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General => Douglas Motorcycles - General Discussion => Topic started by: NeilR on 17 Feb 2013 at 17:24

Title: Specialoid Pistons in MkV
Post by: NeilR on 17 Feb 2013 at 17:24
The MkV engine I have started to rebuild had a pair of Specialoid pistons fitted. There's quite a lot of surface marking on the crowns - probably down to careles decoking, but it looks like the numbers are M-37837, and LL-1. Were these  standard fitment? The cylinder bores measure up standard (at the top), so I was wondering why they might have been fitted otherwise.
They are light at 168g. The JP+30 ones I have are 272g - seems a lot extra for 30 thou. I'm thinking they might have been fitted to wring a bit more power out?

Cheers,

Neil
Title: Re: Specialoid Pistons in MkV
Post by: tck on 18 Feb 2013 at 17:28
If they are really Specaliod then they are light . When fitted to Vincents, Specaloids were lots lighter than the Hepolite alternative. Alas I belive Specaloids have long ceased to be available for anything.  :cry:If they were available and still being manufactured then the Vincent owners would have tracked them down. :lol:
Title: Re: Specialoid Pistons in MkV
Post by: eddie on 18 Feb 2013 at 18:39
Hi Neil,
          The Specialoid pistons were specified for the 'Plus' machines. According to the provisional handbook for the 'Plus', the pistons for the '80' were 'Specialoid Lo ex', and for the '90' 'Specialoid Y Alloy'.
      I have in the past measured standard Mark type pistons and found that they are oval turned, being about 4thou smaller across the gudgeon pin than across the thrust faces, whereas the Specialoid pistons for the 'Plus' were round.
   Hope this helps,
                            Regards,
                                          Eddie.
Title: Re: Specialoid Pistons in MkV
Post by: tck on 19 Feb 2013 at 08:48
Continuing the Specialoid theme they were low expansion. On the Vincent when specialoid stopped production Ron Kemp had a batch made he called 'kempaloid' and they were low expansion as well.
I have had a high comp racing Vincent piston made in low expansion alloy by the USA firm Venolia they make one offs to your pattern and spec (I sent them an old piston with the crown made up in plastecine and bore size of a new barrel) the piston cost 120 (for comparison a stock piston costs 95) so I guess a set of smaller Douglas pistons would be less and if a batch was made up......
Title: Re: Specialoid Pistons in MkV
Post by: NeilR on 21 Feb 2013 at 11:13
Thanks for posting this tck, very interesting. I'll look into it.

Cheers,

Neil