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Well Guys,
Today Grandson and I did a compression check on the Starduster O-470-J with 200 hrs.. since I put it together. It has steel cylinders and 4 ring pistons. We also did oil change and cleaned the plugs. Well I sit back and tell him what to do.
Anyway when someone tells me their Eng.. was 80/80 on comp. I say BS. Today we had 3 cylinders that were 80/80 and 3 79/80. Oh yea the eng was cold, but flew Sunday. So now I guess I need to eat crow.
The Eng. was new in 1965 and had oil burning 3 ring pistons in it. About a quart every 3 hrs. So when I redid the Starduster in 2000 I installed 4 ring pistons and honed the cylinders.
Wow what a difference. before it used a quart about every 3 hours and now it does not use any in 25-30 hours. So bottom line 80-79/80 =no oil burn.
I'm not doubting your story but it reminded of the last time that I got suspiciously good readings. When I turned the outlet valve from the compression gauges completely off, I saw that the outlet gauge read somewhat higher than the input gauge even though they where at the same pressure. I now do this check at the beginning of every test.
I hear you but not the case. I bleed the pressure off each time I connect to the cylinder. After getting the cyl to TDC by putting my finger on the fitting I connect the air and bring the pressure up slowly to 80#. This keeps the prop from wanting to turn and you really don't need to hold it.
A lot safer way than I have seen some people do a comp check.
The same gauge doesn't read this good on my other engines
Isn't the 4 ring piston a "cam ground" piston, meaning that is not round when its cold? The idea of the "cam ground" is to allow for the expansion inequalities caused by the thicker area around the piston pin bosses. So.. , when the cylinder and piston are up to temperature the piston is now round. Checking the compression on a cold cylinder with a cam ground piston greatly reduces the minimum piston skirt clearance, decreasing gaps for blow by. Another design function of the four ring piston is its use of a "half wedge" compression ring. The top of the ring, facing up towards the valves has a slope in it. The half wedge design traps the BMEP, which forces the ring outward into the cylinder wall. When checking the compression on a cold contracted cylinder the half wedge piston ring is driven inward into the tapered piston ring land area. This opposite direction from not being up to temperature creates a tighter seal another gap reduction. Also, the four ring design uses two oil rings. The bottom ring is called a oil scrapper ring, pushes the oil on the cylinder wall down. The other ring is an oil control ring, it distributes oil from passages inside of the piston to the cylinder wall. With this design it is also likely that oil is puddled between the two oil rings causing even less air to blow by.
Two other possibilities of to high of compression readings, both of which I have done and learned from are. Installing the half wedge upside down. Not measuring the ring end gap from new. If the half wedge is installed upside down the ring cannot float in the land, this exerts constant pressure on the ring, which will greatly shorten its lift. The same is true of not having the correct ring gap. When up to temperature it is completely closed with the ends touching, this also puts pressure on the rings and wears them out.
Another thought about checking cold. The lifters are bled down and the pushrods are shorter, causing the valves to only have closing spring pressure on them.
Last edited by Vagabondblues; 11-23-2016 at 02:58 PM.
Another thought about checking cold. The lifters are bled down and the pushrods are shorter, causing the valves to only have closing spring pressure on them.
Todd, Doing cylinder work I have always had to bleed down the lifters, hot or cold.I don't get what else holds the valve closed other than the valve springs. Please explain
After a cylinder swap you have to check the valve circuit cold clearance. You should have a gap when cold so that when it is hot the length of the pushrods does not hold the valve off the seat. I was implying that if there is a correct cold clearance gap the only thing acting on the valve was the closing spring pressure. In other word checking compression hot and the valve clearance is slightly long would cause the valve to be very slightly off its seat. The pressure in the cylinder would act against the valve pushing it in. Kind of a push pull balance.
Hmm, never figured the valve train would unseat the valve. I can usually get the rocker shafts out with my fingers and a small piece of dowel rod when on top dead center.
I believe the situation Tod is referring to is an engine with solid adjustable lifters as in the Lyc O-235C.
The hydraulic lifter insures complete valve seating when closed and zero clearance when open.
When replacing a cylinder you must bleed down the lifter by usually removing and tripping the lifter check valve or constantly forcing the lifter down and relaxing the pressure multiple times until the oil is bled out. Then after installing the cylinder you can check to insure the dry lifter clearances are correct.
Too small a dry clearance could then keep the valve open slightly when it should be closed utilmatenly burning it or if the dry clearance is too high the lifter will be unable to to take up the residual gap and the engine will be noisy; tapped noise.
Last edited by Gilbert Pierce; 11-24-2016 at 12:50 PM.
I believe the situation Tod is referring to is an engine with solid adjustable lifters as in the Lyc O-235C.
The hydraulic lifter insures complete valve seating when closed and zero clearance when open.
When replacing a cylinder you must bleed down the lifter by usually removing and tripping the lifter check valve or constantly forcing the lifter down and relaxing the pressure multiple times until the oil is bled out. Then after installing the cylinder you can check to insure the dry lifter clearances are correct.
Too small a dry clearance could then keep the valve open slightly when it should be closed utilmatenly burning it or if the dry clearance is too high the lifter will be unable to to take up the residual gap and the engine will be noisy; tapped noise.
Gilbert,
Thank you so much. I'm probably stooopid on putting engines together. I have only done 8-10 from 351 Fords, 383 Chevys and Continentals. But when you buy brand new push rods from Continental, have the rockers rebushed, cam reground and new Hyd. lifters. Just what is there to check for cold clearance? A hyd. lifter takes up all the slop. If the clearance is to great or to loose what to you adjust??? Just listen to a big bore Continental at start up. Sounds like a race car until the lifters pump up. Yea we check the cold clearance on our solid lift Chevy race engine.
All I know is ours run great.
As far as the 3 ring piston and the 4 ring piston. From what I have been told was the early big bore Continentals had the 3 ring. My O-470 J is a 1954 design. The IO-470-N and the IO-470Ls in our Bonanza and Baron both have 4 rings. Watched eng builder guys put rings on these type of engines and no top or bottom to the rings.
The up grade which I did to the 4 ring was to keep it from using oil. Without the lower ring you would get a lot of blow-by. Oil would turn black in about 4-5 hours. Must be why Continental allows the master orifice compression test which is about 80/45 ..
As stated before I'm really happy with no oil use and 80/79-80 and a perfect clean screen.
Last edited by wyandot jim; 11-24-2016 at 09:18 PM.
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Originally Posted by Vagabondblues
