Refurbishment of B25 cylinder head and rocker box

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Cocker Cox
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Refurbishment of B25 cylinder head and rocker box

Post by Cocker Cox » Wed Jul 01, 2020 8:01 am

I’ve recently got a 1971 B25SS and have spent the last month checking it over. After finally getting it to run well, I was concerned about a puff of black smoke on start up but put it down to it being unused for years and a silencer full of soot. Then a sticking valve raised my concerns further and I initiated a top-end strip down. My first discovery was that the (standard) piston was a Starfire item, as was the cylinder head and rocker box (with 1/4” studs instead of the upgraded 5/16” items on the SS). All went ok until I tried to separate the rocker box from the head on the bench. There was no gasket, but a cement-like concoction glueing the two together. I had to finally break the bond by carefully jacking the two apart using an aluminium spacer and an expanding bolt. The next issue was the refusal of the two to separate more than 1/4”. I removed the studs, suspecting corrosion, but still no joy. I then realised that the exhaust valve spring collar was impacting on the wall of the rocker box and had to partially open the valve, wedging it in postion with nylon shims under the head of the valve. With the head and rocker box now separated, I could finally inspect the individual components, finding a loose exhaust valve guide, a stripped exhaust spindle thread in the rocker box, a ground-down exhaust valve tip and scuffing to the mating rocker arm pad.

So, I’m looking for a reputable specialist company that I can advise me on what needs to be done and be trusted with the job. I would also appreciate member’s opinions on what work needs to be done.
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Last edited by Cocker Cox on Wed Jul 01, 2020 10:03 am, edited 3 times in total.

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Re: Refurbishment of B25 cylinder head and rocker box

Post by minetymenace » Wed Jul 01, 2020 9:24 am

<135 to the forum Cocker Cox, about time you started posting here, we need some more 250 riders...
For info this is how you can post pictures.
I have added Barnsley as your location (I normally ask new posters to add their location, but as the moderator knows where you are, I thought I'd do it for you!)

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Re: Refurbishment of B25 cylinder head and rocker box

Post by beat » Thu Jul 02, 2020 8:20 pm

Well, it dos not look to bad to me.
Others find Fish and Crabs in there and they still run.

some cleaning, some small parts and some glue - and there it will go :!:

beat

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Re: Refurbishment of B25 cylinder head and rocker box

Post by Cocker Cox » Thu Jul 02, 2020 9:17 pm

👍

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Re: Refurbishment of B25 cylinder head and rocker box

Post by gunnag » Thu Jul 02, 2020 11:26 pm

I can see several issues which you might want to address as follows:-
- the valve seats look slightly pitted, I would get them re-cut ideally using a 3 angle valve job which helps efficiency, get the valves refaced at the same time
- there is evidence of burnt oil on the inlet valve stem, this probably the reason for the puff of smoke as the guide is worn and oil is being sucked down it, I would get the guides relined using K line valve guide liners which is cheaper and easier than replacing the whole guide. Get the guides honed to suit the OD of the valves.
- on one of the rockers (inlet ?) the pad looks worn, get a replacement rocker or get it resurfaced.
- the carbon deposits in the head look black and oily, check the rings aren't worn, if they are you may need new rings and a re hone or even a re bore. Also check the mixture isnt too rich.

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Re: Refurbishment of B25 cylinder head and rocker box

Post by Cocker Cox » Fri Jul 03, 2020 6:12 am

Spot on! Thank you so much for your observations. I’ve taken the head and rocker box to a BSA specialist and your comments confirm his....

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