Oil Light / Bearing Clearance Q.

The oil light on my '89 Daihatsu Charade (1.0L w/ 250K miles) has been
coming on at idle for the last month or so, but only after about 20 minutes
of freeway driving. My first thought was worn main bearings. (Rod bearings
were changed 100K ago, so I don't suspect a problem there.)

But after pulling the pan and inspecting one lower main bearing, I don't see
much sign of wear. Calipers measure the same thickness to within .001" at
various points across the bearing, and they are smooth to the fingernail. I
know that calipers are a backward way of measuring bearings. I'm off to the
auto parts store this morning to get some plastiguage.

But I have a question that an experianced mechanic might be able to help
with. Various books call out .002" as "normal" clearance for main bearings.
But how much clearance would it take to cause my symptoms? To pick some
hypothetical numbers, is .005" enough to cause the problem, or might they
open up to .020" before causing problems? If my bearings are a bit worn, but
something else is causing my problem, I want to get on to the scent now
rather than after reassembly.

Your comments are appreciated.

On Sun, 2 Jul 2006 10:51:13 -0700, "Juan A. Gato"
> wrote:

>The oil light on my '89 Daihatsu Charade (1.0L w/ 250K miles) has been
>coming on at idle for the last month or so, but only after about 20 minutes
>of freeway driving. My first thought was worn main bearings. (Rod bearings
>were changed 100K ago, so I don't suspect a problem there.)
>
>But after pulling the pan and inspecting one lower main bearing, I don't see
>much sign of wear. Calipers measure the same thickness to within .001" at
>various points across the bearing, and they are smooth to the fingernail. I
>know that calipers are a backward way of measuring bearings. I'm off to the
>auto parts store this morning to get some plastiguage.
>
>But I have a question that an experianced mechanic might be able to help
>with. Various books call out .002" as "normal" clearance for main bearings.
>But how much clearance would it take to cause my symptoms? To pick some
>hypothetical numbers, is .005" enough to cause the problem, or might they
>open up to .020" before causing problems? If my bearings are a bit worn, but
>something else is causing my problem, I want to get on to the scent now
>rather than after reassembly.
>
>Your comments are appreciated.
>

Very small changes in main bearing clearance make for large
changes in oil pressure. .003-.004" clearance is enough to cause your
low-pressure lamp to light at idle.
FWIW, plastigauge may not tell the whole story, with the
engine in the car. The crank will be lying in the bottom if the
bearing saddles, and you may get an indication of smaller clearance
than actually exixts.
I'm not familiar with Daihatsu in particular, but .001"
undersize bearings used to be available for most makes in the U.S. I
installed .001" undersize mains in an old Chrysler with the
low-pressure-at-idle symptom. Its main bearing clearances measured
..0025" (new spec was .0015-.002"). Oil pressure at idle was restored
to normal levels.

"the fly" > wrote in message
...
> On Sun, 2 Jul 2006 10:51:13 -0700, "Juan A. Gato"
> > wrote:
>
>>The oil light on my '89 Daihatsu Charade (1.0L w/ 250K miles) has been
>>coming on at idle for the last month or so, but only after about 20
>>minutes
>>of freeway driving. My first thought was worn main bearings. (Rod bearings
>>were changed 100K ago, so I don't suspect a problem there.)
>>
>>But after pulling the pan and inspecting one lower main bearing, I don't
>>see
>>much sign of wear. Calipers measure the same thickness to within .001" at
>>various points across the bearing, and they are smooth to the fingernail.
>>I
>>know that calipers are a backward way of measuring bearings. I'm off to
>>the
>>auto parts store this morning to get some plastiguage.
>>
>>But I have a question that an experianced mechanic might be able to help
>>with. Various books call out .002" as "normal" clearance for main
>>bearings.
>>But how much clearance would it take to cause my symptoms? To pick some
>>hypothetical numbers, is .005" enough to cause the problem, or might they
>>open up to .020" before causing problems? If my bearings are a bit worn,
>>but
>>something else is causing my problem, I want to get on to the scent now
>>rather than after reassembly.
>>
>>Your comments are appreciated.
>>
>
> Very small changes in main bearing clearance make for large
> changes in oil pressure. .003-.004" clearance is enough to cause your
> low-pressure lamp to light at idle.
> FWIW, plastigauge may not tell the whole story, with the
> engine in the car. The crank will be lying in the bottom if the
> bearing saddles, and you may get an indication of smaller clearance
> than actually exixts.
> I'm not familiar with Daihatsu in particular, but .001"
> undersize bearings used to be available for most makes in the U.S. I
> installed .001" undersize mains in an old Chrysler with the
> low-pressure-at-idle symptom. Its main bearing clearances measured
> .0025" (new spec was .0015-.002"). Oil pressure at idle was restored
> to normal levels.

All very true. I just wanted to add that the viscosity of the oil can also
have a great affect on the loss of pressure through worn crank bearings, and
worn cam bearings are another source of pressure loss on some engines.

Kevin wrote:
> "the fly" > wrote in message
> ...
> > On Sun, 2 Jul 2006 10:51:13 -0700, "Juan A. Gato"
> > > wrote:
> >
> > > The oil light on my '89 Daihatsu Charade (1.0L w/ 250K
> > > miles) has been coming on at idle for the last month or
> > > so, but only after about 20 minutes
> > > of freeway driving. My first thought was worn main
> > > bearings. (Rod bearings were changed 100K ago, so I
> > > don't suspect a problem there.) But after pulling the pan
> > > and inspecting one lower main
> > > bearing, I don't see
> > > much sign of wear. Calipers measure the same thickness
> > > to within .001" at various points across the bearing,
> > > and they are smooth to the fingernail. I
> > > know that calipers are a backward way of measuring
> > > bearings. I'm off to the
> > > auto parts store this morning to get some plastiguage.
> > >
> > > But I have a question that an experianced mechanic
> > > might be able to help with. Various books call out
> > > .002" as "normal" clearance for main bearings.
> > > But how much clearance would it take to cause my
> > > symptoms? To pick some hypothetical numbers, is .005"
> > > enough to cause the problem, or might they open up to
> > > .020" before causing problems? If my bearings are a bit
> > > worn, but something else is causing my problem, I want to
> > > get on
> > > to the scent now rather than after reassembly.
> > >
> > > Your comments are appreciated.
> > >
> >
> > Very small changes in main bearing clearance make for
> > large changes in oil pressure. .003-.004" clearance is
> > enough
> > to cause your low-pressure lamp to light at idle.
> > FWIW, plastigauge may not tell the whole story, with the
> > engine in the car. The crank will be lying in the bottom
> > if the bearing saddles, and you may get an indication of
> > smaller clearance than actually exixts.
> > I'm not familiar with Daihatsu in particular, but .001"
> > undersize bearings used to be available for most makes in
> > the U.S. I installed .001" undersize mains in an old
> > Chrysler with the low-pressure-at-idle symptom. Its main
> > bearing clearances measured .0025" (new spec was
> > .0015-.002"). Oil pressure at idle was restored to
> > normal levels.
>
> All very true. I just wanted to add that the viscosity of
> the oil can also have a great affect on the loss of
> pressure through worn crank bearings, and worn cam bearings
> are another source of pressure loss on some engines.

And sensors, too.

"the fly" > wrote in message
...
> Very small changes in main bearing clearance make for large
> changes in oil pressure. .003-.004" clearance is enough to cause your
> low-pressure lamp to light at idle.
> FWIW, plastigauge may not tell the whole story, with the
> engine in the car. The crank will be lying in the bottom if the
> bearing saddles, and you may get an indication of smaller clearance
> than actually exixts.

Thanks for the warning. I had no idea the clearance was that critical. That
doesn't leave much room for poor technique.

I wonder whether I could shim two of the bearings to push the crank shaft to
the top while measuring the clearance of the other two bearings? Is there a
standard appraoch to this, or is doing the bearings with the engine in the
car just bad practice?

You could try pushing up on the crank very slightly with say a floor jack
and a block of wood, kinda a " mickey mouse" way here, recheck the rods too,
100k is a lot of miles. As mentioned the op should have been tested with a
known accurate mechanical gage.
"Juan A. Gato" > wrote in message
...
>
> "the fly" > wrote in message
> ...
>> Very small changes in main bearing clearance make for large
>> changes in oil pressure. .003-.004" clearance is enough to cause your
>> low-pressure lamp to light at idle.
>> FWIW, plastigauge may not tell the whole story, with the
>> engine in the car. The crank will be lying in the bottom if the
>> bearing saddles, and you may get an indication of smaller clearance
>> than actually exixts.
>
> Thanks for the warning. I had no idea the clearance was that critical.
> That doesn't leave much room for poor technique.
>
> I wonder whether I could shim two of the bearings to push the crank shaft
> to the top while measuring the clearance of the other two bearings? Is
> there a standard appraoch to this, or is doing the bearings with the
> engine in the car just bad practice?
>



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"Juan A. Gato" > wrote in message
...
>
> "the fly" > wrote in message
> ...
>> Very small changes in main bearing clearance make for large
>> changes in oil pressure. .003-.004" clearance is enough to cause your
>> low-pressure lamp to light at idle.
>> FWIW, plastigauge may not tell the whole story, with the
>> engine in the car. The crank will be lying in the bottom if the
>> bearing saddles, and you may get an indication of smaller clearance
>> than actually exixts.
>
> Thanks for the warning. I had no idea the clearance was that critical.
> That doesn't leave much room for poor technique.
>
> I wonder whether I could shim two of the bearings to push the crank shaft
> to the top while measuring the clearance of the other two bearings? Is
> there a standard appraoch to this, or is doing the bearings with the
> engine in the car just bad practice?
>

You can measure bearing clearance with plastigage while the engine is still
in the car, but you do need to support the crank so it is not laying on the
bearing cap being checked. Special tool number XXXXXXX can be used, but a
floor jack and wood block works as long as you are sure you get the weight
of the crank off of the bearing you are checking.

Kevin