Hitchcocks Forum

I've noticed that my bike get rather buzzy above 4250rpm, but I'm planning on adding the hotter cams and valves to the engine to compliment the open pipe, filter, and PCV I've already fitted. I'm a little worried about using that power, since it's made at and above 5,000rpm. Having had a piston fail and let go of the piston pin previously on another bike, do you think the stock piston and rod will happily handle the increase in rpm and hence increased forces? Also, is the vibration a sign that the stock piston weight is too much for those higher rpm? Are there any options for a lighter 535 piston?

Thanks for your advice,
Gavin.

Hey mate. i also have added the open exhaust, k and n air filter and power commander 5 from hitchcocks. but the internals of this cafe racer need a major overhaul. lighter and higher compression pistons are a must have. as for the engine, i think it will be just fine if u plan to add hotter cams and valves to it as the engine is running at really a lower state of tune the internals can take a lot more. and hitchcocks guys can u guys please make lighter and higher compression piston for the conti GT. its a must do mod in this bike. guys we r waiting. i am sure there will be many customers for that piston. i am a group member of a conti gt club called chd cafe racers [ check us out on facebook]. and we all are already looking for options for a lighter and more higher compression piston. if hitchcocks make one for the gt there will be amny takers of it. i would want one for sure

A lighter piston will not itself reduce vibrations and may make the problem very much worse. Vibrations may be due to the crankshaft not having been balanced accurately (whether the engine uses lighter or heavier pistons). But vibration may also be due to any number of other causes, like a less than perfectly true crankshaft. Added to which a ‘big’ single will always vibrate at certain frequencies. These vibrations may be reduced by the use of a balance shaft to counter the forces that contribute to vibration, a sensible devise that Enfield chose not to use

I must concur with Presto. The laws of physics mean you can't fully balance a single without using balancer shafts, so they will always vibrate. However, the manufacturers of singles do make the crankshaft weight lopsided in order to partially balance the conrod and piston. Balancing a single means choosing between strong horizontal vibrations, strong vertical vibrations, or an elipsoid vibration mode at some angle between horizontal and vertical. It's called the "balance factor" and choosing it is, to be honest, a bit of a dark art. I think a lot depends on the compliance of the particular frame it's mounted in.



Fitting a lighter piston will alter the balance factor and MAY make the vibration feel WORSE. Of course, it may well improve it, too, because you've got less reciprocating mass. Certainly a lower mass piston will put less strain on the reciprocating parts, which must be good.



In my experience of RE the vibration varies enormously between apparently identical bikes. Mine changed radically just be removing and refitting the exhaust system - I must have altered the pre-load on the brackets and the coupling between the exhaust and the frame in some way. It is all tremendously difficult to predict.

Balance factor on Enfield singles is between 63% - 65%. With the heavy flywheels used in the Enfield engines there is little need to worry about rebalancing the cranks when fitting alternative pistons or over-size pistons or even alternative con-rods.

I had the crank balanced on my 500 iron before 25 mins ride and my fingers were numb after balancing to 63% sweet as have done 300 to 400k trips no sweat went to 535 with one of burtons pistons balanced again after a good few years went to omega 8.5 piston it was 25 grams heavier so back to the shop.
I run a 19 tooth on the gearbox drops the revs on the motorway little vibe period as you go through 110klm then pretty good up to 130. Trick is to get any vibration outside the speed you normally run at just a matter of suck it and see. Chap I use has a balancing machine does each flywheel on its own.

Thanks for all your feedback guys. I also quizzed Hitchcock's themselves on this, and their reply was consistent and helpful:

"Vibration on the GT 535’s and indeed on the other models too, is down to crankshaft balance and varies from bike to bike depending on how well the crank was balanced at the factory. Piston crown was a problem with iron barrel Bullets a while back and this has been rectified and the EFI pistons are a far better quality. Redditch bikes were balanced to a max runout to one thousenth of an inch, Indian cranks can runout by as much as 6 thou. This is why some bikes vibrate more than others. The Royal Enfield engine is a very long stroke unit which possibly should of been fitted with a balance shaft to alleviate vibration, so vibration is not a piston weight problem but more an inherent feature of the design.
[The tuned engines] are not revving that much higher [than stock] which they will cope with easily. The bulk of the power is generated in the existing rev range."

I also noticed upon further though that my bike's vibrations are strongest at about 1,500rpm, with another vibration area around 3,000rpm [stock mirrors a blur], and it’s getting buzzy again towards 4,500rpm. Each time the amplitude is less. It now seems obvious that I should be fine in the region 5,000-5,500rpm, only meeting problems again at 6,000rpm I predict.

All in all I feel more confident know. I will continue my weight loss program (both the bike and myself!) and will try saving for the cams and valves.

More mass at the end of the handlebars (weights or mirrors) is an easy method of dampening down vibration.
I opted for the cheap and invisible £4.99 option which was to pump a tube of metal neutral lead flashing sealant down inside the handlebars. Certainly reduced vibration to the point where you can make out the make and type of vehicle behind you instead of a coloured blob. I guess time will tell how non reactive it is by the length of time it takes for the bars to fall apart!