Iron Piston Ring Surface Prep

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cl350rr

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I have spent most of the winter in search of compression on my Odds and Ends engine with minimal success. I have now produced 4 different sets of iron rings for it and the last set are about as good as I will be able to do. I installed them and tried the engine and the compression is better than any I have tried before but is still insufficient for the engine to run.

my method of testing the rings is with the valve block and spark plug removed and finger over the ports. this eliminates leakage from these parts for the time being and only leaves the piston to bore seal.

I did not make the cylinder so I am not sure what material it is made out of but I assume cast iron and have no reason to believe it is anything else. I checked the bore and it is straight and round. I tried honing it with a brake hone and that improved compression slightly but not enough.

the question is: what surface prep is recommended in a 1" bore cylinder for running an aluminum piston and iron rings?


do the rings need to break in like on a larger engine or are you guys pollishing the ring and cylinder with laps to create the seal?

thanks
 

Jasonb

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The cyl hone should be enough. Run the engine in for an hour or so with a pully and belt off the lathe or similar and plug removed with plenty of oil this will bed the rings in and you should notice the increased compression.

Jason
 

gbritnell

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You say that you tested the compression with the spark plug and valve block removed. When you did this did it have good compression? If not then I suspect that the bore is in fact not round. If it has a slight taper it won't bother it too much but if it's not round it won't work at all. When you add up the two elements, bad bore and possibly leaky valves then it will be awful hard to get it running. If you have followed some of the other fellow's build threads you will understand that when building an I.C. engine the tolerances are much more critical than a steam/air engine.
How did you check the roundness of the bore? How did you make the piston rings? What is the piston to bore clearance?
gbritnell
 

cl350rr

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thanks for the input.

run in... I have but not for a long period. I will try that next.

when I tested with valve block and plug removed I did not get sufficient compression. that's why I am asking about the bore and not the valves as that's the first thing everybody points to when I say I am having trouble with this.

the rings were made from gray iron using the no heat method (turn to 1.030, bore to .860 slice off, cut gap with .032 saw, close on jig and return to 1.000, rejig and bore to size, pollish, chamfer id and install on piston).

I checked the bore with snap gauges and a mic. it is round with very little taper.

I understand that the tollerances are critical, I have kept them as tight as possible. I confirmed yesterday that when I coat the rings with a stiff grease, I get good bounce (seal) until the grease goes away, indicating that the rings are the source of the leakage.


please keep the ideas coming, I am running out of my own.

Randel
 

gbritnell

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Randel,
With a bore of 1.00 I would make the piston diameter no smaller than .9985. If the bore is 1.00, concentric, with no taper then the rings should seal quite well even with no run in on them. By adding the grease you are in effect sealing an imperfections in the rings, piston and bore. When the grease thins out or dissipates and the compression goes away then I would start suspecting the cylinder bore. I know you said you have checked and double checked but that is where I would start looking.
gbritnell
 

LongRat

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How wide is your ring groove relative to the ring? If the ring is able to rock about in the groove when the piston changes direction you may compromise the seal. This effect would also be alleviated by the grease.
 

cl350rr

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just came in from measuring:

bore is 1.0030 at most a .0005 taper front to rear
piston is .9975

ring to groove clearance is .0030

so... I guess a new piston is in order :)
 

Lakc

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How much wall pressure do the rings have? That piston may knock a little when cold but should have plenty of compression. Ring gap?
 

gbritnell

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Were the rings made to the 1.003 dimension or were they made for a 1.00 bore? This would cause a small gap also.
gbritnell
 

cl350rr

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I have no idea how to test wall pressure on rings this size. outside of the bore they have about .030 gap. installed in the bore they have .002 gap. I assume that should be sufficient wall pressure.

the rings were made 1.000.
 

stevehuckss396

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cl350rr said:
I have no idea how to test wall pressure on rings this size. outside of the bore they have about .030 gap. installed in the bore they have .002 gap. I assume that should be sufficient wall pressure.

the rings were made 1.000.
If I were in your shoes I would remake the piston and ring. Piston would be 1.0015 - 1.002 and the ring should be right on.
 

Mike N

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How close does the piston come to the head? Maybe the design doesn't have a chance to get good compression. I have built several IC engine kits with 1 1/4" bores. The kits have called for any where from 1/8" head to piston clearance to almost an 1" of head to piston clearance. I found that on most engines that run the best the piston is approx. 3/16"ths clearance. I also hone the cylinder with a good cross hatch. I run the the engine in the lathe for 3 hrs to get the rings seated & then re-hone the cylinder with a cross hatch again before I even try to start it for the first time. I take out the spark plug & squirt a shot of WD-40 in the cylinder it acts like starting fluid. I set the ignition timing at TDC. This method works for me.
 

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