Tuesday, 24 January 2012
ARSE FIRST. REVERSE HEAD TRIUMPHS
If you Google reverse head Triumphs you will come up with various internet experts spouting their knowledge on forums and telling you that it doesn't work, they run hot, they burn exhaust valves out, you cant ride in the wet etc etc. I have built and ridden two without problems so make your own choice of who to believe. Sorry if that sounds a bit harsh but people whose only mechanical knowledge comes from sitting in front of a screen then claim to be an expert piss me off.
There is no logical reason for doing it but the general opinion is that drag racers did it first to get a kind of ram effect for the carbs although when they continued to build them reversed with blowers it was more a case of there being more room at the front of the engine for the supercharger and the motor could stay as far back as possible in the frame to aid weight transfer. Whatever they look the tits and that's all we are bothered about here.
I quite often get asked how to do it so here are the basics. before you start you have to get things straight in your mind as to what is going to happen, the crank will rotate as normal and the pistons will go up and down the same as before, they do not know you have altered the flow of the mixture into and out of the combustion chambers.
First off you have to consider the camshafts, on most early engines they are the same profile and are also symmetrical so can be left as they are. If you have a unit motor with points or an early pre-unit with distributor you may have to think it out a bit further, basically the cam at the exhaust side will now be your inlet and vice-verse. I have plenty of info on all Triumph cams so if you are in doubt get in touch.
By far the easiest motor to use is a pre-unit 8 stud with magneto ignition. The ignition has a wasted spark so no problems there, the piston valve cut outs are same back and front and barrels are also symmetrical so can be left as is. Just take the head off and put it on backwards. If you have a 9 stud set up you will need to turn the barrels round also, possibly the pistons as well if they have larger cutouts for the inlet valves.
Next job will be to re-time the cams, as they run at half the crank speed you need to dial them in at 90 degrees from the original marks. This is a pretty basic statement as it will depend on the cams you are using but with mild (say standard Thunderbird) cams it will get you going. One problem you will find straight away is that the number of teeth on the cam wheels is not divisible by 4 so unless you start filing or using offset key ways you cannot get the timing to be exactly 90 degrees from stock. In practice i timed both engines (a 500 unit and a 650 pre-unit) to run with half a tooth of advance on each cam, they both ran fine so i left it at that.
Exhausts are dead easy to make as there are almost no bends. Carburetion is trial and error, on the 500 i had the standard manifold and Concentric which luckily fitted between the frame tubes, on the gold bike i made up an angled manifold with a Monobloc which was probably more useable, it ran without problems even in pouring rain. The only issue was condensation and icing on the longer inlet.
The only real problems you will have are from people who havent a bloody clue what has gone on ! Messing with heads will mess with heads, the ones who think they understand will want to know how you have altered the gearbox to stop the bike running backwards...
If you have ever done one send me a photo and some info and if you want to have a go get in touch at firstname.lastname@example.org and i will help if i can although i am not a mechanic so no hard questions !