While I don't consider myself to have near the skill or ability of Randy Ryan when it comes to rebuilding old engines, his results are an inspiration. See here for pictures of his restoration of an old Webra 2.5 Mach I.

http://www.rcuniverse.com/forum/m_4224809/tm.htm

Mine isn't a restoration so much as a tear down, major cleaning and rebuild. I never thought of taking a picture before I started, next time.

Picture if you will a 40 year old PAW 2.49cc DS plain bearing diesel that has been well used and is quite literaly covered with gooey and baked on castor oil from many hours of use. Overall general condition was good with no dings, bangs or major scratches. I managed to start the engine before I started my rebuild. It took quite a bit of work to get it going, needle valve was very touchy as was compression, spit black gooey stuff from every crevice all over my test bench and just sounded like a sick dog.

After tearing down the engine, the cleaning began by soaking overnight in straight kerosene followed by a scrubbing with toothbrush in hand and a margarine container of general purpose degreaser. I literally scrapped about 1/100" of carbon off the top of the piston.

After a good scrubbing with kerosene a good scrubbing with warm water and dish soap followed with the result being a nice clean engine.

Piston cylinder fit is good but certainly not like new.

I also took the time to "customize" my rebuilt engine. I replaced the head bolts with m3x.50pitch x 30mm long socket head cap screws. The original 6BA head were OK but a but the cheese heads were a bit rough looking, I can get replacements from the UK but used the metric as they are much easier to get. The crankcase tapped out very easily with a 3mm tap as the original 6BA threads have a pitch very nearly the same, .53 . The holes on the top of the head had to be drilled out to clear to the cap screws heads. I used a #1 drill for this

I also replaced the the traditional PAW compression adjustment screw with a 10-32 x .5" socket head cap screw. The original thread is 2BA and very close to 10-32 in diameter and pitch. Makes a for nice light interference fit. I drilled out the end of the cap screw to help prevent the compression setting from drifting as the engine runs. Look at the bottom of the PAW screw and you should this on the originals.

I also put 3 small (1/8") holes in the head, 1 would have been enough to help relieve any pressure that might build up in that area when adjusting the contra up and down. 3 has a nice look so this was a way to customize my engine.

The head was then polished up to remove some small scratches and make it look a wee bit brighter.

The muffler was refitted with new gaskets (easy to make, 3/4" diameter to fit over the cylinder and 1-1/4" diameter overall)

I also replaced the PAW needle valve assembly with a Super Tigre G20/.15 assembly. The PAW needle assemblies work just fine but I do find that they are a bit coarse and often times a bit loose for my taste.

The prop driver crankshaft taper seat was relieved slightly to allow the prop drive to sit closer to the crankcase. The taper is 7 degrees. I went for a clearance of about .020 to .025 and fitted a .010 brass washer so that I could use a starter on this engine if I wished.

Took a good days work to clean up, fit new parts and reassemble. All in all, I like the results. It looks much better, hand starts easily, needle valve and compression adjustments are smooth and postive and sounds much happier. Compression is not like a newer engine but really isn't all that bad.

Now runs up to 9020 rpm on a Kavan 9x5 prop, before rebuilding I was lucky to get it any where near steady at about 8500 on the same prop. I have a small remote thermometer like the car guys use, before the rebuild temperatures where around 200F after 160.

I have another old PAW 2.49 in dire need of rebuild. This too will be a custom rebuild rather than a restoration. This one has a worn out piston/cylinder and will need some work. I am scrounging up some .15 car engine parts and plan on mixing and matching connecting rods and pistons to get a good set. I will hopefully be able to try an aluminum piston in a steel cylinder and maybe even try a ringed aluminum piston.

cheers, Graham in Embrun near Ottawa Canada.

Graham -

Wow. Thanks for sharing.

Could you say a little more about how this works?
"I drilled out the end of the cap screw to help prevent the compression setting from drifting as the engine runs."

Cheers - LeeHop

I had noticed the ends on the compression screws of my PAWS where drilled out and made hollow. I didn't really know why and no one I asked could provide a suitable answer. I discovered the answer on Ron Chernich's Model Engineering web site in a story about Peter Burfords .03 diesel engine

http://modelenginenews.org/pb/index.html for the entire entry on Peter Burford's marvelous little engine, the bit on the compression screw is about 1/3 down the page with a picture of the compression screw from Peter's engine.

Ron's site is a cornucopia of interesting stuff on small engines and well worth spending much time reading.

Hope that helps clarify.

cheers, Graham

Nice job on the clean up. Ron's website is fantastic for model engine knowledge.

Nice job Graham, nothing like giving one a new lease or look of life is there? Thanks for your kind words, I look forward to getting back to it this winter and doing some more old clunks. To tell you the truth, my restorations are experiments, I start out with a subject that leaves me with nothing left to lose. If it works out I have something like the Mach I, if not, I have the same junk I started out with.