Motivation is high to get this monkey off my back so am going to post progress on here. Warning, there may be some gaps in progress but hopefully not another 20 years. Actually now that the internet has been invented progress should be sped up somewhat in research and procurement of goodies etc. But first a brief history. My Dad and me purchased it,( Btw its a Benelli 250ss from about 1973-ish) from a guy down the road when we lived in Greymouth for $125 in 1984. It had been run out of oil, but Dad managed to get parts from overseas including a big end and various bearings and a camshaft. All was fitted up except the cam as it didn't look lumpy enough so he resurrected the old one which is still there. We got it going and rode it round for a few years with bugger all problems. Me and a mate used to race around the local kart track until the karters chased us away. The old man eventually retired to Napier and in about 1992 sent it down to me as he wasn't using it. I immediately pulled it apart and put it in boxes, more interested in footy and seeing how much beer i could consume. At that time I was working as an engine reconditioner so did some porting work on the head (unfinished) and bored it out 4mm to suit a piston that alludes me now. Anyway various moves later and the odd burst , painting frame, rebuilding (badly) wheels etc my motivation is good thanks to you BMW guys and your projects so bare with me as progress is updated. Some photos for perspective, its a tiny bike. This weekend i made some valve guides out of brass, bronze is better so will make some more later. Yes this has been mentioned on the off topic thread. I heated the head on the gas hob so the guides would go in betterer. Also fitted an old tyre to the DR rim I laced up years ago.The wheel and tyre are too big but will do for the meantime to set things up. More later
Awesome stuff Woodman. Benellis are my second favourite marque!! I love that purple tornado - but i've already said that. You inspire me so much I'm gonna go down & assemble the welding table I bought from SCA & maybe even use your welder
Yep, but that one is not original, actually in their original form they are not that flash looking at all IMHO. Mine had clip ons and rear sets put on it pretty much as soon as we got it going, but while the old man was good at making stuff work, errr function came well before form and the rear sets were angle iron and the linkages were bent steel rod, even the clip ons were just the old bars brazed up to fit. Everything worked perfectly and even the clip ons stayed intact after quite a few lay downs on the kart track, and one while trying to impress a girl with my Freddie Spencer cornering technique. Original here. note high bars and ugly chrome strip under the tank.
It's the 4T 250 isn't it that was built to take on the MVA & Aermacchis? Bill was saying today they won the world champs 3 times running ? or some useless bit of information like that.
Thanks for the encouragement. Progress has been made, new bronze guides are in my lathe awaiting me to get some tool steel to grind up a cutting tool. hopefully sorted out ignition and coil so it will run, carby rebuilt, but will need re-jetting cos of increased capacity, compression ratio has been worked out so I know how much to remove from top and bottom of barrel to suit xr piston, will be 9:1-ish.Removed gudgeon bush, but can't spin up a new one till piston arrives so I know pin size. Was working out of town last week and next week so that hasn't helped, but could have done more on the body and controls but am trying to focus on getting the engine in and running before moving on to the next phase which will be making rearsets and linkages to suit. Still not sure whether to run left or right gear change yet. So not much progress but not completely idle either.
Yes 4t Heavily modded Motobi is the racing version,(Motobi is basically rebadged Benelli) which is the same bike really, but standard has the egg shaped motor, but internally identical. They still making and racing them. http://www.motobi.com/frame.htm
XR piston arrived yesterday ooooh lovely. So when I get back from Dunedin next friday I will mock the engine up to work out more accurate how much to remove from the barrell. Shouldn't have to remove as nuch as first thought cos the piston is a high compression one and has high dome whereas the original is flat. Anyone want a xr280 head and base gasket? came with the piston.
Does the dome not have to match the shape of the head? I take it you are shortening the barrel because the distance from the crown to gudgeon pin is less than the original piston??? I had the local mech have a look at Berlinas pistons & barrels. He reckoned the pistons looked mint but hasn't measured them yet & the cylinders looked pretty good too. He also had a looksie at the 100 pistons & barrels I got & didn't really want to know about them as they were smoking pretty badly which is why they were replaced with a seibenrock set. Anyway they were high compression performance ones from some flash manufacturer with a raised dome. He showed me how on the flash ones the skirt was narrower than the old stock 65 ones, bloody interesting stuff for a slow learning wannabe mechanic
Ideally all piston tops should be flat and the combustion chamber dome shaped but reality isn't like that. yes the barrell is being shortened for the reasons you say, I will still have to check piston to valve clearance with plasticene or something and maybe cut valve reliefs in the crown (using a drill, supaglue and sand paper) but pretty confident they will not collide.
This ends up with a long thin combustion chamber (like half an orange peel) - not good shape, but it's what you got with old timey two valve engines - to get decent size valves they need to be set widely splayed; to get decent compresion you needed a high dome piston. Detonation then becomes an issue because it's such a shitty shape. (Methanol helps) My old speedway bike motors (J.A.P and Jawa) were like this - then Weslake came along with four little valves set almost parallel and a flat top piston and two valve motors were history (on speedway, anyway)
Excellent lil reply old no.99. So the 4 valves pushed more mixture thru in a smaller area of the combustion chamber & a better detonation happens in a 'squarer' chamber? Why are the valve recesses there on top of the pistons? if the cam etc are timed right there shouldn't be any chance of the valve hitting the piston. Am I right in guessing that the space in the cylinder is limited perhaps due to a short stroke & the further the valve opens the more mixture can pass thru however the only place to gain depth for the valves is the top of the piston? Rotary valves or sliding valves no good? Wouldn't the valve recesses affect the efficiency of the combustion?
If the valves open further then they will allow more air/fuel to enter but piston to valve clearance is harder to achieve so valve recesses and dome shaped pistons need to be used so a decent copression ratio is achieved. The other way is to have a cam with a long duration but again compression ratio is affected because the valve is also closing later therefore there is less volume of fuel/air mixture (e.g.the piston is further up the stroke when the valve closes) to compress to achieve decent compression so generally pistons are domed. Yes there are a lot of compromises in engine design.
Thanks I went & did a bit of research after posting & learnt a lot. benelli content When I first saw that pic I thought it was some kind of dogs face???
Don't panic, haven't lost interest, just a little angry with myself for being a dum ass. Made a new gudgeon bush up yesterday with my new lathe tool set (don't tell Mrs Woodman). After machining up the bush and polishing it all smooth I took it out of the lathe and went to fit it into the small end and that when i realised that I had machined it for clearance instead of interference. Still have some stock bushing left so will try again today, or take the KLR for a hoon. Probly this. Another issue discovered yesterday was that the benelli small end is wider by about 4mm than the distance between the gudgeon bosses on the xr piston. Dremmel time. Padmei? did you have an electric dremmel? Anyway not major issues , just annoying challenges .
Bugger. How much do you allow for interference fit? a thou or same dia? Yep I've got a ryobi dremel machine thing. i'll drop it out to you with your welder - i'm gonna buy my own one.
LOL.. Wasn't hot when yer done the final cut was it.. then shrunk when it cooled.. I have done that before... wonder why the bearing suddenly be come a lose fit..
