Engines > Restoration of Model Engines

Fitting a piston for F2C engine

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Neil-Lickfold:
This is trimming a piston to  fit for an F2C diesel team race engine. The crown has a short taper, about 0.5 deg per side for the crown thickness of about 2mm.
The main part of  the piston is just parallel turned. At .5 deg angle of the topslide, every 0.05mm movement is 1 micron in diameter cut. The only PCD inserts I have found to be any good for this sort of work has been the positive geometry PCD inserts from Summitomo in Japan.

Niels Abildgaard:
Thank You for some very interesting pictures.
The holy grail has been to lap piston an liner together and You do it by plain turning.
How do You aply feed fine enough?
When do You use three jaw and when ER collets for holding mandrel?

Jasonb:
Nick the last photo looks to have a much better finish than the one 3rd up from bottom, have you done something other than turn to get to the last photo? My thinking is that if magnified a fine turned surface is a fine screw thread, lapping tends to flatten the crests but just turning to size does not.

Also do do you find that those inserts will repeatably take off 1 micron for 1 micron advance of the tool, other inserts would tend to just run with that shallow a cut.

Finally how do you measure the bore and OD of the piston to know there is X microns to come off?

J

Ramon Wilson:
Thanks for describing your technique on this method Neil.

I was surprised to find you are using tip tools for this op - which goes to prove (to me) just how sharp tips are compared to my day.

You don't say what the material is - it appears to be steel as opposed to my usual use of cast iron.

As Jason refers to,  a lapped finish removes most if not all of the machining marks so I can only assume your surface finish is very fine - by usual standards that is - what feed rate are you using ?

Once you have the fit in the bore at the lower end do you have a degree of taper in the liner to work tighter at the top end? If so how did you achieve that.

Sorry for all the questions but 'if you don't ask.....

Regards - Tug

Neil-Lickfold:
So these pistons are made from RSA 444 a 30% silicon material from the RSP company in Holland.
The pistons are on mandrels, that allow the liner to be fitted over the piston to near TDC.
The back part of the mandrel is about 0.1mmin diameter smaller than the finished diameter of the piston.
Sometimes I have the mandrels in an ER40 collet, other times they are in a Grip True adjustable 3 jaw chuck.
These liners that I fit a piston to, are all Aluminum liners hard chrome plated,Except for the F2A speed liners,(they are either bronze or BeCu Beryllium copper)  and have been factory honed and lapped.
Sometimes I do make laps, and re lap the liner a very small amount. But on the most part they are near perfect as they are.
If I am re trimming or reworking an existing piston, then the  mandrel goes into the adjustable 3jaw. If it is a new trim and has like 0.1mm on diameter
or so, then I will often use the ER40 collet to hold the mandrel.
These PCD inserts are the sharpest I have ever come across, and do last quite  along  time so far. I do have a spare insert, but so far have never needed it yet. SO this insert is about 3 to 4 years old now. Only ever gets used for the finish outside passes, and I never use it to trim the end face with it. So I never cut to zero with this insert. For the crown face I use just the regular sharp carbide Al inserts or the Kyocera 06 coated high rake insert. They leave a really good top face finish. Hope t his will help out some people down the track. I have more info in the hints section of the Continuous slide way oiler.
I hand feed the carriage at about 0.01mm per rev or so. so at around 600 rpm, it takes about close to 2.5 mins for a pass down a F2C piston that is about 15mm long. As you feed the tool, you see this very fine sand coming off the workpiece and onto the end of the tool. It can be cut dry, or you can use rice bran oil for a lubricant. Either way, it needs cleaning with break clean to make it very clean to try the piston fit. It also needs to warm back up to room temp as well for the fitting, quite important actually. The chilling of the break clean will result in a smaller piston being fitted, that once warmed up, will be too tight.
Been there and made that mistake.
Neil

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