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Re: Rockerbox switching [Re: Gordo in Comox] #761340 01/03/19 5:03 am
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Kerry W Offline
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Without looking up the angles, etc involved, the big-fin engines had a narrower valve angle (as I recall - easily wrong!). I don't know about valve length, but the essential dimension would be the distance between the valve stems ends, compared to early/late engines - that would drive my decision regarding which rocker box. As you sy, it could be a development towards the later heads...which could easily include the later valve angles, especially with the steep inlet downdraft.



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Re: Rockerbox switching [Re: Gordo in Comox] #761391 01/03/19 5:08 pm
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Gordo in Comox Offline OP
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Kerry;

Strange how it is hard to find out info on what took place in the BSA factory over sixty five years ago, any of those involved in the development would be fairly old now.

The best reference that I have is the Golland book "Goldie the development history of the GOLD STAR". The development in the book links advances by years and not by models and even then there is always a problem with GS history books in that calendar years and model years are not consistent.

With that in mind Golland states that in 1952, with the separate rocker box introduction, the valve angles went from 37 degrees and 30 minutes inlet and 37 degrees 30 minutes exhaust to 33 degrees 45 minutes for inlet and 33 degrees for the exhaust. That would be the late ZB and then BB engines I would guess. There were many small changes in valves and seats over the years but it is not well chronicled. In 1954 the inlet tract was lengthened by 9/16 inches, could this have required a taller head? That would most likely be the CB engine. With the introduction of the eccentric rocker adjustment the valve stem length had to be increased according to Golland but there is not explanation as to why. Maybe it was the taller head and not the eccentric type rocker setup that necessitated the longer stems. I have one of the CB 32 inlet valves and it is quite a bit longer than the BB type valves.

Years ago I measured the two types of rockers and the dimensions seemed to be about he same. I will revisit those measurements to see if there is any difference.

Gordo


The roadside repairs make for the best post ride stories.
Re: Rockerbox switching [Re: Gordo in Comox] #762523 01/14/19 7:22 am
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devon Offline
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Gordo


A sandcast 350 clubman(april-1952) with xtra downdraft (compared with ZB32GS head) exists in Ch.Ch. here. I used to own it. Looked bulky compared to a die cast BB head. Perhaps I can locate it, if so will get back to you.


devon.





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Re: Rockerbox switching [Re: Gordo in Comox] #763800 01/28/19 9:24 pm
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Mal Marsden Online Content
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Gordo
Today I received a BB34 head and have put it side by side with a BB32 head which I bought and have modified to fit on my Daytona replica. The differences between these two are almost the same as photos you posted. I am certain the 350 head is correct as the combustion chamber is definitely smaller to suit 350 bore.

Differences I see easily are.
1) height, 350 is 95.5mm and 500 is 85.7mm
2) 8 fins on 350, 7 on 500
3) 350 is about 15 degrees downdraught, 500 is is nearer 7 degrees.
4) 350 casting thicker, less defined, sand cast? 500 is neater, thinner fins, better detail etc.
5) 350 exhaust outlet, ø42mm, 500 is ø46mm
6) 350 part number on bottom face, 500 part number on side of inlet valve housing,
7) gap between fins at front of pushrod tube not as deep on 350 (1/2”) and nearer 1” on 500
8) all holding down threads on 350 are ø3/8” x 26tpi, 500 has ø7/16” on the long head to crankcase bolts
9) valves are different but my 350 has non standard valves so cannot compare dimensions
10) 350 head might have cast number 65-1757 & 67-1805, very hard to determine with confidence
11) 350 inlet port ø28mm, 500 is ø29.7

[img]http://06C990AE-FD8A-4E03-9C52-46DD4DAC7880[/img]
[img]http://127DA4B2-4BAB-46F5-B3F0-38CC1FE1A41D[/img]
[img]http://6DFE9A4F-DA46-460C-AFC4-D485C569B8B8[/img]


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Re: Rockerbox switching [Re: Gordo in Comox] #763811 01/28/19 11:27 pm
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Gordo in Comox Offline OP
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Mal: Thanks for the info. Are those links above photos as I cannot open them.

My BB 500 head has all 7/16 threads, the 350 all 3/8 inch. My CB500 head has five 7/16 and three 3/8. Of course those engines had a long bolt in the pusrod tube area. My 350 inlet port is about 28 mm.

Thanks again for all the info, it helps us to sort out the transitions from one model to the next.

Gordo


The roadside repairs make for the best post ride stories.
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