Heat Dispersal

[pitley]

Howdo.

I am in the process of doing a motor rebuild on my majesty, whilst its in bits i wonderd about getting the head n barrel vapour blasted to get the old paint off.

Question is do i leave it just plain alloy or do i get it sprayed up?

If its left plain alloy will the motor have same ablity for the heat to disperse as it was when painted?

Any one had their 250 Majesty ported to get a little more power output?

Was going to try birkett motor sport, word is they can improve it a tad.

Any help appreciated

Edited November 17, 2006 by pitley

[woody]

Not sure personally whether it will make any difference to heat dispersion whether you paint it or not. I only say this because there are many bikes at trials with unpainted barrell/head that don't suffer from overheating - ie; all the Greeves/James etc using Villiers square barrells.

Cosmetically, vapour blasting leaves a nicer finish than bead/sand as it polishes as well as blasts so you get a shiny (ish) finish. I had a Bultaco barrell/head done locally a couple of years ago and it was an impressive finish. Costs a bit more than just blasting. To keep it looking that way though is always going to be difficult as the mud will bake on during the event and probably stain the alloy, as it does paint - easier to clean off from the paint than bare alloy? So if you're going to paint it you may as well just have it blasted as the cheaper option, unless there is no price difference between the two where you are. Thing with sand/bead blasting is you can normally find someone who will do it in their 'lunchtime' for a few quid. Vapour blasters are thin on the ground.

I think for porting on the 250, Birkett is the only option with the spec and previous experience to do it. Shirty is the only other option but they've no interest in Yams now I would guess. Funnily enough I was chatting to Mick Thurman last week at the Classic Experts and he said his mate has a 250 done by Birkett and it is a nice bike. Still think the best option is the softly tuned 320 though.

[timp]

What is vapour blasting?

[woody]

It's still very fine bead blasting but in a jet/spray of high pressure water/steam so as well as stipping the crud off it mildly polishes as well, leaving a shiny finish as opposed to the dull finish of normal blasting. If you google it you'll get a better explaination than that though......

Works well on engine cases/cylinders on stuff like 70s Jap road bikes such as Z900 to restore the casings to original type finish - shiny but not mirror polished

[andy m]

Fins and crankcases painted flat black radiate heat considerably better than silver shiny surfaces. Remember also that heat radiation is retarded by the presence of mud. Low speed engines lose much more heat energy than say race engines because each combustion cycle is longer. It has also been found that blasting the head, barrel and cases with aluminium oxide will increase the surface area about 5 times. They will need a Kal guard coating and oven baking to enable to keep the aluminium oxide on the surface, operating temp will then be reduced by around 10- 13 % . Thanks

[pitley]

Christ Bo!

Reckon you been digging your old books out of the roof space to come up with that answer, but thanks all the same mate.

Woody how does the 320 compare to a standard Pinky, as i have not had the chance to try the 320.

I too spoke to Mick at Rhayader, his words 320 better in muddy stuff as you can pull bigger gears, but he said his mates ported 250 was better on the rocks.

Probably stick to the 250 ported verson as it suits my ability

Edited November 18, 2006 by pitley

[atomant]

Fins and crankcases painted flat black radiate heat considerably better than silver shiny surfaces.

Black surfaces will radiate heat faster than a shiny surface but this can have a negative effect as black absorbs heat fast too! so therefore if the fins are in close proximity then painting black wont help much especially when the bike is stationary with no air flow between the fins to dissipate the heat via convection.

Low speed engines lose much more heat energy than say race engines because each combustion cycle is longer.

Can you explain this in more detail please? Are you saying because of the longer stroke the frictional characteristics create more heat?

It has also been found that blasting the head, barrel and cases with aluminium oxide will increase the surface area about 5 times.

I am interested in this info for another project. Can you point me in the direction of the technical data on it please?

[andy m]

All info can be found in a book; Performance Tuning in Theory and Practice, 2 strokes. A. Graham Bell. First published in 1983, which is around the era of Pitleys Majesty. Obviously things have moved on since then. Basically a trials engine cycle runs a lot slower than a race engine TZ or WHY, not all but most of the time. Thanks

PS. There is a copy on Ebay, finishes tomorrow, bargain.

Edited November 18, 2006 by bo drinker

[atomant]

Thanks for the info

[bob]

A while ago there was someone who had done a test using an IR thermometer to test the temp of modern and older air cooled bikes during a trial. If I remember rightly, the modern bikes ran hotter, but at a more consistent temperature.

What I'm trying to say, is that if this is true then aircooling is somewhat too effective on trials bikes, perhaps because they put plenty of big fins in just in case the bike never gets any good air flow round it; so you should be fine whatever, even if it does run sightly hotter then it probably won't matter too much. I have seen plenty of aircooled engines both painted and unpainter run fine for long periods.

Bob

[woody]

Woody how does the 320 compare to a standard Pinky, as i have not had the chance to try the 320.

I too spoke to Mick at Rhayader, his words 320 better in muddy stuff as you can pull bigger gears, but he said his mates ported 250 was better on the rocks.

Probably stick to the 250 ported verson as it suits my ability

Difficult to say how it compares with a standard Pinky to be honest. The mono engine should theoretically be better all round than the old TY motor due to it's more modern design. Certainly holds true when compared to a 250 TY/Majesty but when the old 'un is converted to 320 it transforms it. People say they're too powerful but it depends how it's set up. It's possible to get it to run pretty much how you want. When I bought mine it was a snappy power delivery, very sharp and a wheelspinning nightmare. I've softened it right off and I run with no tickover so I can shut it off to an absolute crawl without the clutch. With the flywheel weight fitted it's amazing how slow it will go without stalling (1st 2nd or 3rd). Timing is retarded, bigger exhaust and airbox, all help to soften the delivery. It's quite docile but very torquey and the power is there when you want it. Grips well and no problems with it on rocks. In fact, you can pick your way up a rocky stream quite nicely on the throttle without having to use the clutch - one less thing to concntrate on - throttle/clutch coordination - which is nice.....

If there are any trials down your way over December period that I can get too you should have a try on it. You probably won't like the bike's set up but it will give you an idea of how soft a 320 can be

[pitley]

Woody

Thanks for the offer to try your yam, will be doing next rhayader trial on 26th, and most events in the south west midlands, and mid wales between now and the new year.