Would appreciate knowing what the experience(s) of others are in regards
to this. It supports experience here. One thousand miles with clearance
@ .0010 ( manual stated .0007 ~ .0014 ). Machinist at the time of boring
the cyls. stated that oil occupies .0009 so how could the fit be so
tight? Well after the 1,000 miles and go to throttle up , Bam, #2 piston
partially siezes. Rebore and fit up @ .002 ( and careful attention paid
to ring end gap ) = no problems. What’s the general concensus ?
Paul Spitzer---------------------------------------------------------------------------------------
I can shed some experienced light on the piston/cylinder clearance
issue. On my first rebuild I tried to follow the book to the letter,
which indicated .0015 clearance, which I had done, and the car ran fine
up to the 1000miles at 50mph (per the book) I was off on a trip and
pushed her up to 60mph and three of the pistons collapsed (went round)
and started knocking.
The key is: the book says .0015, but means on EACH side of the
piston. That is .003 or so, and I believe allows up to .006 including
wear.
I’ve always gone by the .001 per inch of bore and always had success
ever since. I have better luck by making the bore the size that gives
the proper ring gap clearance to avoid excessive oil use. Assuming the
rings are already properly sized.
Also , any dents or scratch in the bores must be cleaned up or re-
sleeved, and for Gods sake put in those new type valve guides/ seals.
Bob
Would appreciate knowing what the experience(s) of others are in regards
to this. It supports experience here. One thousand miles with clearance
@ .0010 ( manual stated .0007 ~ .0014 ). Machinist at the time of boring
the cyls. stated that oil occupies .0009 so how could the fit be so
tight? Well after the 1,000 miles and go to throttle up , Bam, #2 piston
partially siezes. Rebore and fit up @ .002 ( and careful attention paid
to ring end gap ) = no problems. What’s the general concensus ?
Hello Paul,
Another one ? Did you follow the recent thread on this subject? If not,
check the archives. You don’t say which engine you are talking about, but
the piston manufacturer of oversize 3.8 pistons changed the recommended
clearance to 0.004". I suggest contacting your piston manufacture for their
recommendation. One old rule is 0.001" per inch of bore size, but most
factory recommendations are in the 0.001-0.002" range and Jaguar’s original
recommendation is not unusually tight. The new pistons must be of a
different material or shape, or perhaps the block expands more after material
is removed, I don’t know. Jaguar also recommends a rebore at 0.006"
clearance, so we are cutting it close here. Some of the racers on the list
talk about 0.004-0.005" for a race engine. I have a 3.8 that’s been running
for ten years starting at 0.0025", any chance it was your ring gaps ? As to
the thickness of oil, your machinist might be right, but he should not be
surprised, millions of Fords have 0.0008-0.0019". I recently read some
research comparing aircraft engines with modern car engines and frictional
losses. The tight clearances on car engine pistons results in a continuos
oil film around the piston which increases the drag, which is one reason the
old, big bore, wide clearance aircraft engine still have a better BSFC at a
constant speed. I found this surprising given the emphasis on efficiency,
perhaps it is a noise or emission issue. If your engine is together with
0.002" clearance, don’t worry, just use a thin oil and try to get some miles
on it and avoid overheating. Maximum bore wear is usually on a cold start.
Maybe some others on the list can recommend a break in procedure, I usually
like to load the engine to seat the rings, but in this case, maybe there is a
better idea out there.
Paul - this question has been fairly well hashed out over the last couple of
months. Check the clearance specs that come with the pistons and use them.
The conclusion is that the current AE (now owned by Federal Mogul Corp.)
specification is +.004 clearance on a new set of pistons. Those of us who
have set clearances at Jaguar specs have merely been very lucky to avoid
piston seizures if pistons have been acquired in the Federal Mogul era. Most
shops in my area are using .003" to .004" clearances for the FM pistons.
Bob Grossman----- Original Message -----
From: “Paul Spitzer” tyrnbiter@flaglink.com
To: xk-engine@jag-lovers.org
Sent: Thursday, March 01, 2001 2:05 PM
Subject: [xk-engine] piston clearance
Would appreciate knowing what the experience(s) of others are in regards
to this. It supports experience here. One thousand miles with clearance
@ .0010 ( manual stated .0007 ~ .0014 ). Machinist at the time of boring
the cyls. stated that oil occupies .0009 so how could the fit be so
tight? Well after the 1,000 miles and go to throttle up , Bam, #2 piston
partially siezes. Rebore and fit up @ .002 ( and careful attention paid
to ring end gap ) = no problems. What’s the general concensus ?
I can shed some experienced light on the piston/cylinder clearance
issue. On my first rebuild I tried to follow the book to the letter,
which indicated .0015 clearance, which I had done, and the car ran fine
up to the 1000miles at 50mph (per the book) I was off on a trip and
pushed her up to 60mph and three of the pistons collapsed (went round)
and started knocking.
The key is: the book says .0015, but means on EACH side of the
piston. That is .003 or so, and I believe allows up to .006 including
wear.
I’ve always gone by the .001 per inch of bore and always had success
ever since. I have better luck by making the bore the size that gives
the proper ring gap clearance to avoid excessive oil use. Assuming the
rings are already properly sized.
Also , any dents or scratch in the bores must be cleaned up or re-
sleeved, and for Gods sake put in those new type valve guides/ seals.
Bob
.001 to .004 is considered acceptable for old pistons in old bores and
for oversize pistons in new bores. a lot depends on how one measures the
piston clearance. that specification is taken from “automotive engine
rebuilding” by j.g. hughes, copyright '84. i would think that a “tight”
.001 is not enough clearance.
dave g.On Thu, 01 Mar 2001 14:05:04 -0700 Paul Spitzer tyrnbiter@flaglink.com writes: Would appreciate knowing what the experience(s) of others are in regards to this. It supports experience here. One thousand miles with clearance @ .0010 ( manual stated .0007 ~ .0014 ). Machinist at the time of boring the cyls. stated that oil occupies .0009 so how could the fit be so tight? Well after the 1,000 miles and go to throttle up , Bam, #2 piston partially siezes. Rebore and fit up @ .002 ( and careful attention paid to ring end gap ) = no problems. What’s the general concensus ? Paul Spitzer From the Saloons list: Robert Pfost jagdoc@earthlink.net wrote:
I can shed some experienced light on the piston/cylinder
clearance
issue. On my first rebuild I tried to follow the book to the
letter,
which indicated .0015 clearance, which I had done, and the car ran
fine
up to the 1000miles at 50mph (per the book) I was off on a trip
and
pushed her up to 60mph and three of the pistons collapsed (went
round)
and started knocking.
The key is: the book says .0015, but means on EACH side of the
piston. That is .003 or so, and I believe allows up to .006
including
wear.
I’ve always gone by the .001 per inch of bore and always had
success
ever since. I have better luck by making the bore the size that
gives
the proper ring gap clearance to avoid excessive oil use.
Assuming the
rings are already properly sized.
Also , any dents or scratch in the bores must be cleaned up or
re-
sleeved, and for Gods sake put in those new type valve guides/
seals.
Bob
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I wouldn’t be afraid to go with the old rule of .001 per inch of bore as
the piston is a long skirt traveling along in a long stroke.
The tendency to cock the piston in the bore is minimal and with any
decent oil film on the walls, audible slap should not be appearant.
I also recognize that the tendency with a rebuild is to keep everything
on the tight side to insure the longest wear for all the expense.
New alloys used in piston manufacture have different expansion
characteristics than the stuff used in the 50’s. These alloys may grow
more or even less, but with our engines many have found they expand more
than originally expected. IMHO some of these pistons don’t even “grow”
in a streight line with the temp increase.
It is best to go directly to the piston maker and get his take on
clearances explaning that folks have experienced engine failures with
recent piston replacements. Since the stroke is so long by todays engine
standards it is imperative that the shop mic the entire bore to insure
all taper is trued in the overbore process. Again IMHO that many shops
have little experience with long strokes or how critical the bore
diameter can become. One of our mates (and I apologize for not
remembering which) said that he thought some clearances were expressed as
the space between the piston and cylinder wall and therefore actually be
doubled when machining is needed. This could be very well true, as in
one case the text is describing the piston as it resides in the bore
during engine operation.
Many pistons are actually designed to grow at different rates themselves.
That is they expand into ovals instead if cylinders. This nonlinear
expansion is purpose built to compensate for bore wear on the thrust
sides of the block. The integrity of the compression actually is a
combination on how the rings fit the bores during their total travel
distance and how these rings fit in the lands (their ability to
efficiently seal). The actual bore to piston (ring carrier if you will)
is not as critical as many people are led to believe. The piston needs
to carry the rings and keep the mating surfaces in contact to seal
properly. Obviously if too worn it could wobble, but more likely the
land to ring seal weakens resulting in the “tired engine” and mosquito
smogger.
Hope it helps, mind you I still use a BFH when things don’t fit!! Ha!
Many pistons are actually designed to grow at different rates themselves.
That is they expand into ovals instead if cylinders.
Hello Rick,
I think you'll find that most pistons are cam ground in an oval shape and
expand at the pin axis into a circle, unless you are talking about Honda
Grand Prix motorcycle engines. This is caused by a heat differential.
Pistons should be measured across the thrust face because of this difference.
True oval pistons in oval cylinders are banned from racing because they are
very expensive and allow more valve area. If 106 mm is long today, what do
you think the bore clearance was on a 1911 Fiat with a 200 mm stroke ? Does
anyone know why the normally aspirated engine seems to have a limit of 90
ft/lbs per liter ? All the effort is then taking the 90 and multiplying it by
higher and higher rpms. I’ve heard this several times and can’t find a
reason. Anyone know of a NA engine with a higher output ?
Great point Paul. To my knowledge, the only real improvement in
torque/displacement has come from improved burn characteristics in the
combustion chamber; i.e., turbulence, plug placement, friction losses. I
must expose my ignorance of modern torque/displacement specs and you’ve once
again piqued my curiosity! BTW, who in the world could remember when Honda
went to the exorbitant expense of producing and sealing an oval cylinder!
Oh, well, more technology for the dusty archives of recip motors.
Bob Grossman----- Original Message -----
From: Tlsalt@aol.com
To: xk-engine@jag-lovers.org
Sent: Friday, March 02, 2001 1:16 PM
Subject: Re: [xk-engine] piston clearance
Rick wrote:
Many pistons are actually designed to grow at different rates themselves.
That is they expand into ovals instead if cylinders.
Hello Rick,
I think you'll find that most pistons are cam ground in an oval shape
and
expand at the pin axis into a circle, unless you are talking about Honda
Grand Prix motorcycle engines. This is caused by a heat differential.
Pistons should be measured across the thrust face because of this
difference.
True oval pistons in oval cylinders are banned from racing because they
are
very expensive and allow more valve area. If 106 mm is long today, what do
you think the bore clearance was on a 1911 Fiat with a 200 mm stroke ?
Does
anyone know why the normally aspirated engine seems to have a limit of 90
ft/lbs per liter ? All the effort is then taking the 90 and multiplying it
by
higher and higher rpms. I’ve heard this several times and can’t find a
reason. Anyone know of a NA engine with a higher output ?
Paul is right! they start cold as an oval and go round at temp. I wrote
it bassackwards!!
Paul I wasn’t referring to the oval pistons in Hondas but the slight
oval"ity" you can measure on modern pistons. There isn’t anyone that I
know that could effectively “bore an oval” in a block nor would they have
the tools to do it anyway!!
Don’t know about the 1911 Fiat but the way your text read it seems a step
“up” from steam, perhaps Fiat was using cylinders from that schooling.
Didn’t quite understand your question on pressure and stroke in reference
to natural aspiration, unless you are asking at what time (in RPM) a
cylinder cannot be filled with air (even if the top were completely open)
because the air cannot “travel” fast enough to fill the lower confines of
the cylinder and remain at its normal density. There are formulas for
this bore stroke to ingestion maximums but I don’t know them. The
“topless cylinder” of course would have a Max opening but there are
various combinations of valving and intake designs that tend to restrict
flows in “normal” engines. There are methods that will slightly enhance
the input speeds. This type of “tuned input” enhancement is usually high
band conscious and not well suited for road car engines requiring a wide
range of RPM. There are velocity stacks and manifold designs that have
been avail for this purpose. But each time you get something at one end
you are apt to loose something at another…
I will read it over again if I can find it. I know there are many guys
out there that can answer this stuff “off the top”…
Thanks and regards,
Rick,
677342-DHC
673190-Roadster
Somerville NJ, USofA
There isn’t anyone that I know that could effectively “bore an oval” in a
block nor would they have the tools to do it anyway!!
Hello Rick,
Honda can and did, until it was banned. I don’t know what the tools look
like, but nothing is out of bounds for those guys.
Don’t know about the 1911 Fiat but the way your text read it seems a step
“up” from steam, perhaps Fiat was using cylinders from that schooling.
Bore size was restricted, so go for the longest stroke you can get.
Didn’t quite understand your question on pressure and stroke in reference
to natural aspiration, unless you are asking at what time (in RPM) a
cylinder cannot be filled with air (even if the top were completely open)
because the air cannot “travel” fast enough to fill the lower confines of
the cylinder and remain at its normal density. There are formulas for
this bore stroke to ingestion maximums but I don’t know them. The
“topless cylinder” of course would have a Max opening but there are
various combinations of valving and intake designs that tend to restrict
flows in “normal” engines.
This is apparently the problem with current F1 engine at 18,000 rpm. There
just isn’t enough time to fill the cylinder to rev any higher. Something to
do with the speed of sound (where’s my Beemans ?). They all want to lift
the limit of ten cylinders to go to 12s, 14s or 16s to get smaller cylinder
displacement. Apparently the ideal cylinder size is somewhere below 300 cc.
Honda was running 24,000 rpm with 25 cc cylinders in the early sixties.
Where does that leave the XK with up to 700cc ?
There are methods that will slightly enhance
the input speeds. This type of “tuned input” enhancement is usually high
band conscious and not well suited for road car engines requiring a wide
range of RPM. There are velocity stacks and manifold designs that have
been avail for this purpose. But each time you get something at one end
you are apt to loose something at another…
Porsche and others (Mercedes in 1955) have used variable length velocity
stacks
to get around a fixed limitation. The longer the tract, the lower the VE peak
and the sharper the slope falling off at high speed. Jaguar worked this out
on the D Type out to 32 inch pipes on the Lucas FI in late 50’s. Most new
versions involve several sets of throttle plates but my favorite is the
rotating drum intake.