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Dork Tube fitted to venturi intake

Care and Feeding of PAW Diesel Engines
By Adrian Duncan

PAW model diesels, like other model engines, are NOT toys - they are in fact examples of miniature precision engineering, and need to be treated as such. In fact, the required standard of fits and tolerances for a model diesel to perform well is substantially higher than that required for many "full-sized" machines.

The intent of maintenance is to maintain the fits and tolerances at peak levels for as long as possible. This is a challenge in a combat setting, where engines are subject to long, hot runs in dirty conditions, along with the ground impacts and dirt ingestion which inevitably results from combat competition.

Although the life of an active combat diesel will inevitably be shorter than that of an engine used in a "non-contact" application (such as stunt), the owner can do much to minimise the effects of wear and tear, thus maximising the return on his/her investment. The following are some simple steps that can be followed in this regard. 

(1) Inspect the new engine upon delivery to ensure that the end-float on the crankshaft is not excessive. This is a very common problem with new PAW's. The key point is that the rearward movement of the shaft should be constrained by contact between the rear of the prop driver and the front of the main bearing, NOT by binding of the con-rod or contact with the backplate.

The test is - if you push the shaft back as far as it will go and you can still see a gap between the rear of the driver and the front of the bearing, there's too much end float. An end-float setting of 5 to 10 thou is usually ample, and more than this is excessive. If the end float is too large, the driver can be removed with a small gear puller (the three-jaw variety is best) and a steel shim washer of the correct dimensions fitted to reduce the end float.

If you have access to a small lathe, the taper in the prop driver can be cut out a little to move the driver back to achieve the same objective, but DON'T try this unless you really know what you're doing! The shim approach is much easier if you don't have a lathe.

2) Use the proper fuel - this means, in our book, a fuel containing not less than 24% castor lubricant. See elsewhere in this web site for detailed information on model diesel fuels.

(3) Break the engine in properly BEFORE mounting it in a model and flying it! See the section on break-in procedures elsewhere in this web site.

(4) Set the venturi angle so that the needle is angled back as far as it can be without actually making contact with the crankcase. This will greatly reduce the tendency for the spraybar to break in a crash. DON'T over-tighten the set screw since this can distort the venturi bore . Also, a slightly looser fitting allows some "give" in a crash, further reducing any tendency towards breakage. But ALWAYS check the set-screw after a crash!! 

(5) Use a flexible "dork tube" (a simple length of silicone tubing) fitted to the venturi intake and secured with a small ziplock fastener or thin wire. This should extend some half-an-inch beyond the end of the venturi . It will bend over the end of the venturi in a crash and temporarily close it off, thus minimising any tendency to ingest dirt in the crash.



Adrian Duncan.

(6) Avoid over-compression and lean runs like the plague! If the engine starts to sound "laboured" or "harsh" in flight (often accompanied by excessive smoke from the exhaust), ditch the model IMMEDIATELY to get a re-set of the controls. Running in the overheated or overcompressed condition will wear out the rod bearings in no time flat! Every combat pilot needs to become expert in the noble art of non-destructive ditching of a distressed model!

(7) An engine which is known to have ingested a significant amount of dirt in a crash should NOT be re-started until it has been flushed out THOROUGHLY with raw fuel or some other cleaning liquid. Not a big deal if you're just testing, but if you're in the middle of a close match with a seriously mud-balled engine, you have to decide on the spot if the potential damage to your engine is worth the potential match win! Extreme cases require removal of the backplate for cleaning.

This is no joke - dirt can quickly ruin both the shaft bearings and the piston fit. On a PAW .15 BR, particular attention also needs to be paid to the gap between the prop driver and the front of the main bearing. The shaft runs directly in the aluminium case at the front end, and any dirt entering the bearing quickly turns the soft case material into a very efficient lap, wearing out the shaft journal in very short order! A "wrap-around" prop driver is very effective in minimising this tendency. The stock PAW BR driver can easily be modified by recessing and re-tapering to enclose the front of the bearing if you have access to a small lathe. 

(8) Some fuel residuals are quite corrosive. If you're using castor oil, you're ahead of the game since castor oil is an efficient rust inhibitor. Regardless, one should always inject some after-run oil through the venturi following a session, and the engine should then be turned over to distribute this oil throughout the working surfaces. 

This is particularly important if the engine will not be used for some time, since it reduces the tendency of the castor oil residues to "gum up" the works and makes starting easier the next time out.

I always squirt some kerosene over the OUTSIDE surfaces after a session since this prevents the build-up of baked-on castor gum over time and helps keep the cooling fins clear so that they function as intended in cooling the motor in flight. I also inject some straight kerosene through the tank and out through the venturi to eliminate any potential gumming and consequent blockage of the fuel system.

Dismantling of PAW Diesel Engines

Our first piece of advice in this area is - DON'T, unless it's absolutely necessary! Once an engine is fully broken in, any disturbance of the parts will inevitably result in them being re-assemble in a fractionally different relationship to one another, and some of the benefits of a careful break-in may be lost. There is also the potential for the introduction of dust or dirt unless conditions of absolute cleanliness are maintained. At the very least, there will be a short-term period of accelerated wear while the parts get re-acquainted. So follow the old dictum of "if it ain't broke, don't fix it!"!!

This said, even the best cared-for engine will inevitably require some replacement parts at some point. The most common replacement requirements on a PAW .15 or 19 diesel in our extensive experience are the con rod and wrist pin. This is simple wear and tear given the very high loadings under which these components operate. We have also needed to replace the rear ball bearing on a few engines, probably due to dirt ingestion during a match where we decided to go straight back up (see above!!). At some point, too, the engine may require a rebore when the piston fit becomes too slack for good starting.


Typical engine pod with PAW 15 BR Deluxe recessed
into leading edge.

Unless you REALLY know what you are doing (as opposed to THINKING that you know!), we strongly recommend that you return the engine either to your supplier or to PAW themselves for all repairs requiring complete dismantling of the engine. 

Over the years, we have seen a depressingly large number of engines that have been seriously damaged or in some cases ruined by ill-informed attempts to effect repairs without the necessary know-how or tools. This applies particularly to attempts to "tune" the engine for more power. 

The Vintage event rules do not allow any form of performance-enhancing modifications, so this is not an issue for us. 

For those of you in other areas where tuned engines may be permitted, we can only say that, while our own experiments have shown that it is certainly possible IF YOU KNOW WHAT YOU'RE DOING to tune a PAW .15BR (or any other PAW for that matter) for measurably greater performance, there is usually a cost in terms of reduced flexibility, consistency and ease of handling.

Most of the "tuning" efforts we've seen by others have resulted in a motor that is neither user-friendly nor consistent and runs little if any better than a good stock motor - sometimes worse, in fact! We find that a well-treated stock motor works just fine in the right model, and strongly recommend that you follow suit.

Oliver Twist mk.VI designed by Martyn Cowley 1969.
Model built by Paul Dranfield


If you MUST take the engine apart (apart form the removal of the backplate, which is perfectly straightforward and is sometimes necessary for cleaning), here are a few things that you should know:

(1) DON'T tackle this work unless you have the right tools to do the job and are fully conversant with the correct methods for approaching the dismantling and rebuilding of miniature precision machinery.

The most important factor is CLEANLINESS. Great care must be taken to keep dirt and other potential abrasives out of components such as ball races and rod bearings, and parts should be scrupulously cleaned during the re-fitting and re-assembly stages in particular. 

(2) Remove the backplate first, followed by the three head screws. Do NOT lose these screws - they are a British Association (BA) thread of a type not found much any more outside Britain. Replacements will be hard to find!

(3) BEFORE removing the cylinder liner, mark the front of the cylinder PERMANENTLY with an engraving tool or a small Dremel grinding bit. It is ESSENTIAL that all parts be re-assembled as closely as possible to the original configuration if accelerated wear is to be minimised, and clear marking is a requirement if this is to be achieved.

(4) Before removing the piston/con rod assembly, note that there is a punch-mark near one edge of the piston crown directly above one of the wrist pin bosses. This punch mark is CRITICAL - the piston MUST be replaced with the punch mark towards the REAR of the engine (NOT the front, as many "tinkerers" seem to think!).

(5) The wrist pin is a fairly tight press-fit in the piston bosses. Use a drill press with a suitable mandrel to press it out if the rod has to be replaced. Be VERY careful not to distort the piston while doing this - it's easy to get it wrong, and a rebore is the only fix if you blow it! ALWAYS press the wrist pin out TOWARDS the punch mark on the piston crown, and re-install it from the punch mark side. This is because we want to keep the wrist pin boss as tight as possible at the front to prevent the wrist pin from moving forward and snagging in the front transfer port. There is no transfer port for it to snag in at the rear, so movement in this direction is less critical (although still highly undesirable!). Note that the rod too is a one-way fit - the FRONT of the big end bearing on the rod has a small chamfer incorporated, and the big end oil hole must be correctly oriented. So carefully note the way the rod is fitted before attempting to remove it from the piston, and re-fit it or its replacement in the same orientation..

(6) If you are replacing the rod, use a new wrist pin as well . A loose top bearing is often as much a reflection of wear on the wrist pin as of the rod eye, and wrist pins are cheap anyway!

(7) The prop driver fits on a simple taper on the front of the crankshaft - there is no split collet. Also, the shaft is a sliding fit in the inner race of the rear ball bearing. Hence the shaft can be removed simply by pressing it out from the front using a vice (NOT a hammer!!). ALWAYS protect the jaws of the vice with a non-marring (non-metallic) material to maintain the rear sealing surface onto which the backplate will be tightened upon re-assembly, otherwise you'll lose the vital crankcase gas seal.

(8) If you have to remove the rear ball race, first heat the case in an oven to around 200 deg. C or so (no hotter!). Often a sharp rap on the rear of the heated case on a non-marring surface (wood or similar) will cause the bearing to simply drop out. Failing this, you may have to force it out using light pressure applied with a suitable tool. BE VERY CAREFUL not to mar the plain bearing surface while extracting the ball race, otherwise you may lose all the lovely in-flight fuel suction which the BR is so good at!


Adrian Duncan with his September Warrior. (Aeromodeller Plan) designed by Bas Bunstead  in 1963.                                                



* The engine is re-assembled in the reverse order, making absolutely certain that all parts are re-assembled in the correct orientation. Cleanliness is paramount at this stage. I always clean all parts in an ultrasonic cleaner filled with kerosene prior to assembly. Oil all parts as they are re-fitted, and re-oil the entire engine once assembly is complete. Treat the re-assembled engine as a new motor for at least the first 4 or 5 runs if you want to maximise engine life. 

Again, we do NOT recommend that you dismantle any PAW (or any other diesel for that matter) unless absolutely necessary and you truly know what you are about. But adherence to the above steps will help ensure a successful rebuild if this is unavoidable. 

Good luck!! 


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This page was last updated on 02/21/04.