<VV> Engine building - bearing clearance

Mark Durham 62vair at gmail.com
Thu Jan 21 22:19:03 EST 2010


All, I have built 100's of air cooled engines (mostly Lycoming and Contental
Aircraft engines, some corvair, but you will *never*, get this, *never*,
 get accurate readings on the bearing insert journals using a inside mike.
It is tough enough to get accurate readings in larger cylinder bores using
inside mikes, where a half thousands off is no big deal. But it is a big
deal on crank clearance. Just don't go there, it is waste of your time, and
as I can see, frustrating.

Plastiguage is the only reliable way to go, but should only be used as a
verification/confirmation of calculations you have completed.

You cannot put bearing shells of different thicknesses in the same journal.
That will put side pressure on the crank and wear out the tighter
bearing insert and possibly the crank journal itself and failure of the
unit.

The clearances allowed for the crank main and rod journals by GM are more
than adequate for adjusting in .001 increments.

You are making this way to complex. A selection of which bearing to install
is done by comparing the size of the crank journals to what new std journal
sizes are. You add the closest amount of undersize bearing to keep bearing
to journal specs in limits.

Step 1. Measure and record the crank main and rod journals after polishing.

Step 2. If the journals are in the new range, a std bearing is in order.

Step 3. If the journals are outside the range smaller, how far?  less than
.001? Then a .001 would fit.

If .0015, then probably a .002 would be necessary to make up the .0015 and
bring the clearance .0005 into the clearance range. Remember that the
tightest clearance is .0007, so bringing the clearance .0005 into the range
will not be too tight.

Once you have done these sizing measurements and comparisons, and have
selected your bearing size, std, .001, .002, undersize, then is when to
confirm with plastiguage.

I usually only have to plastiguage one time as a confirmation.

Mark Durham

On Thu, Jan 21, 2010 at 5:35 PM, ray smith <turbovair at earthlink.net> wrote:

> If the crank is exerting any force to the bearing other then gravity - you
> are down to zero bearing clearance.
> Snap gauges are for ballpark checks. You need an inside mic for this job.
> from my experience if your end bearing clearances are within spec on a
> block , and all the crankpins check out , but the inner bearing clearances
> are tight , your block needs to be line honed.
> Ray Smith
> Sent via BlackBerry by AT&T
>
> -----Original Message-----
> From: "corvairduval at cox.net" <corvairduval at cox.net>
> Date: Thu, 21 Jan 2010 16:54:33
> To: <virtualvairs at corvair.org>
> Subject: Re: <VV> Engine building - bearing clearance
>
> I'm also agree that .001 shells add .0005 per shell, .001 total undersize.
>
> I think the reading error is because the crank was not in the bearing
> shells when you torqued down the block, hence why plastigage works. The
> presence of the crank will affect the crush of the shells.
>
> Awaiting with flame suit on. ggg
>
> Frank DuVal
>
> Original Message:
> -----------------
> From:  corvair at mts.net
> Date: Thu, 21 Jan 2010 14:55:36 -0600
> To: hyarnell1 at earthlink.net, bobhelt at aol.com, virtualvairs at corvair.org
> Subject: Re: <VV> Engine building - bearing clearance
>
>
> I am measuring the inside of the bearing bore in a torqued block, using
> snap gauges and a micrometer.
>
> I measure the crank, the journal is (for example, and from memory) 2.0985".
> I measure the corresponding bore with snap gauge (.001 shells installed),
> then mike the gauge, and get 2.0991".
>
> So my clearance is .0006 - too small.
>
> I want .0010 more clearance, so I substitute 2 STD shells instead of .001
> shells.
>
> Remeasure, and now the bore measures 2.1011 and clearance is .0026" (pretty
> big).
>
> That's what makes me think that a single .001 shell takes away .001
> clearance and a pair of them takes away .002.
>
> It's confusing me. Assembling with plastigauge will assuage my concerns.
>
> Les
>
> >
> > From: "Harry Yarnell" <hyarnell1 at earthlink.net>
> > Date: 2010/01/21 Thu PM 02:37:08 CST
> > To: <corvair at mts.net>,
> >       <bobhelt at aol.com>,
> >       <virtualvairs at corvair.org>
> > Subject: Re: Re: <VV> Engine building - bearing clearance
> >
> > What  are you measuring with? Plasticgage is the only way I know of.
> > Measuring with mechanical calipers gets you close for the crank. Forget
> > about measuring the bearings with calipers...
> >
> >
> > Harry Yarnell
> > Perryman Garage and Orphanage
> > hyarnell1 at earthlink.net
> > ----- Original Message -----
> > From: <corvair at mts.net>
> > To: "Harry Yarnell" <hyarnell1 at earthlink.net>; <bobhelt at aol.com>;
> > <virtualvairs at corvair.org>
> > Sent: Thursday, January 21, 2010 3:08 PM
> > Subject: Re: Re: <VV> Engine building - bearing clearance
> >
> >
> > > That's what I thought too, and it's why I'm scratching my head. It's
> hard
> > > to measure the thickness of the bearing to know for sure.
> > >
> > > I swap ONE shell, and my clearances are good - added .001 by swapping
> one
> > > shell.
> > >
> > > I will plastigauge as a last step but this is confusing me.
> > >
> > > Les
> > >>
> > >> From: "Harry Yarnell" <hyarnell1 at earthlink.net>
> > >> Date: 2010/01/21 Thu PM 01:56:59 CST
> > >> To: <corvair at mts.net>,
> > >> <bobhelt at aol.com>,
> > >> <virtualvairs at corvair.org>
> > >> Subject: Re: <VV> Engine building - bearing clearance
> > >>
> > >> I thought that a .001 undersize bearing was the TOTAL, not .001 for
> each
> > >> shell .
> > >>
> > >> Harry Yarnell
> > >> Perryman Garage and Orphanage
> > >> hyarnell1 at earthlink.net
> > >> ----- Original Message ----- >
> > >> > Am I correct that a .001 US bearing is supposed to take away .001
> > >> > clearance for EACH bearing shell (.002 total if you use one on each
> > >> > side)?
> > >> > That's what my measuring looks like.
> > >> >
> > >> > I have a nice .0010-.0016 across all four bearings now, all .001
> except
> > >> > for 1 side STD in #2 and 1 side STD in #4.
> > >> >
> > >>
> > >>
> > >
> > >
> >
> >
>
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