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350/500 Conrod #762873 01/18/19 5:55 pm
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Gordo in Comox Offline OP
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I appreciate that the GS family of engines had several conrod lengths over the years with the 350 not always being the same length as the 500 for some early years. That said, can one assume that for the years where the length was the same it was a common item for the two engines?

Any help would be appreciated

Gordo


The roadside repairs make for the best post ride stories.
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Re: 350/500 Conrod [Re: Gordo in Comox] #762921 01/19/19 4:39 am
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Magnetoman Online Content
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Piecing together information in several places in Chapter 3 of Golland may have the answer. He says most of the factory attention was paid to the 350cc, that the 350cc had two changes of rod length (i.e. three total lengths), while the 500cc had only two. Further, "In time for the 1954 season however, the 500cc model was modified in one operation, using knowledge and techniques developed for the 350cc version." It was the 1954 BB-to-CB transition that the rod length of the 500 was changed. Although not completely certain, I infer from this that the 350cc already had made the switch to the sorter rod. This would mean that the early 350 and 500 ZBs had the same length, and from the 500 CB on they had the same length, but in between they didn't.

Re: 350/500 Conrod [Re: Gordo in Comox] #762923 01/19/19 5:09 am
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Gordo in Comox Offline OP
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MM I agree with all the above. What I am looking for is confirmation that for the years (early and late) where the length was the same length for both the 350 and the 500 was it actually the same rod. Can I use a 500 rod in a 350 engine if it is the length I need? I have been offered a 7 3/8 GS rod most likely from a 500. Can I use it in a 350?

Gordo


The roadside repairs make for the best post ride stories.
Re: 350/500 Conrod [Re: Gordo in Comox] #762925 01/19/19 6:37 am
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Kerry W Offline
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Interchangeability between pods of the same length? Yes. I've never seen a reference to 350-only or 500-only rods and have had them side by side year ago - no noticeable differences. (I only have '500cc' parts books to hand..)

Interestingly, the later B33's had the medium length rod from the late Zb and BB 350's..


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
Re: 350/500 Conrod [Re: Gordo in Comox] #762954 01/19/19 3:30 pm
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Magnetoman Online Content
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Originally Posted by Gordo in Comox
Can I use a 500 rod in a 350 engine if it is the length I need? I have been offered a 7 3/8 GS rod most likely from a 500. Can I use it in a 350?
I looked in the 1951-52 and 1954 parts books.

In the 1951-52 book the 350 and 500 cc engines both used the same rod, 65-1321. This was the 7-3/8" rod you're asking about.

In the 1954 book the Road, Trials and Scrambles 350 and 500 cc (which would be the BB version of the 500) used different rods: 65-1613 for the 350 but the same 65-1321 (7-3/8") as earlier for the 500.

In 1954 the Clubmans & Road Racing 350 and 500 cc (which would be the CB version of the 500) both used 65-1836 rods, which was the shorter rod.

This shows that you can use that 500 rod in your 350.

Re: 350/500 Conrod [Re: Gordo in Comox] #762972 01/19/19 7:07 pm
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Gordo in Comox Offline OP
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Kerry and MM: thanks for the confirmation. Very helpful.

I have attached a summary of part numbers I just got from the Ian, GSOC, that he sent to help me sort some parts issues. I think he compiled them from parts books. We have addressed many of these things previously but I think it is useful to have them altogether.

Gord

PART NUMBERS: ZB / BB / CB /
ZB31 / 32 / 33 / 34 T/S CRANKCASE 66-1616
ZB31 / 32 / 33 / 34 D/S CRANKCASE 66-1642
ZB34 GS D/S CRANKCASE 66-1646 BIG BEARING

BB CLUB/RD/TR/SC 350 T/S CRANKCASE 66-1650
BB CLUBMAN 350 350 D/S CRANKCASE 66-1652
BB CLUBMAN 500 T/S CRANKCASE 66-1648
BB CLUBMAN 500 D/S CRANKCASE 66-1654
BB32 / 34 T/S CRANKCASE 66-1616
BB32 / 34 D/S CRANKCASE 66-1656
BB CLUB / RD / SC 350 HEAD 65-1604
BB CLUB/ RD / SC 500 HEAD 65-1501 65-1502 casting
BB32 TR HEAD 65-1756
BB34 TR / SC HEAD 65-1749
BB GS 350 BARREL 65-1601
BB GS 500 BARREL 65-1503
BB32 TR BARREL 65-1601
BB34 TR BARREL 65-1503
BB32/34 ROCKER BOX 65-1506
CB CLUB + RR T/SCRANKCASE 66-1676
IN 1954 BOOK D/S CRANKCASE 66-1677
350 HEAD 65-1802
350 BARREL 65-1810
500 HEAD 65-1805
500 BARREL 65-1825

DB 350 HEAD RD / SC 65-2471
[IN CB BOOK SUPPLEMENT] 500 HEAD RD / SC 65-2467

DBD [RED BOOK] T/S CRANKCASE 66-1676 =66-1681 assembled
D/S CRANKCASE 66-1677 =66-1681 assembled
HEAD CLUB 65-640
HEAD SC 65-664
BARREL 65-1825



The roadside repairs make for the best post ride stories.
Re: 350/500 Conrod [Re: Gordo in Comox] #762978 01/19/19 8:03 pm
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I guess it depends on how hard you intend to rev this motor but Phil Pearson told me there are no new pistons he would recommend out there at the moment for a BB34 and advised me to use a DBD type Omega piston which he reckons are bullet proof. Problem is you need to alter the length of the con rod to make these work on BB34 to get the piston at the right height since gudgeon pins are at different height.

His way round this is to get a special rod from Carillo that is an intermediate length between the short and long rods, this puts the piston in the correct place to give you some decent compression. I can give you the part number if you want it, took me 6 weeks wait as they are made to order. Might be a little cheaper for you in the States but it cost me £300+, I’ll let you work out the dollar price but bet it is less. End price for me was a little over £900 for the finished crank but by hell, it looks good, shame nobody ever sees it! I’ve got a genuine BB34 bottom end to build up now and most likely have to go this route except that I have new AE Hepolite piston for the BB34 with it, what length rod that needs I’ll try and measure up in the coming months.

Mal


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Re: 350/500 Conrod [Re: Gordo in Comox] #763029 01/20/19 11:52 am
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To add a little...I ran a BB34 years ago, with a (badly) shortened cylinder. The rod in the engine as it came to us, was a medium length version, a la BB32/late B33. The piston was about 8:1 and marked '11744'. The cylinder had been shortened to suit the piston/rod combination. The bike came with a van-load of spares, including about 10 of these pistons. All gone now. However, I always thought the piston was an aftermarket version to suit the BB33, with the medium length rod.

Only yesterday did I spy this in eBay:

https://www.eBay.co.uk/itm/bsa-bb33-30-genuine-hepolite-piston-new-old-stock-some-gold-stars/153344614838?hash=item23b40d21b6:g:8owAAOSwwDJcQkAM:rk:3:pf:1

Yes, tha same piston as I'd used 30+ years ago. The split skirt wasn't encouraging when we pulled it apart, though it stayed together at 7000rpm! Eventually, for other reasons, the alloy top end was replaced with a very nicely ported B33 setup, cut down to suit DBD piston and rod. A stock short rod was used in turned-down B33 flywheels, with OEM GS crankpin,. The rod was very carefully polished and shot-peened and was used regularly for the next few years and seemed happy at 7000rpm too - the big-end was rebuilt at one point and the rod crack tested - all OK. I think it is still untouched and used periodically as a road-0racer by the new owner. I should never have sold it..



No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
Re: 350/500 Conrod [Re: Gordo in Comox] #763043 01/20/19 6:18 pm
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Gordo in Comox Offline OP
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Kerry

Interesting machine for sure. I am not against modifying bits to make an engine work. Some twenty years ago a very nice BB31 cases and bottom end came my way. I was already working on a BB34A Clipper engine which is really just a basic BB33 with an alloy GS trials/scrambles top end so I figured why not get an alloy top end for the BB31. Except for the lack of BB32A code on the cases it would be identical to the real BB32A.

With that in mind I obtained a 350 GS head and a NOS cylinder and BB GS piston from the GSOC parts scheme. I had the bottom end rebuilt and then got into the details. I noticed that the piston that came with the cylinder was a lot higher above the pin (even when ignoring the higher crown ) than the BB31 piston that came with the long rod crank. Calculation showed that the deck of the piston would be too high at TDC. The rod is 7 3/8 inches and the cylinder is the the same height as the ZB long rod engines. The cylinder is most likely for the 6000 series ZB32 GS engines with the separate rockerbox that finished out the ZB run in 1952. The piston is for the shorter BB32GS rod that came out when the BB engine started out. What I really need is a ZB32 GS piston to match the rod and cylinder pairing but they are not easy to find. It turns out that the head I bought was a non standard development head that is somewhere between a BB32 and CB32 head but it can be made to work.

I am not too keen to modify the NOS cylinder as it is just too nice to change.

[Linked Image]

My backup plan is to use an early 80 thou over T100 piston that would be the correct 71mm and with a slightly smaller conrod bushing would work just fine. Here is the T100 piston alongside the B31 piston. It would have the same deck height but with a better crown to give reasonable compression. I would change out the BB31 rod for a real GS rod to give the engine a better chance to survive.

[Linked Image]

It is fun!

Gordo


The roadside repairs make for the best post ride stories.
Re: 350/500 Conrod [Re: Magnetoman] #763239 01/22/19 10:17 pm
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devon Offline
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Hi Gordo, sounds right to me. However the BB33 used the 2nd rod with lower gudgeon pin type piston, while the BB31 kept to the long 1st rod. The reverse of the Goldies, before the 3rd(shortest) rod CB arrived. Seems for touring the BB33 got more initial punch while the BB31 remained restrained. These No2(mediun length) rods are unobtaniun here in NZ, not so the piston. A G80 rod can be used, length being the same. Cheers.





ZA10 R
XA7
B50mx
FB197
AJS S5
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I could see daylight thru the Chequered Flag so I rode another lap.


Re: 350/500 Conrod [Re: Gordo in Comox] #763240 01/22/19 10:20 pm
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devon Offline
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Yes so long as the 350 ain't the April 1952- clubman ZB type, which used the No2 rod, as did the BB33.





ZA10 R
XA7
B50mx
FB197
AJS S5
ZC11 S 4
21/37 sc







I could see daylight thru the Chequered Flag so I rode another lap.



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