Knurl an undersize piston or build it up?
I just got the head off the Waterloo, which some may recall had a LARGE
amount of blow by. Slide a ring about midway and took a measurement.
.085 was the gap. Pretty close all the way down. Looks like maybe it
was honed too much. Dave Reed looked at the rings at Portland, and said
it was uniform wear, which means no egg shape to the bore. So, my
question is how much slop can I take out if I get the piston skirt
knurled?
I would hate to screw up the piston or the bore, and do not want to
sleeve it. OR, my other thought is to see if I can find a piston that
is CLOSE to the correct size.
What ya all think?
PS: I was so dejected that I did not to make my road trip, I heated the
Waterloo nuts instead. 
William J. Pfeiffer Jr.,
Sharon A. Cook,
Freckles the gutless wonderdog &
Sugar the tongued terror
rusty_iron@ameritech.net
Woodstock, Illinois, USA
So what do you think? Should Bill knurl the piston to reduce the clearance,
replace it, or have it built up (eg. by metal spraying)?
Can you post the piston diameter and the bore diameter? Give folks an
idea how much clearance you have. Another alternative to knurling is to
build-up the piston with metal spray.
My experience has been that I could only pick up a few thou with knurling. I am not a fan of it.
As far as sleeving goes, a lot of guys sleeve the engine and do nothing about the piston. My
opinion on that is that is only a partial job.
If you can get the engine bored just enough oversized to end up with a clean, straight hole, there
are a few options on the piston.
If you want to go all out, you could get a new piston cast and size it to fit. Lots of work.
You could send it to Niagara with the dimensions of the new hole and have them spray weld it and
turn it to fit as well as making up a set of oversized rings. When they sprayweld them, they just
build up the skirt. The lands are left as is. I am a fan of this method.
I have a buddy who does it a little different, he puts the piston in the bore, runs the engine to
BDC to see how far the skirt comes out of the bore and then he builds up the portion of the skirt
that does not come out of the bore with brazing and turns it to fit. It works good for him.
Sometimes you can shrink a piece of cast iron sleeve over the skirt and resize it that way,
althoug most of the time the sizes don't work out so that that is a good option.
Then, of course, is the option of using an aluminum piston out of something else. If that is
something you would consider.
Do you have a bore mic to measure the bore and see what you really are dealing with? If it were
mine, that is where I would start. If you don't have one, I could lend you one. I will be at the
Baraboo swap meet and could bring one along and you could mail it back when you were done. I have
a lot of other stuff to do for this year and won't miss it for a long time. If I got in a pinch, I
could always use Pa's, his toolbox sits next to mine, anyway.
Good Luck, keep us posted on what you find!
Joe
Isn't this maybe .0085
> amount of blow by. Slide a ring about midway and took a measurement.
> .085 was the gap.
Jim
Nope, .085 is what it reads. THAT is where all the blow by came from.
This is not the side clearance between the piston and the cylinder, it
is the ring end gap. Which means, I am about .070 to .075 inches too
wide. I can put the piston in, and see a daylight at the top.
You might get as much as 0.030" or so from knurling but it won't be a permanent fix. Knurling doesn't last very long. Beings the Waterloo is gonna be used for show, it might not matter.
A couple of years ago at Portland, I saw an open crankcase (I think an Economy) about 3hp that the guy had brazed the piston skirt on to reduce clearance. He gobbed braze on the skirt then turned the brass to give a little clearance. I was impressed. Especially at the bottom of the stroke when the skirt showed it's colors (brass!). Ran nice and quiet.
85 mils is really a bit much for ring gap. It should only be a few mils per inch of bore diameter.
So far as your engine and blowby due to oversize bore, ("Slop, I think you
call it.", I really doubt knurling the piston will any effect at all. Knurling
is done on the piston skirt, and it does raise ridges that reduce the distance
between the piston and the cylinder. It does nothing for the top end of
the piston around the rings. That is where you are losing your compression,
probably much of it through those little holes you found under the rings.
Knurling was, and I believe its main purpose was, to reduce or stop
piston "slap" from worn pistons when people were replacing piston rings and
did not want to spend the money for new pistons. Would a "real" auto mechanic
comment on this, please?
It's pretty much as Twin says, knurling was intended to reduce piston slap in
worn engines, it doesn't do much for crankcase blowby caused by worn rings and
cylinders.
You can use a coarse knurling tool to get greater raised areas, but they will
wear down quicker as the thrust of the piston is taken on a smaller area (points
of the knurl)
It was always a temporary measure, used a lot in the years when the cost of
engine repair was relatively high, and people wanted to get the most out of an
engine.
Regarding sleeving, I have done that a few times, especially the Rolls-Royce
Silver Ghost (40/50hp) engine that I overhauled back in the late 1970's. That
was fitted with Bedford truck liners, then they were bored back to standard
size. The original cast iron pistons were well below size, so we had a set of
alloy pistons fitted which improved the pick-up of the engine considerably.
I would always prefer to sleeve/line and engine and fit new pistons than try and
bodge something by knurling, but I have had it done for people who simply had to
get a few more thousand miles out of an old banger that wasn't worth spending
money on.
Also, don't forget that the piston's main function in life is to carry the
piston rings square in the bore and fill the hole in the middle ! It also
obviously transmits the combustion pressure to the con-rod, but its sealing and
ring-carrying functions are of the top importance.
This article is part of the Stationary Internal Combustion Engine Frequently
Asked Questions (FAQ). This series is a combination of my views and knowledge
and the views and knowledge of other people - most of them members of the
Stationary Engine Mailing List (More info on ATIS).
Those articles which were written by others are © Copyright to the
author. Those articles written by myself are © Copyright to Paul
Pavlinovich.
Got comments?If you would like to comment on any part of the FAQ please do so by using the
Disclaimer:It should be noted that the information
given in this document is considered to be good advice by the people who give
it, however any legal liability lies strictly with the reader. The contributors
are hobbiests not professionals.
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