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Author Topic: how to straighten a bent fin on the cylinders and head?  (Read 5703 times)

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Offline Martin Thomassen

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how to straighten a bent fin on the cylinders and head?
« on: February 17, 2014, 12:47:27 AM »
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  :D
« Last Edit: February 17, 2014, 01:07:16 AM by Martin Thomasen »
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Offline skidooextreme

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #1 on: February 17, 2014, 03:45:12 AM »
high heat and patients, increasing pressure. 
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Offline lrutt

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #2 on: February 17, 2014, 06:29:41 AM »
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.
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Offline Martin Thomassen

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #3 on: February 17, 2014, 12:13:51 PM »
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?   
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Offline LesterPiglet

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #4 on: February 17, 2014, 12:38:07 PM »
Too easy to do damage with oxy/acetylene and I think a heat gun wouldn't be enough. One of these would be better.

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Offline Trad

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #5 on: February 17, 2014, 12:49:23 PM »
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.  :P
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Offline Martin Thomassen

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #6 on: February 17, 2014, 12:52:06 PM »
Quote from: LesterPiglet on February 17, 2014, 12:38:07 PM
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  :D

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?
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Offline Trad

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #7 on: February 17, 2014, 12:57:16 PM »
Just use a chisel or a flathead. Trust me, they have some give. I wouldn't use a wedge and hammer.
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Offline Don R

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #8 on: February 17, 2014, 02:38:58 PM »
Sheet metal pliers, the wide flat ones. That way you're not prying on something else.
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Re: how to straighten a bent fin on the cylinders and head?
« Reply #9 on: February 17, 2014, 04:36:33 PM »
Quote from: Trad on February 17, 2014, 12:57:16 PM
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.
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Offline lucky

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Re: how to straighten a bent fin on the cylinders and head?
« Reply #10 on: February 18, 2014, 09:22:47 AM »
Quote from: Martin Thomasen on February 17, 2014, 12:47:27 AM
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  :D


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.
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