Drilling lightening holes in cylinder barrels

Status
Not open for further replies.
'Because I can' is never a good reason for doing anything. Some people believe that drilling holes in everything makes a motorcycle look like a racer. But I would say this - in a commando engine, the hottest part of the barrels is probably the part in between the two cylinders, how much heat is conducted away elsewhere, is probably irrelevant. If I wanted more weight saving, I would fit aluminium barrels. But because I always use methanol fuel, I won't. Getting enough heat into the motor prior to racing would always be a problem. Even when using petrol as fuel, you need to warm the engine before thrashing it. I think those holes would simply cause hot spots - uneven heating. The standard barrels seem to work OK - drilling holes in the fins might create an unknown.
I would never drill a disc rotor - that is how my friend was killed at Bathurst - cast iron disc with holes - the discs exploded off his RG500 Suzuki at the end of Con-Rod straight. He ended his life among the spectators.
 
Eddie
When you were a kid and you saw a Wasps nest did you throw stones at it?
Or drill holes in it?
 
How often have you seen guys walk up to a motorcycle which has a stone cold motor, kick it into life then rev it up straight away without even letting it idle for a few minutes ? When I have my 850 at a race meeting, it usually sits idling for quite a while before I ride it. Heat build-up is something which makes motors slower, but running a cold motor hard is not good for pistons, especially when using methanol fuel. My motor is on standard comp. and loves methanol. I would never race using petrol, simply because of the heat build-up. Even with methanol, - on a hot day the motor probably gets too hot. Quite often, if you look closely at a worn barrel, you can see high spots in the cylinders where the threads which hold the ends of the head studs are. I would not drill holes in the fins.
 
After discovering what poking holes with a stick earned me longer sticks or stones, when available, and fast sprinting were utilized from then on.
 
Some more info on this topic here:

www.accessnorton.com

Holey Cylinders, Batman!

I've seen several Commando cylinders that have had the fins drilled to lighten them, but never knew how much weight could be saved. I was thinking about using a set of iron 850 cylinders for my short stroke 750 build, and decided to find out if drilling was worthwhile. I drilled the fins from...
www.accessnorton.com www.accessnorton.com

Ken
 
TomU said:
May as well cut off the fins entirely :p

The fins are there to promote heat transfer away from the barrels. How is heat supposed to transfer if there's no surface for the heat to transfer to?
Click to expand...
As posted by Davamb in the thread Ken posted a link to:


For what it's worth - little bit of math:
surface area lost = 2 * pi * d^2, surface area gained = t * pi * d
where d is the diameter of the hole and t is the thickness of the fin.
For an improvement in passive convection cooling, the area gained shall be greater than the area lost:
t * pi * d > 2 * pi * d^2
or
t > 2d
or
d < t / 2
i.e. you will increase the overall surface area available for cooling as long as the diameter of the hole is less than half the thickness of the fin.
 
Talking about holes in the cilinder, it makes sense to enlarge these holes between the cilinders somewhat, as they easily clog up.

Drilling lightening holes in cylinder barrels
 
Last edited:
Whole fins will better conduct heat away from the cylinder. To what degree (!) the increased surface area of drilled fins offsets that effect, I’m not clever enough to work out.

Tell you this though- if your engine is overheating, those holes won’t fix it!
 
acotrel said:
How often have you seen guys walk up to a motorcycle which has a stone cold motor, kick it into life then rev it up straight away without even letting it idle for a few minutes
Click to expand...

Riding on the road is different. Start- and ride off immediately, at sensible speed.
 
  • Like
Reactions: baz
Fast Eddie said:
As posted by Davamb in the thread Ken posted a link to:


For what it's worth - little bit of math:
surface area lost = 2 * pi * d^2, surface area gained = t * pi * d
where d is the diameter of the hole and t is the thickness of the fin.
For an improvement in passive convection cooling, the area gained shall be greater than the area lost:
t * pi * d > 2 * pi * d^2
or
t > 2d
or
d < t / 2
i.e. you will increase the overall surface area available for cooling as long as the diameter of the hole is less than half the thickness of the fin.
Click to expand...
I believe what you are saying is that you are gaining vertical surface are with the holes. So the math is what you loose in horizontal area vs. what you gain in vertical area.

That's somewhat simplistic because you need to take into account the airflow which is the medium the heat from the fins is transferring to. You start getting into triple integers doing that math. :cool:
 
Don’t forget the fins lose heat by radiation as well as conduction to air.

If radiating surfaces are very close together, such as the sides of a very small hole, then they radiate heat into each other, which can’t help.
 
Triton Thrasher said:
Don’t forget the fins lose heat by radiation as well as conduction to air.

If radiating surfaces are very close together, such as the sides of a very small hole, then they radiate heat into each other, which can’t help.
Click to expand...
So one big hole beats many small ones :-)
 
As a previous poster stated:
"If you really want good heat transfer and low weight, you need to use aluminum."

I'm dubious of the claimed benefit in cooling or weight reduction, but sure looks bitchin'
But if you have sacrificial iron cylinder barrels, and looking for justification for owning a drill press, then have a go at it.
For sure those barrels will be a conversation piece where ever they are noticed.
I'd love to watch a video of the whole process.
 
TomU said:
That's somewhat simplistic because you need to take into account the airflow which is the medium the heat from the fins is transferring to. You start getting into triple integers doing that math. :cool:
Click to expand...

Which, in essence, was the basis of my opening question...!
 
Before the fins can release heat first it needs to get into them. I suspect that the drilling would reduce the ability of the fins to conduct heat away from the source, and reduce the quantity of heat which the fins could remove/hold, so that whether they then are better worse at releasing it would be irrelevent. Just my seat of the pants 5c worth.
 
ntst8 said:
Before the fins can release heat first it needs to get into them. I suspect that the drilling would reduce the ability of the fins to conduct heat away from the source, and reduce the quantity of heat which the fins could remove/hold, so that whether they then are better worse at releasing it would be irrelevent. Just my seat of the pants 5c worth.
Click to expand...
That's kind of my point. And aluminum transfers heat significantly better than steel (but not as good as copper)

www.metalsupermarkets.com

Which Metals Conduct Heat Best? - Metal Supermarkets

Metals conduct heat, called thermal conductivity. It is important to consider in applications with high temperatures. But which metals conduct heat best?
www.metalsupermarkets.com www.metalsupermarkets.com
 
Status
Not open for further replies.

Similar threads

Peter R
  • Locked
Cooling holes in Cylinder casting
Replies
4
Views
1,056
robs ss
robs ss
Back
Top