Dennis Mulhair Posted September 13 Share Posted September 13 Hello, By any slim chance does anyone have an exploded parts diagram of an SC/ASP/Magum 25A aero engine. I have access to the diagram for the later model SC 25A-S but its the older version I need. The label on the box is SC 25A AERO RC ABC ENGINE I need to find out if : A. It has 2 head gaskets ( as my model does not) B. It has a gasket on the back plate ( as my model does not) Hope someone can help. Dennis Quote Link to comment Share on other sites More sharing options...
Rich Griff Posted September 13 Share Posted September 13 Have a look on sceptreflight, it may show components. Quote Link to comment Share on other sites More sharing options...
Dennis Mulhair Posted September 13 Author Share Posted September 13 Thanks Rich but they dont do that model! Quote Link to comment Share on other sites More sharing options...
John Wagg Posted September 13 Share Posted September 13 Is this what you need :- https://www.justengines.co.uk/shop/engine-spares/two-stroke-spares/a-s-p-two-stroke-spares/a-s-p-s25-spares/?v=79cba1185463 Shows one gasket on the back plate and two on the head. 1 Quote Link to comment Share on other sites More sharing options...
kevin b Posted September 13 Share Posted September 13 Can anybody explain to me why a single cylinder 2 stroke engine should require 2 head gaskets please ? The purpose of a gasket is to seal between 2 interfacing parts. As far as i'm aware it should serve no other function if the engine was designed properly. I'm sure Engine doctoer will be able to explain, or Jon H if he is about. Quote Link to comment Share on other sites More sharing options...
Rich Griff Posted September 14 Share Posted September 14 Compression ratio and its associated problems/advantages.... Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted September 14 Share Posted September 14 My ASP 180FS was a bit ill mannered and rough until I added a second head shim, which transformed it. Quote Link to comment Share on other sites More sharing options...
Dennis Mulhair Posted September 14 Author Share Posted September 14 Thanks Kevin for your reply but the Just Engines diagram show the 25A-S model not the slightly older 25A. Regards Dennis Quote Link to comment Share on other sites More sharing options...
Caveman Posted September 14 Share Posted September 14 (edited) Dennis, if you look on the Just Engines website there is an exploded diagram of the 28 two stroke which is the older model with the round fins and head. It also has a gasket for the rear cover and two for the head, so it looks like they’re all like that. GDB Edited September 14 by Caveman Typo 1 Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted September 14 Share Posted September 14 21 hours ago, kevin b said: Can anybody explain to me why a single cylinder 2 stroke engine should require 2 head gaskets please ? The purpose of a gasket is to seal between 2 interfacing parts. As far as i'm aware it should serve no other function if the engine was designed properly. I'm sure Engine doctoer will be able to explain, or Jon H if he is about. As mentioned above the number of head gaskets will effect the compression ratio. One factor will be the fuel you are using as you want to avoid pre-detonation as it can knock a hole through the top of the piston + other damage. Another factor is the difficulty to start the engine or its ability to chuck the prop off as it pre-detonates and rotates backwards. For completion engines you are looking for the absolute advantage without engine destruction before the finish line, but for most modellers reliability and ease of starting is more important. I used to race air cooled 2 strokes (250cc) when the water cooled competition with reed valves & power valves were becoming dominant. We were tuning the air cooled ones to the point where reliability was being sacrificed. Anyway one engine was due a strip down new rings and check over as we had stopped using Castrol R and moved to a fully synthetic 40:1 and my dad said we would need to anneal the gasket (the methanol engines didn't have one as it ran higher compression +16:1), but it didn't have one! Much head scratching as it had been going really well, then closer inspection of the oil ingredients and the last item on the list "octane booster" but a head gasket did go back it. You can put 2 gaskets in and have an easy life, one and it might chuck a prop at you or lap the cylinder head to the barrel and go like the wind...to A&E PS When starting the 350cc single it once fired backwards and took my knee up into the underside of the handlebars...that took about 20 minutes for the pain to subside...oh the fun of well advanced static ignition timing and more port area than cylinder wall! Quote Link to comment Share on other sites More sharing options...
kevin b Posted September 14 Share Posted September 14 Thanks Chris. So, the reason why this manufacturer fits 2 head gaskets is a "bodge fix" for getting the design / manufaturing wrong. That is what I assumed in the first place. I couldn't fine Laser, or Saito ever suggesting fitting 2 gaskets anywhere. 😁 Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted September 14 Share Posted September 14 Its not a bodge but head shims are to compensate for different fuel mixes. 1 Quote Link to comment Share on other sites More sharing options...
Jim Hearnden 1 Posted September 14 Share Posted September 14 (edited) Head gaskets affect the squish, which on a glow 2 stroke is the timing adjustment. Normally as nitro level increases you reduce gaskets & increase the compression & squish. I suspect the engine is designed for all world markets & the two gaskets are used in markets where Nitromethane content is lower or non-existent. Europe tend to use straight or 5-10% whereas the USA markets tend to use 25-35% as their standard. I have found SC/ASP engines of all capacities etc happy on 5% fuel in all weathers, although in cold winters I used to use a small amount of unleaded petrol, which improved starting & idle when it was brass monkeys. Too old to fly in that these days! Edited September 14 by Jim Hearnden 1 Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted September 14 Share Posted September 14 As ED says its not a bodge, it gives you and anyone else an option depending what fuel or performance they would like vs the chance of visiting their A&E department if it goes wrong.. Life is full of choices ...do I keep the throttle nailed or not.... Quote Link to comment Share on other sites More sharing options...
Nick Cripps Posted September 14 Share Posted September 14 1 hour ago, Jim Hearnden 1 said: Normally as nitro level increases you reduce gaskets & increase the compression & squish. Actually, that's the wrong way round. Increasing nitromethane effectively increases compression ratio so you have to add head shims to reduce compression to avoid pre-detonation. Quote Link to comment Share on other sites More sharing options...
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