelot sent Wed 9 Oct 2013:
Since we are talking about precision machining and the life or
death of an engine, can we try to get the numbers right please? We
need to watch our typing and maths Gergo, because your decimal
points are all over the place. Rounding up,
35 thou is not 0.088 mm, it is 0.89 mm
7 thou is not 0.018 mm it is 0.18 mm
In your previous post you said 0.1 mm was ‘huge’. I don’t think
it’s all that huge (about 4 thou) and at those clearances the
engine doesn’t sound like bag of nails, depending on piston.
Meanwhile, if it’s all that huge, why are you saying here that 7
thou is definitely not enough?
I’m dropping out of this one. Ask Mahle and follow their suggestion.
Pete–
The original message included these comments:
35tou equals to 0.088mms, and You used it in a drag-race
PS: 7 thou is deffiniatly not enough, it equals to 0.018mms,
–
1E75339 66 D, 885958 62 FHC,1R27190 70 FHC, 1R28009 70 FHC
Gaithersburg, Maryland, United States
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In reply to a message from PeterCrespin sent Wed 9 Oct 2013:
Sorry, obviously my misstake I usualy use mms, not (thou)
inches.
Correct munbers: 3.5 thou and 0.7 thou.
Thanks for correcting me.
Gergo–
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