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1917 roadster - .025 pistons? Anyone have a spare? Ideas on replacement


dbtravis
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Hi All,

 

Wanted to start a new piston-related thread here for an issue I've encountered on my 1917 roadster engine.  I know many of you like thorough details when troubleshooting, so I've tried to provide as much info here as possible - so thanks for bearing with me as you read through this.  As you may have seen from a different post, I've removed the head to explore potential valve issues causing lack of power and carb backfiring.  Compression prior to disassembly was very good, even on the high side, with an even 60psi across all cylinders.  I did notice a little oil pumping at the #1 cylinder when it was hot, although as mentioned compression on this one checked out good.  I've always heard a little knock from the engine, but nothing major.  However, upon removal of the head, I noticed that the rear 3 pistons are stamped .025, yet the #1 cylinder has nothing stamped on it.  As I began to remove carbon, the rear 3 pistons only wiggled ever so slightly in their cylinders at top dead center, if at all.  However, the #1 piston just floats in the bore, I can move it right-left enough that it actually makes a loud slapping noise!  Visually I wouldn't be surprised to say it moves at least 1/16" or more.  Although this is likely not a cause of the issue I set out to fix, I don't want to button things back up without addressing it, so I've dropped the pan and removed that piston.  After several measuring repetitions and as careful as I can be, the piston seems to measure 3.875" and the bore by some stroke of luck appears to be round, un-tapered, and in good condition measuring 3.900" on both top and bottom. 

 

The underside of the pistons in the back three cylinders all say "Lynite" and something else which was obscured from my viewing angle by the wrist pin but looked somewhat like "Wilson".  The front piston is a completely different one that says "Permite 702" on the inside of the skirt.  Given that no oversize numbers were stamped on the #1 piston, and it's a completely different piston brand, and looking where the measurements come out, my guess is that whomever worked on this engine before me had it bored .025, but only had 3 correct pistons so they used a standard size in #1 and probably tried to compensate by putting oversize rings on it.  ???  Maybe?

 

Anyhow, I'm now looking for advice on exactly what I should do.   All the pistons seem to be some aluminum comp type of material.  It seems that replacement new pistons only come in oversize increments of .010, and since apparently I've got an oddball bore to .025, I'm right between.  I've come up with 4 options...  1) See if I can find a used .025 aluminum piston with good rings. (not likely to find).  2) buy a new .020 piston  (but then what do I do for rings?).  3) See if I can hone the cylinder up to .030 (I have a rigid micrometer hone) and then buy a .030 piston and rings (I don't trust myself on accuracy here and currently the bore looks pretty good and even, but maybe it's not that hard?). 4) Buy the .030 piston and see if a machine shop can turn it down .005 and then file the rings to fit.

 

I'm eager to hear your feedback on the best idea out of what I've proposed or anything else you can come up with.  Goals are to not mess with the rear 3 pistons since they seem great, and do this with the engine in the car - unless anyone can convince me that either of those are extremely necessary.  Ideally it seems the best route would be to find a .025 piston and rings, but that seems like wishful thinking... unless any of you have one laying around?! 🤔  Although the compression checked out good on this one, it seems with this much play it needs to be fixed as I'm concerned putting it back as-is would cause damage to the bore with continued usage that would necessitate removing the entire block for a re-bore... something I'd like to avoid, if at all possible.

 

Thanks in advance for your thoughts and advice - and check those parts piles for a 025 piston if you happen to have one (or a set) you'd be willing to sell!

 

Travis

Edited by dbtravis (see edit history)
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Even if you do find another .025 OS, it will need to be matched for weight to the other 3. Back in the day, you could buy a set of pistons and grind them to your hole/need. The size could then be stamped on the top. For rings, start with .030 OS and grind the slot for fitting, which may have been done on the .025 OS.

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Look up EGGE Machine, they may be able to help. I agree with adding the full set but I guess it could be done. I once bought a V-8 car that ran ok, but had one cylinder bored oversize and a larger piston fit. I couldn't tell by driving, and I don't know if they tried to balance it with the others but it is a true story.

Edited by JFranklin (see edit history)
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As you said they usually bore to even .01s, .025 is unusual. I would have it bored or professionally honed to .03 and buy new pistons. That way they will match in weight and last 100 years. I'm not going to say you will never hone it by hand .005 because someone will always argue. But you won't have a round cylinder that doesn't have ugly taper if you try it. An automotive machine shop can power hone it and control roundness and taper. 

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Thanks to all for the opinions and advice.  I think you are making great points and since I already have at all opened up, it's a small investment in time to really do this right so it will last another 100 years.  Equally weighted pistons hadn't even crossed my mind since the #1 is currently a different one, but with these old engines, I think anything to get a little smoother running at high RPM and less wear and tear on the crank/bearings is definitely a good thing.  Hopefully when I get it all back together it'll run even better than it did before, and I'll have the peace of mind that #1 isn't destroying the bore with the extreme clearance that currently exists.

 

Keep the ideas coming, but you guys have pointed me in the right direction.  Might be a week or so before I have time to dig back into it, but I will certainly re-post here as progress is made!

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