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Posted by Steve on April 26, 2008, 6:52 pm
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Finally, I've finished my attempt at building Malcolm Stride's Nemett15S.
Never really made anything significant before, but Malcolm's article was
that detailed I thought it was worth a shot. It fired up and ran tonight a
bit of fiddling left to do with the carb, but it works.
I didn't stretch to spark ignition but left it as a glow - motor. It's not
exhibition standard, there are a few careless mistakes, but still pretty
pleased with it.
Here are the pics of the finished engine:
www.btinternet.com/~steve.withnell/lathe/Nemett/nem2.JPG
www.btinternet.com/~steve.withnell/lathe/Nemett/nem3.JPG
The files are big <2M.
So almost exactly two years to complete, I'll write down what I learned
that wasn't in the books when I get chance.
Anyway, big thanks to John, Peter(s), Dave, Tony, Keith and everyone else
that provide help along the way and of course "Nemett" for an excellent
constructional article.
Oh and a question - anybody know where I can get a copy of the magazine
article with Bruce Satra's valve lapping tool?
Thanks Guys
Steve
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Posted by Richard on April 27, 2008, 7:11 am
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Congratulations Steve! If that's your first attempt at something
'significant' you should be very satisfied. IC engines, especially at
model scale, are a lot more taxing than steam engines.
I haven't got a copy of the article you request, but I have made both
piston and valve laps by simply making a replica(ish) of the mating
part from a soft-ish material (usually alloy for convenience) and
lapping the component with an appropriate abrasive. I have some 1 and
5 micron diamond paste I acquired moons ago and which works well. Also
simple old-fashioned 'Brasso' does a fair job.
The lap works better if you create some grooves (two or three or so)
perpendicular to the direction of surface movement. This allows the
abrasive slurry to get between the surfaces and do its job.
You don't need a lot of pressure holding the parts together.
Richard
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Posted by Steve on April 27, 2008, 12:39 pm
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"Richard" <sharkface-pilot at toucansurf dot com> wrote in message
> Congratulations Steve! If that's your first attempt at something
> 'significant' you should be very satisfied. IC engines, especially at
> model scale, are a lot more taxing than steam engines.
>
> I haven't got a copy of the article you request, but I have made both
> piston and valve laps by simply making a replica(ish) of the mating
> part from a soft-ish material (usually alloy for convenience) and
> lapping the component with an appropriate abrasive. I have some 1 and
> 5 micron diamond paste I acquired moons ago and which works well. Also
> simple old-fashioned 'Brasso' does a fair job.
>
> The lap works better if you create some grooves (two or three or so)
> perpendicular to the direction of surface movement. This allows the
> abrasive slurry to get between the surfaces and do its job.
>
> You don't need a lot of pressure holding the parts together.
>
> Richard
So if I made a dummy valve from copper or brass to the same spec as the
"real" ones but left a long stem to get hold of, cut the perpendicular
grooves to carry the abrasive then I should be good to go?
I'm planning a more challenging motor and want to get a good seal around the
valves from the off. With this one I made also sorts of duff assumptions
about how to get a good seal - I went to huge trouble to ensure the valve
seat and valve were cut at identical angles (wrong!) I tried lapping with
the valve in situ (wrong!). I assumed you needed a broad seat (wrong!).
So now I know how the experts do it, and it just the lapping trick I was
short of to complete the toolkit. In the end I cut new seats and didn't
lap them in to get a reaonable seal.
Thanks
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Posted by Richard on April 27, 2008, 7:24 pm
Please log in for more thread options >So if I made a dummy valve from copper or brass to the same spec as the
>"real" ones but left a long stem to get hold of, cut the perpendicular
>grooves to carry the abrasive then I should be good to go?
Broadly 'Yes'. You would probably be better off using brass rather
than copper as it's somewhat harder and a lot stiffer - you don't want
it moving about or you'll end up with the seat pissed. Secondly, You
will probably find the valve stem too small to be useful to 'drive'
the hone with so use it as a pilot, but machine the hone so you have a
rod left on the valve 'head' to drive it with.
Depending on what you have available a small, light drill or possibly
a drill press can be used. I have found a speed of somewhere around
500RPM (but it's not at all critical) works quite well for finishing
pistons, less for valves as they're smaller and you don't want to rip
off so much material. You need to make sure you have plenty of fluid
with the abrasive.
You are really using the fluid as a 'lubricant' which keeps the two
surfaces floating out of contact and the abrasive is somewhat larger
than the film thickness and thereby gets to grind the surfaces away.
It also gets embedded into the softer surface and continues cutting a
bit like a microscopic grinding wheel, which is why you must NEVER be
tempted to use the sometimes quoted technique of running-in a tight
engine 'with a bit of Brasso in the fuel'. It's curtains in an instant
and it's utterly irreversible. For a valve you obviously want to
arrange it so you don't get a load of it up the stem/guide.
As for seat width, models can't be scaled to use 'scale' contact
widths, it'd be a few thou (at a guess) to get sensible contact
pressure. But the relatively cool running temperature of methanol and
the likely running hours means it's not a major issue in practice.
A very slight deliberate mis-match in angles between valve and the
seat can help to make sure the contact is round the outer rim of the
valve contact area so that gas pressure in the cylinder doesn't get
under the edge and help to lift the valve off its seat.
>
>I'm planning a more challenging motor
I look forward to seeing your progress
Rgds Richard
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Posted by Dave Baker on April 27, 2008, 7:59 pm
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> So if I made a dummy valve from copper or brass to the same spec as the
> "real" ones but left a long stem to get hold of, cut the perpendicular
> grooves to carry the abrasive then I should be good to go?
>
> I'm planning a more challenging motor and want to get a good seal around
> the valves from the off. With this one I made also sorts of duff
> assumptions about how to get a good seal - I went to huge trouble to
> ensure the valve seat and valve were cut at identical angles (wrong!) I
> tried lapping with the valve in situ (wrong!). I assumed you needed a
> broad seat (wrong!).
>
> So now I know how the experts do it, and it just the lapping trick I was
> short of to complete the toolkit. In the end I cut new seats and didn't
> lap them in to get a reaonable seal.
As someone who cuts valve seats in race engines for a living I'm not sure
where you're getting the above advice. The valve and seat should indeed be
at the same angle although OE manufacturers sometimes use a very small (less
than 1 degree) difference between them to save having to lap them in and let
them bed into each other on their own. I don't hold with that approach
myself. The more accurately the two faces match from the start the better
sealing and heat rejection you get.
A good choice of seat width is 4.5% of the diameter of the valve head on
inlets and use the same seat width as the inlet on the smaller exhaust
valves which automatically creates a slightly higher percentage width to aid
in heat transfer. This normally equates to about 5% to 5.5% of the exhaust
valve head size. By seat width I mean the perpendicular width not the
horizontal.
For lapping you just need a stick with a rubber sucker on the end and fine
lapping paste. However lapping in general is bad n'kay. It actually creates
concave surfaces on both valve and seat as the majority of the grinding
action takes place in the centre of the contact patch. You can see this very
clearly when you reface a heavily lapped valve on the grinder and the
initial contact is only on the inner and outer edges of the seat. Any
valve/seat that needs a lot of lapping hasn't been properly cut and really
the lapping operation is mainly to check the seating and not to correct poor
cutting.
--
Dave Baker
Puma Race Engines
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> 'significant' you should be very satisfied. IC engines, especially at
> model scale, are a lot more taxing than steam engines.
>
> I haven't got a copy of the article you request, but I have made both
> piston and valve laps by simply making a replica(ish) of the mating
> part from a soft-ish material (usually alloy for convenience) and
> lapping the component with an appropriate abrasive. I have some 1 and
> 5 micron diamond paste I acquired moons ago and which works well. Also
> simple old-fashioned 'Brasso' does a fair job.
>
> The lap works better if you create some grooves (two or three or so)
> perpendicular to the direction of surface movement. This allows the
> abrasive slurry to get between the surfaces and do its job.
>
> You don't need a lot of pressure holding the parts together.
>
> Richard