classic motorcycle forum

Motorcycle Discussions => British Bikes => Topic started by: andyw1260 on June 02, 2007, 06:23:32 AM

Title: Rough casting finish T120
Post by: andyw1260 on June 02, 2007, 06:23:32 AM

Hi all  could anybody please inform me of the type of finish the crank cases on a T120 1968 should have, should it be the rough finish you get after casting or a smooth semi polished finished.   I have seen both types so I wondered how they left the meriden factory.
Thanks for your  ???time      Andy W    

Title: Re: Rough casting finish T120
Post by: TBS on June 02, 2007, 07:03:59 AM

They are a rough finish, never a smooth semi- polished finish. I have all my cases professionally vapour blasted which brings them back to a perfect as new finish, silver with a slight sheen.

Title: Re: Rough casting finish T120
Post by: andyw1260 on June 02, 2007, 04:49:13 PM

Hello TBS and thanks for your reply, my cases have had that rough surface taken off except next to the raised engine number step so at least I can prove the number is correct.
  Why I raised the issue is that even beneath the engine the rough surface has gone.
  Another point is that my cases show no sign of a join beside the engine number step (just forward of the primary chain oil filler point (which other T120s show also the join in the case 9 inches back of that at the fixing point which is standard still stands out clearly. Somebody had a lot of spare time on there hands years ago.    
  Lovely weather going for a ride today to find a country pub sit out side have a pint and eats and admire the bike how sad is that!!     Andy W  

Title: Re: Rough casting finish T120
Post by: Old-Nail on June 23, 2007, 11:05:07 PM

I read somewhere that the rough casting actually aided engine cooling, whereas a polished surface made the bike run slightly hotter.

This may be an 'urban myth' as I can't recollect where I read it, it may have been Triumph twins by WC Haycraft.

Title: Re: Rough casting finish T120
Post by: 33d6 on June 24, 2007, 08:45:01 AM

You're right. A rough casting finish gives better cooling-as long as it is kept clean.
The rough finish presents a greater surface area for heat exchange with the surrounding air. A polished crankcase has less surface area plus does not radiate heat as effectively.
I think the difference is a bit academic for ordinary road riding unless you commute through Death Valley every day but why go to all that bother of polishing a crankcase when you know it doesn't help?
Cheers, Bob