Hi 'Guys',
Just a short(?) update on whats been happening over the last couple of weeks and one which will bring this particular engine project to a close.
Along with finishing the said article on the Mk1and 2 for the ME mag which has now been sent off I've been making parts for a 'Stockton and Jehlik' version of the 15d only this one is correct size - 2.5cc. All the details of this highly modified engine were published in a 1967 Aeromodeller magazine however unlike all the other 5cc versions made so far including an S&J this one is destined to be used as intended and is to be fitted to a replica of S&J's team race model 'Jefe'.
As per the original (modified engine) it utilises some 15d parts, namely the crankcase and piston and liner however though the original also used a standard but slightly modified crankshaft this one had a new one made to suit the prop driver/spinner arrangement.
It's now finished so for those with an interest here are a few pics and captions...
The front bearing housing and backplate. These were machined from a higher spec aluminium (Alumec 89) than the usual 6082
mainly for the fact that the rear valve bearing has no sleeve - how well it stands up remains to be seen.
The induction system differs from the original rotor disc to that of a rotary valve which is still driven by the crankpin extension.
The original crank case. Unfortunately the front and rear faces were well out of parallel by some 0.2mm so after machining the as cast mounting lugs (a considerable fault with the original) true to the exhaust register the case was carefully set up, a very fine cut taken through the bore and all faces brought square and parallel
It had also had the cylinder bolt holes opened to 5BA, one of which was drunken and all slightly out of line to the bore. After setting up to the register for the exhaust flange the holes were opened up to 4BA and threaded ali inserts screwed in with JB Weld. After 24 hours these were re drilled and tapped 6BA - a successful reclamation.
This one is for Per
- it is the composite contra piston - the outer flange is a tight press fit to the liner and the inner moving part - 8.9mm dia - a tight but moveable fit within it
The original article called for a (relatively) large diameter cylinder head with fins of 25 thou thickness and a 25 thou gap cut "with a tool ground from a hacksaw blade". A tool holder was milled up which worked extremely well but unfortunately an attempt to grind side relief into the blade led to the blade disappearing on the first cut. Thereafter the tool had no relief but the rubbing caused pick up which pushed the fins over - as called for they are quite deep 7.9mm. After 'not a little anxiety' as each fin was achieved - the fourth but last is hanging on by such a small amount it can be deflected quite easily - it did not take much to decide that it should go in the bin. Not only that but cleaning the swarf from the fins proved a real a pain and quickly lead to the realisation that as such, despite the extra surface area, any build up of detritus would soon lead to ineffective cooling anyway.
A decision was made to do the same as the 5cc version and reduce the number of fins and open the gap. These were cut using a cutter ground from a discarded FC3 cutter shank which allowed some side relief and the whole thing was so much easier.
Incidentally - the bolt holes are put in first - this makes deburring the holes much much easier and is much less likely for the drill to wander when going through separate fins
I think you'll probably agree it was the right thing to do
The crankshaft was turned from En24T but this time (for a first time too) the crankpin was cut from a hardened steel pin and pressed into the web with about .0125mm interference. Again time will tell how this works out
The Prop Driver, Sleeve Nut and Spinner parts. The brass spinner-nose screws into the front of the sleeve nut
The original used the nylon front end of a Cox TD-049 and its venturi/spray bar system. The intake parts here are original Cox but the case is machined from hard plastic.
All parts were given a thorough cleaning .....
.....and it was finally assembled today. The backdrop is a copy of the original 'Aeromodeller' article
and this is it with it's big brother
Well 'Guys' thats the 'Eta' project over
and for me time to take a break from machining - (Is that okay Eric?
)
I guess by now you've all worked out that I'm not a true blue, dedicated, dyed in the wool, model engineer like most of you are but someone who likes a bit of a change from time to time. On that note I'm going to change tack (pun definitely intended
) for a while and do something completely different as plans are afoot to build a Marblehead yacht.
That does not mean to say I'm not intending to make any more engines - theres a couple of Italian beauties waiting in the wings for starters - but needs must
I would however just like to say a big Thank You to all of you for your kind comments and encouragement throughout this thread - all extremely valued and much appreciated.
On that note then I shall 'slip me cable' for a while and return at some stage refreshed and ready to 'have another go'
Back 'afore too long then - Ramon