Topic: Broken/Mashed Fin Fix Recommendations

[TheGreek]

Hello everyone and happy Sunday. I have a mashed fin on the right side of the head that was damaged when I purchased the bike. What is everyone’s recommendations to fix. I originally planned on sending it out for a full rebuild/restoration but rethinking that after looking at shipping costs and not sure if someone in Oregon can do the job.

[bryanj]

WHAT BIKE

[754]

 CB 750  it's a toss up, if you can repair on bike for reasonable, it's worth it but heads are around 100 bux or maybe less

[maxheadflow]

You could try and straighten the fin with something like this then file the fins flat.

https://www.amazon.com/IRWIN-VISE-GRIP-Original-Locking-23/dp/B0000TFGFU/ref=asc_df_B0000TFGFU/?tag=hyprod-20&linkCode=df0&hvadid=198070022856&hvpos=1o4&hvnetw=g&hvrand=17205247538821484207&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9031309&hvtargid=pla-381941437341&psc=1

If it breaks off, a good welder can build the fin back up.

The main issue should be the condition of the head.  If it's good, it might be worthwhile to fix. Worn out? Find another.

[scottly]

It's only cosmetic; if the bike is functioning well, don't rebuild the engine just to fix the fin.

[jaytee-nz]

I've straightened fins before using pliers etc. If you add some heat (not too much) and are patient, you can work the fin slowly back in to place and get reasonable results. Going slow is the key.
Use a small flat file to tidy up the edges.

[disco]

I've straightened fins before using pliers etc. If you add some heat (not too much) and are patient, you can work the fin slowly back in to place and get reasonable results. Going slow is the key.
Use a small flat file to tidy up the edges.

+1 to this.

[calj737]

Use MAPP gas to heat the fin. Then gently pressure to bend it. Whatever you use to "grip" the fin with will leave marks in the surface. If you get the aluminum hot enough to move, it will be soft enough to be scarred.

It would be possible to repair that single area while on the bike, though not as easy. But it is doable. If you find a competent welder, they will TIG weld it. Suggest to them they use 4047 filler, not 4043. -47 melts at a lower temp, works a bit better on cast aluminum, and should be an easier repair.

Afterwards, some sanding and scuffing and the entire repair might simply disappear.

[Redline it]

it gives it character, it looks alright the way it is. they might only bend 1 time.

[TheGreek]

it gives it character, it looks alright the way it is. they might only bend 1 time.

Hahaha yes I love it. Unfortunately the “character” I’m looking for was well established in 1971 which didn’t include a broken fin.

[754]

The last pic shows correct head, yours is 73 to 76 head..

[BenelliSEI]

Work it after the bike gets a hot run. I’ve had good results with wide  nose pliers or Channell Locks. The ones with a nose like a hammer head shark. Brew up some JB Weld with enough of the hardener for a decent colour match and fill any cracks. After it cools and hardens, small file.

I’ve also managed to fill in a few decent size gaps (chips missing from fins) with JBWeld. Built up slowly and patiently (winter project), good results.

[Don R]

 I've had luck with the pliers listed above. I've also used heat with caution. Go slow, some fins are brittle and will crack, some are softer and will form right back to shape. I haven't learned how to see the difference.

[Don R]

 If it breaks you can have someone weld the edge of the fin with a tig. Stack beads until it's built back up then carefully sand and file until it matches. I've had good luck with a muffler cut off disc on an angle air tool, then a sanding disc and a thin flat file. I have a diecast head in the shop waiting for me to grind it now.