Topic: how to straighten a bent fin on the cylinders and head?

[Martin Thomassen]

Hi

I got a few bent fins on my cylinders and cylinder head, which annoys me a little, and I would very much like to straighten them out.
I have found this guide on the internet:
http://w6rec.com/duane/bmw/bent_fins/
but I´m not sure if the aluminium on the Honda´s are equally tough? Part from the bent fins, the cylinders and head is pretty fine (nothing broken), and I would of course like them to stay that way.
Anyone in here that has experience with straighten bent fins on their CB 750?

I know the pictures suck, but at the moment I only have my IPAD and some bad ambient light 

My plan is to have the cylinders, head and valve cover blasted and painted, and having the head portet and the cylinders paired with a set of 849 cc pistons. I have both a spare head and a spare cylinder, but the cylinder has several broken fins, so I´ll either use the cylinder on my bike (if the fins can be straighten) or buy a spare cylinder that hasn´t got bent fins 

[skidooextreme]

high heat and patients, increasing pressure. 

[lrutt]

those bends are pretty mild and you should be able to straighten them without breaking, I know I have on other Jap bikes, but not on an SOHC.

[Martin Thomassen]

Hi

Thanks for the replies 
When you say high heat, is a heat gun enough then, or will I need  oxygen/gas (not sure what it is called in English):


Should I use a tire crowbar (or something similar in metal) or will a wooden wedge be a better choice?   

[LesterPiglet]

Too easy to do damage with oxy/acetylene and I think a heat gun wouldn't be enough. One of these would be better.

[Trad]

I had worse bent fins from a #$%*ty machine shop who dropped the cylinder head. I never ended up using that head but I was able to bend the fins back. I honestly didn't even use heat. I just used a wide flathead and increased pressure until they bend back. I'm sure heat would have been better but hey, I never broke any fins and you couldn't even tell where they were bent. Minus the corner that broke right off from the initial drop. 

[Martin Thomassen]

Too easy to do damage with oxy/acetylene and I think a heat gun wouldn't be enough. One of these would be better.

Nice, my brother in law has one of those I can borrow 

What about the last part? Could I use a wooden wedge, and then slowly increase the pressure on the fin by gently forcing it in (that came out wrong) with the tap of a hammer?

[Trad]

Just use a chisel or a flathead. Trust me, they have some give. I wouldn't use a wedge and hammer.

[Don R]

Sheet metal pliers, the wide flat ones. That way you're not prying on something else.

[AJK]

Just use a chisel or a flathead. Trust me, they have some give. I wouldn't use a wedge and hammer.

+1

*Go gently* with some light taps on the peaks, patiently correcting any minor bumps here and there. No heat required, just some gentle/mild tapping.
  Pliers would probably work as well as DonR has mentioned. I think i also used these to some extent.

I have broken fins as well (previous owner), so used jbweld and some popsticks on some minor missing fragments to fill them and shape them before a coat of paint.

[lucky]

Hi

I got a few bent fins on my cylinders and cylinder head, which annoys me a little, and I would very much like to straighten them out.
I have found this guide on the internet:
http://w6rec.com/duane/bmw/bent_fins/
but I´m not sure if the aluminium on the Honda´s are equally tough? Part from the bent fins, the cylinders and head is pretty fine (nothing broken), and I would of course like them to stay that way.
Anyone in here that has experience with straighten bent fins on their CB 750?

I know the pictures suck, but at the moment I only have my IPAD and some bad ambient light 

My plan is to have the cylinders, head and valve cover blasted and painted, and having the head portet and the cylinders paired with a set of 849 cc pistons. I have both a spare head and a spare cylinder, but the cylinder has several broken fins, so I´ll either use the cylinder on my bike (if the fins can be straighten) or buy a spare cylinder that hasn´t got bent fins 

When silicon is added to aluminum it makes the expansion rate much less.
But it also makes the aluminum more brittle.
A piston is the hardest and has about 22% silicon.

Do not confuse silicon with silicone.

If you have a bent fin you may be able to bend it if it is heated slightly.
But I would just leave it alone or paint the cylinders black and then it will not be noticeable.