REFLECTOR: Cylinder repair followup

[Scott Derrick]

I'm having a bit of trouble understanding your logic about higher oil
pressure causing a decrease in oil cooling?  Yes, you "might" be pumping
more oil through the cooler, so the same volume of oil might not
exchange as much heat out through the cooler because of the faster
transit time, but your pumping more volume through the cooler, providing
more oil to be cooled in the same amount of time.  I would think these
two factors would  pretty much even things out, as far as the total
amount of  heat exchanged over time.  Maybe a little less, maybe a
little more, but probably not enough to significantly change your oil temp.

As far as the cylinder liners are concerned, there is a minimum amount
of ring gap allowed when overhauling cylinders. If the new liners don't
meet that, the liners should be machined to meet that tolerance. 

Its possible there are alternate rings that would meet that tolerance. 
I've also filed the ends of rings to fit tight sleeves in automotive
applications but you have to be very very careful doing that.  If the
rings don't fit thats telling you you have less clearance between the
cylinder and the piston which can result in scuffing and scraping the
cylinder walls. I don't think I would use alternate rings or file the
rings in an aircooled engine because of the greater expansion ratios
between the pistons and the cylinder walls.

Machining the liners to proper tolerance as measured by ring gap sounds
like the right thing to do.

Scott



John Dibble wrote:
> With further investigation of my recent failed cylinder and related
> issues, I have concluded that a combination high oil temp and tighter
> cylinder clearance is the cause and would like comments on how to
> proceed.  First is the oil temp.  My oil temp had been pretty good the
> past year or so.  Then 4 months ago I added 3 washers behind the bypass
> spring to raise oil pressure.  I didn't notice temperature issues then,
> but it was a much cooler time of the year.  As I understand it, the
> reason the pressure was low, was due to engine wear which allowed oil to
> pass through the lubricating areas more easily.  I suppose if the wear
> was uniform, then the lower pressure would not matter, but it's most
> likely that some areas didn't wear as much and need the higher pressure
> for lubrication.  By raising the pressure with the washers, there is now
> more oil being pumped through the engine than before which means more
> oil is also going through the cooler.  That explains the higher oil
> temps since the available cooling air is unchanged, so the oil can not
> be cooled as much as before.
> The 3 washers gave about a 10-12 psig increase in pressure.  However,
> the higher oil temp eliminates most of that increase ( maybe not really,
> since the reason for the decrease in pressure at higher temps is that
> the oil flows more easily and good lubrication is still being
> achieved?), so I'm thinking of removing some or all of the washers.
> What I'm wondering is what is the minimum oil pressure that I can fly
> with?
> The other issue is cylinder clearance.  I've had 2 PZL sleeves replaced
> with American sleeves (I understand that is all that is available).
> Each time I sent the cylinders for repair I was told that the PZL liners
> were beyond the tolerance.  It seems strange for that to have happened
> in as little as 350 hours.  Besides my remaing PZL cylinders have 700
> hours and are working fine.  For that matter the 2 PZL liners that were
> replaced looked in good condition with no scoring.  I replaced them only
> because I was told they were beyond the tolerance limit.  In retrospect,
> I think I should not have replaced them as I feel that the new liners
> have a too tight tolerance.  The i.d. of the new liners is 4.465" and
> the mechanic said I could have them bored out 0.003".  I'm thinking of
> doing that as I think I need greater clearance for my operating
> conditions.
> I suppose I could also install a second oil cooler.  Seems like that
> would be more work.  Anyway, I'm looking for suggestions as to which way
> to proceed.
>
> Thanks,
>
> John
>
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