Author Topic: 550/591 750 pistons  (Read 3421 times)

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Offline dave500

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550/591 750 pistons
« on: December 26, 2011, 01:39:15 AM »
i dont visit here much but i thought this might be the best place to ask,ive searched through all the gentlemans express threads but im still left unclear as to what i have,,its a 550 barrel bored to 61mm,it was assembled by a chimpanzee at some point i think,the pistons wont make it into the bores under gravity,although it was forced together without rings,a few remnants in grooves and one or two half bits of ring in the bottom end,,and some frags embedded in the barrel and case gasket area!,i think ill be able to rescue this top end unit though,the 61mm piston is .65mm taller after the wristpin than a 550 piston,i can see the slightest valve touch on the crown marks,,whats the common method of using 750 pistons in this set up?do you buzz that .65mm off the top(too easy),the longer skirt hasnt fouled the crank,i think someone put this together and it was just bad or something???,there dosent seem to be much wear on the pistons but they had the brown from use look underneath.some photos.
            piston on the left is brand new .25mm oversize 550,i get .65mm here.

     even the smallest piston wont drop through

    no marks show the front

   bk in a circle
      "cs" with the s inside the c

« Last Edit: December 26, 2011, 01:43:23 AM by dave500 »

Offline bwaller

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Re: 550/591 750 pistons
« Reply #1 on: December 26, 2011, 05:25:16 AM »
Chimpanzee is right, man wonder what they thought might happen. Can't help with piston maker. Cometic makes several thickness normal base gaskets. In their viton coated steel base gasket option I believe they offer 0.010, 0.014, 0.020, and 0.031". If you hone to match pistons, then a different base gasket might help.


Offline Big Jay

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Re: 550/591 750 pistons
« Reply #2 on: December 26, 2011, 10:31:08 AM »
Valve pockets would be wrong.

Offline dave500

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Re: 550/591 750 pistons
« Reply #3 on: December 26, 2011, 12:41:47 PM »
thanks fellas,they might have been used pistons from a 750?thats why they had the brown underneath?i cant see how this ever ran?i didnt turn the engine over it was too tight so i dont know if the crown fouls the head,the main bearings are too tight aswell,i opened it up to remove both the chains and just closed it up lightly as a spare,the crank still cant be turned easy by hand,the head and transmission are very good though.

Offline sohc boy

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Re: 550/591 750 pistons
« Reply #4 on: December 30, 2011, 09:19:06 PM »
dave 500,  ive seen pistons with those markings out of eng's yrs ago ,think they a chinese repo ,got thing in my head Bert Kingston in Bris'y used to sell stuff like that ,(still does ) may explain the circled BK(possibly had em cast up himself ?) what size gudgeon pin in 500,same as 750 ?...750 has 15mm ..... 
i live by the voice inside my head

Offline Retro Rocket

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Re: 550/591 750 pistons
« Reply #5 on: December 30, 2011, 09:34:05 PM »
dave 500,  ive seen pistons with those markings out of eng's yrs ago ,think they a chinese repo ,got thing in my head Bert Kingston in Bris'y used to sell stuff like that ,(still does ) may explain the circled BK(possibly had em cast up himself ?) what size gudgeon pin in 500,same as 750 ?...750 has 15mm .....

Dave, Bert Kingston has a shop 5 minutes from my house if you would like me to get some info for you....
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline dave500

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Re: 550/591 750 pistons
« Reply #6 on: December 30, 2011, 10:28:03 PM »
thanks sohcboy and mick,ill send bert an email,hes always prompt with his replies,these have the 15mm gudgeon pin,happy new year!i still cant beleive how someone forced these slugs into the bores?

Offline dave500

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Re: 550/591 750 pistons
« Reply #7 on: January 01, 2012, 09:41:00 PM »
and heres berts promptly emailed reply,hes a good bloke ive never met him only spoken on the phone a few times and emails.


G'day Dave,
 
It is one of our's with "BK" inside the skirt. They are in fact Honda CB/XL125 to 150cc
Someone has done their homework to figure out that these work as a big bore for the 550.
 
I do have rings 4 @ $45.20 each set and head gasket $41.47 both prices plus GST
 
Happy New Year to you also.
 
Regards,
Bert K.

Offline Retro Rocket

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Re: 550/591 750 pistons
« Reply #8 on: January 01, 2012, 09:43:25 PM »
and heres berts promptly emailed reply,hes a good bloke ive never met him only spoken on the phone a few times and emails.


G'day Dave,
 
It is one of our's with "BK" inside the skirt. They are in fact Honda CB/XL125 to 150cc
Someone has done their homework to figure out that these work as a big bore for the 550.
 
I do have rings 4 @ $45.20 each set and head gasket $41.47 both prices plus GST
 
Happy New Year to you also.
 
Regards,
Bert K.

Cool, so its full steam ahead on the big bore build then...... ;D
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline dave500

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Re: 550/591 750 pistons
« Reply #9 on: January 01, 2012, 10:58:51 PM »
ive put it in just bikes classifieds,i have four new first over size 550 piston sets and a complete running but getting smokey spare engine,plus i have gaskets etc for it,so its a hundred bucks for any one who sees it here,the block has been sand blasted and painted gloss black now.

Offline sohc boy

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Re: 550/591 750 pistons
« Reply #10 on: January 02, 2012, 02:19:22 AM »
AHHHHH, so my memory isn't all that mush up after all ,old Bert still there ?, sheees .....
i live by the voice inside my head

Offline dave500

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Re: 550/591 750 pistons
« Reply #11 on: January 02, 2012, 03:16:31 AM »
yeah hes still there sohc boy,im glad ive sorted out these pistons now,,i think if they had have been 750 ones a few mil would have to come of the crowns?i bought xl125 pistons from him and used them in a 500 once,ive had a bit of stuff from him,,hes always prompt with email responses and is happy to talk on the phone with any enquiry,heres a link to the "about us"section of his site.
http://www.bkperformance.com.au/aboutus.php

Offline Scott S

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Re: 550/591 750 pistons
« Reply #12 on: January 21, 2012, 06:43:18 PM »
 So, do they fit the bores now or ???
'71 CB500 K0
'17 Triumph Street Scrambler
'81 Yamaha XS650

Offline dave500

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Re: 550/591 750 pistons
« Reply #13 on: January 21, 2012, 08:54:10 PM »
honed the bores a little more and they slip in.