CHI-COM Panhead cylinders, or max overbore
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mogman
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CHI-COM Panhead cylinders, or max overbore
Anyone have any experience with the Chinese re-pop cylinders, looks like I'm going to need some. Also what is considered the max safe overbore on OEM cylinders?
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Cotten
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OEM cylinders can be blown out as big as you can find pistons for them, ~.100" oversize. However, using torque plates is manditory for any bores over .060".
(Stories of fragged cylinders or running hot result from ordinary "poke and hope" borejobs.)
Whether a large overbore or a fresh imported casting, it is always wise to measure your walls at the thinnest points. Taiwan cylinders of the last decade were subject to horrible core-float.
....Cotten
(Stories of fragged cylinders or running hot result from ordinary "poke and hope" borejobs.)
Whether a large overbore or a fresh imported casting, it is always wise to measure your walls at the thinnest points. Taiwan cylinders of the last decade were subject to horrible core-float.
....Cotten
cylinders
The aftermarket cylinders work okay. One point to watch out for is the area at the base of the cylinder where the oil return line runs. There is very little meat around there compared to an original cylinder. I came across a brand new repop that was perforated.
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jlaley
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Cotten, you mentioned torque plates. What purpose do they do on a pan cylinder. I have a set I've used on my Twinkie and understand the reason for it but this kind of head that you aren't squeezing together I don't get. You bolt a plate to the top and one to the bottom and the middle just goes where ever anyways doesn't it? Can you explain why to me? Thanks, Jim
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Cotten
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Jim!
Twinkies and evo's use plates to compress the cylinder as if installed. The design of the cylinders are intended to minimize distortion from fasteners by spreading their pressure over wider areas.
Earlier designs however, suffer from localized fastener pressures more and more as the casting is thinned by overbores. (A stock bore cylinder should show negligeable distortion.) So plates are torqued to the cylinder as if installed while the bore is machined, usually for only the honing step.
On OHV's, fastener torque from the head pulls the cylinder wall slightly outward at five spots. This compromises ring seal and power, but does not create friction or damage.
The cylinder base, however, distorts the spigot dramatically inward and outward.
It is not unusual to record fastener stress distortions of a thou and a half inward and a thou and a half outward on the springy spigot, for a full three thousandths differential. (I have encountered twice that!)
This means that a conventional poke'n'hope without a plate would rub a piston at .0015" clearance as soon as you installed it. It becomes apparent where tales of fragged and hot-running thin cylinders originate.
So the ancient solution has always been to simulate installation by bolting the cylinder bases to accurate plates for fitting. They go squirrely when you unbolt them, but installation should return them to round again, if you are precise with your torquewrench.
Strong and cool runners can be built at enormous overbores, but as I suggested, the cylinder walls should always be measured.
Attached is a photo of how this can be easily done with an old-fashioned caliper that has the setscrew reversed.
Although I draw the line at .070" left at the thinnest spot, I have a 80's-vintage Taiwan pan cylinder that serviced at nearly zero wall: I put my fingernail through it after disassembly for other problems.
It would have been a great cylinder if not for the core-float that made it shot-out at .030"!
....Cotten
Twinkies and evo's use plates to compress the cylinder as if installed. The design of the cylinders are intended to minimize distortion from fasteners by spreading their pressure over wider areas.
Earlier designs however, suffer from localized fastener pressures more and more as the casting is thinned by overbores. (A stock bore cylinder should show negligeable distortion.) So plates are torqued to the cylinder as if installed while the bore is machined, usually for only the honing step.
On OHV's, fastener torque from the head pulls the cylinder wall slightly outward at five spots. This compromises ring seal and power, but does not create friction or damage.
The cylinder base, however, distorts the spigot dramatically inward and outward.
It is not unusual to record fastener stress distortions of a thou and a half inward and a thou and a half outward on the springy spigot, for a full three thousandths differential. (I have encountered twice that!)
This means that a conventional poke'n'hope without a plate would rub a piston at .0015" clearance as soon as you installed it. It becomes apparent where tales of fragged and hot-running thin cylinders originate.
So the ancient solution has always been to simulate installation by bolting the cylinder bases to accurate plates for fitting. They go squirrely when you unbolt them, but installation should return them to round again, if you are precise with your torquewrench.
Strong and cool runners can be built at enormous overbores, but as I suggested, the cylinder walls should always be measured.
Attached is a photo of how this can be easily done with an old-fashioned caliper that has the setscrew reversed.
Although I draw the line at .070" left at the thinnest spot, I have a 80's-vintage Taiwan pan cylinder that serviced at nearly zero wall: I put my fingernail through it after disassembly for other problems.
It would have been a great cylinder if not for the core-float that made it shot-out at .030"!
....Cotten
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