C90 into st70

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Sparky_col
Posts: 5
Joined: Tue May 14, 2019 5:27 pm
Rides:: St70

C90 into st70

Post by Sparky_col » Thu May 23, 2019 8:31 pm

Hi all,
Anyone advise on any problems doing the above, its a 6v c90 engine.
Many thanks.

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wightegi
Posts: 7626
Joined: Tue Sep 01, 2009 7:23 pm
Rides:: 12v 1988 e start C90, Gl5oo silverwing ,suzuki GS500F
Location: Isle of Wight

Re: C90 into st70

Post by wightegi » Thu May 23, 2019 8:38 pm

50 and 70 engines are the sane size as the 12 volt 90 , the 6 volt 90 will not fit in these frames , the big valve C70 bike is faster than the 6 volt 90 one ;)

Sparky_col
Posts: 5
Joined: Tue May 14, 2019 5:27 pm
Rides:: St70

Re: C90 into st70

Post by Sparky_col » Thu May 23, 2019 9:16 pm

Thanks, next question is where can i get a big valve head, anyone know?
Thanks again.

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steveST100
Posts: 1696
Joined: Tue Oct 25, 2011 3:04 pm
Rides:: 1983 C50LAC 1976 ST100 1980 ST70
Location: Clitheroe Lancs

Re: C90 into st70

Post by steveST100 » Thu May 23, 2019 10:16 pm

Yes it can be done.
Lot of work and some cutting of the frame to fit it.
Will be a pain to do the wiring.

Image

Image

Stick to the 6v 50/70 and 12v 50/70/90/110/125 for it.

steve g

Sparky_col
Posts: 5
Joined: Tue May 14, 2019 5:27 pm
Rides:: St70

Re: C90 into st70

Post by Sparky_col » Fri May 24, 2019 7:44 pm

Thanks for that info, i want to keep my bike as original as possible so i really dont want to cut the frame, i will hunt for a c70 head, i understand its pre 79 bikes that had them, any pointers towards one would be gratefully received.

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steveST100
Posts: 1696
Joined: Tue Oct 25, 2011 3:04 pm
Rides:: 1983 C50LAC 1976 ST100 1980 ST70
Location: Clitheroe Lancs

Re: C90 into st70

Post by steveST100 » Fri May 24, 2019 8:17 pm

Bit i did in the guides on the big valve heads.

viewtopic.php?f=43&t=25040

steve g

Barnsey
Posts: 92
Joined: Fri Aug 11, 2017 12:31 pm
Rides:: Honda C50 BMW R100
Location: Portsmouth, Hampshire

Re: C90 into st70

Post by Barnsey » Thu Jun 06, 2019 2:09 pm

Check this out:

Ebay part number 322387613092 .

I ordered one of these, and I was relieved that it bolted straight onto my 1973 C50 engine. I just can't say how well it runs as I haven't started it yet.

Regards,

Ian

Sparky_col
Posts: 5
Joined: Tue May 14, 2019 5:27 pm
Rides:: St70

Re: C90 into st70

Post by Sparky_col » Tue Jun 25, 2019 9:36 pm

Steve g, thanks for the big valve link, can you tell me if all big valve heads are bi metal, its just i looked at my spare c70 motor, its an early one with a downdraught carb, the head is not bi metal but the valves are marked with 7's not 7's in circles, the top of the inlet valve measures 26.45mm, thats accross the top in situ, so is it a big valve one?
Many thanks.

Sparky_col
Posts: 5
Joined: Tue May 14, 2019 5:27 pm
Rides:: St70

Re: C90 into st70

Post by Sparky_col » Tue Jun 25, 2019 10:00 pm

Apologies, they are 25 and 22mm not as i said before.

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steveST100
Posts: 1696
Joined: Tue Oct 25, 2011 3:04 pm
Rides:: 1983 C50LAC 1976 ST100 1980 ST70
Location: Clitheroe Lancs

Re: C90 into st70

Post by steveST100 » Thu Jun 27, 2019 9:12 pm

Most but not all the bi-metal big valve 70 heads i have had were from the early downdraft carb models. Think some may have been swapped for later 6v heads or ones from the ATC70 engines. Have had a lot of good fast engine builds using the smaller of the 2 big valve heads,work best with a good fast road cam. One non bi-metal head i had that took the bigger valves was from a CT70H engine.

steve g

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