<VV> Larry's low compression engine and the surprising reason for several issues
larry at forman.net
Mon Jan 8 00:42:29 EST 2018
I pulled the left cylinders to measure cylinder taper and out of
round. Both were within specs with taper reading from 1 to 4 mil and
out of round right at 1.5 mil. Then I replaced the cylinders and had
time to really look at the engine and found a very surprising thing the
last shop did ... they replaced the normal stock raised plateau piston
in cylinder #4 with a FLAT one, no raised area at all. I am sure with
that dramatically lower piston top the compression ratio and overall
compression would be a lot lower. And that likely explains why the
engine did not want to idle balanced very well. I suspected that they
might have done the same thing for cylinder #1, but the right side all
appear to be the raised plateau so that is somewhat of a relief.
Now I need to pull off the fan mount/engine cover so I can pull that #4
piston and cylinder. Fortunately I have a set of used stock late
pistons and will need to check them carefully and move the rings and rod
over there and then I should be good to go. I will need to carefully
mark the rings so they go on the same orientation (top to bottom) and
same location. I know a couple of my used pistons have compressed ring
gaps, so they are likely out as possible replacements. I just want to
ensure that the top does not come off my replacement piston, as I know
that has been an issue.
I was still waiting another week or more for the shop to complete the
valve seat and valve work as they suddenly got very busy during what is
normally their lull period. That will give me plenty of time to get
the piston changed and get everything back ready for the cylinder head.
Now my feelings about that shop were really confirmed.
On 12/28/2017 12:56 AM, Larry Forman via VirtualVairs wrote:
> Here is an update:
> I received an excellent recommendation from Howard Pilon, who is a
> great Corvair mechanic located in Vacaville. He is on the CORSA
> recommended list of Corvair mechanics. I know that since I
> recommended him several years ago when he inquired of our Sacramento
> Corvair club as he wanted to start working on his part time Corvair
> repair business, which quickly became full time. His normal machine
> shop in Vacaville recently closed, but he had used a machine shop in
> Sacramento, ironically when he did an engine rebuild for my wife's
> 1954 Nash Metropolitan, and they did a great job. This was a
> difficult machine shop job as many of the parts were difficult to
> locate since my wife wanted to retain the original 1200 cc, 42 hp
> engine that was only used for less than one year on the first year of
> the Metropolitans, versus migrating to the larger 1500 cc engine that
> was used for the vast majority of the Metropolitan's model run.
> When I took the cylinder head in there today, they looked at it and
> noticed that there were indications of the #4 cylinder having the jug
> hitting the surface of the cylinder head as when the head gasket
> mating surface recedes into the head. This is the surface that needs
> to be fly cut so the head gaskets mate properly. I am sure that this
> is what was causing the low compression on #4 and was pointed out by
> several of the VV & UV experts. It most likely was what was causing
> the low compression on #1. I should have noticed that myself.
> This shop does not advertise and does most of their work for other
> shops especially work on cranks.
> This shop has about a one week backlog until they can start working on
> my head. They have extensive experience on air cooled heads. They
> will replace all the valve seats and do a proper valve job.
> I want to thank everyone for the great suggestions and
> recommendations. I feel I am in good hands now. If this works out
> well, then we can recommend this shop for other Corvair head work in
> Sacramento as the other Corvair machine shop in Sacramento takes
> forever to complete the same work. We really need a capable and
> responsive shop like this for this type of work in Sacramento. I will
> let everyone know how this works out as I work other issues like
> possible ring compression on the #2 cylinder.
> Have a Happy and safe New Year,
> Larry Forman
> On 12/26/2017 9:59 PM, Larry Forman via VirtualVairs wrote:
>> Hi All,
>> I recently purchased a nice 65 Monza Vert PG at Hot August Nights car
>> auction with about 80 K miles on the odometer. It was claimed to
>> have had a recent engine rebuilt about 5K miles ago (now about 7K
>> miles). I measured the cylinder compression and got the following
>> cyl 1: 100 and w/ oil at 115 cyl 2: 120
>> cyl 3: 125 cyl 4: 90 and w/ oil
>> at 95
>> cyl 5: 125 cyl 6: 125
>> As luck would have it, it just dropped an exhaust valve seat on #2,
>> but fortunately the valve seat remained whole but wallowed out the
>> valve seat mounting area and gouged the piston top. I will smooth
>> out the piston top so there are no sharp edges where it was gouged.
>> The cylinders show the cross hatching evidence of a recent rebuild.
>> I have not yet measured the cylinders on the driver's side for
>> taper. I have only removed the driver's side cylinder head and am
>> not planning on removing the passenger side since the engine is still
>> in the car.
>> I am not too impressed with whatever Reno area shop did the rebuild
>> since they did not do ANY head deflashing, which likely contributed
>> to the dropped valve seat. That same shop also rebuilt the carbs and
>> left out one power enrichment needle, which explains why I had
>> trouble getting the carbs balanced until I redid the carbs and found
>> that problem. I have now done a proper head deflashing on both
>> heads so hopefully once the engine is back together it should be more
>> I am now looking for a Sacramento area air cooled engine shop who can
>> do a proper valve seat repair and check the head out for any other
>> issues or possible loose valve seats.
>> My question is: What are the recommendations for determining why the
>> #4 cylinder compression is low? And what should I do to make the
>> best of the current situation?
>> I assume I should check cylinder taper, ring conditions and
>> orientation, and look for any valve issues on #4. Any other
>> recommendations? I have not yet determined if the pistons have been
>> replaced from the stock ones that can lose their tops. I certainly
>> hope so.
>> Any recommendations would be very welcome.
>> Thanks in advance and have a Happy New Year,
>> Larry Forman
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