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Bsa gold star forum Jump to new posts
Pre-Unit Singles Pistons Magnetoman 1 hour ago
Gold Star Pistons: Overview

I'll try to make sense of the piston situation by first simplifying as much as possible without, I hope, creating any errors by oversimplifying. This thread had its origin in a different thread where some background information can be found. For the "ZB" and "BB" I use the .GS suffix when referring to the early Gold Star models since there are differences between the pistons and rods of those and the "common" ZB and BB models, but I don't use it on the CB/DB/DBD because there isn't the same possibility of ambiguity.

The 350cc Gold Star engines used three rod lengths for early ZB.GS, late ZB.GS and BB.GS, and CB/DB. I'll leave those aside for now to reduce the complexity of this introduction. The 500cc Gold Star engines used only two rod lengths: 7-3/8" for ZB.GS and BB.GS, and 6-15/32" from CB through DBD.

Again, the following only applies to the 500cc Gold Stars. I'll deal with the 350cc in a later post.

Since I'm particularly interested in BB.GS pistons at the moment, I'll start with those. BB.GS pistons were supplied in five compression ratios from 6.8:1 through 11:1, all available in standard, ½ mm oversize and 1 mm oversize, for a total of 15 part numbers for the complete assemblies. BSA Instruction Manual 00-4010 covers 1946-1955 single cylinder models and shows the same pistons were used in the ZB.GS and BB.GS. It seems possible that even lower CR pistons might have been available in early years, although they would have been dropped in a 1955 publication if no longer supplied at that time. However, I haven't researched that yet. In any case, an important conclusion is that ZB.GS and BB.GS pistons are interchangeable.

The CB had an oval flywheel to clear the piston skirt, necessary because of the CB's shorter rod, but its pistons have different part numbers than either the ZB.GS/BB.GS or DB/DBD Gold Star models. At this point I don't know if the overall height of the CB pistons are the same as the earlier ones, in which case having different part numbers would indicate other differences (e.g. expansion slots). I eagerly await hearing from someone with a CB piston.

DB/DBD pistons definitely are shorter than ZB.GS/BB.GS, which was necessary for their skirts to clear the non-oval flywheels.

As more people contribute data to this thread one question to be resolved is if the pistons for later 500cc models have the same pin-to-deck distance as the ZB.GS/BB.GS. If they do, an important secondary question is if the later, shorter pistons can be used in the ZB.GS/BB.GS (although, vice versa definitely isn't possible due to the flywheel).

Piston Dimensions

Although the main topic of this thread is Gold Star pistons, many of us have other pre-unit BSA singles so it would be nice to have data on them as well. With that in mind, please fill in as many of the blanks as you can on as many BSA singles pistons as possible. Although I only discussed 500 cc Gold Stars above, definitely of interest are data on pistons for all 350 cc and 500 cc pre-unit singles (e.g. ZB31, BB33, etc, as well as both sizes and all series of Gold Star). Once there is enough data to work with I'll try to combine and format it in a way for easy future reference. But, please be clear whether you are reporting data on a, say, ZB32 or a ZB32.GS.

To repeat something I wrote in that other thread, I believe what will be most useful aren't precision engineering drawings, but rather the dimensions that are easy to measure with a set of calipers in order to check a given piston. So, for example, although the distance from the center line of the gudgeon pin would be used on an engineering drawing, it is the distance from the top of the hole that is easily measured with calipers.

In addition to the list below, the three images are a blank diagram for ease of printing, a diagram with letters assigned to the important dimensions, and a partially filled-out diagram to indicate that not all blanks have to be filled out for the information to be quite useful. Post your results in whatever way is easiest for you.

Application, if known (e.g. ZB32.GS, BB33, etc.):
Casting number(s):
Stamped number(s):
Manufacturer, if known (AE = Hepolite, MC = M-C Supply, etc.)
Compression ratio, if known:
Weight, complete assembly with rings and pin:
Weight, piston alone:
Gudgeon Pin diameter:
Top of pin to piston deck:
Top of pin to piston crown:
Total height of piston:
Ring groove dimensions:
__ top:
__ middle:
__ oil:
__ from bottom land to top of top land:

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Garage sale - The private buy & sell board Jump to new posts
Re: 67 A65 / A50 Parts hacksaw 1 hour ago
whats the inside of the chrome tank look like?
15 1,296 Read More
Garage sale - The private buy & sell board Jump to new posts
Re: T-Lite After market hacksaw 1 hour ago
is that lens pitted? whats up with it?
1 114 Read More
The triple forum Jump to new posts
Re: Clutch cable stretched? Woodsie 1 hour ago
My T160 's setup is exactly the same as the parts book. My parts book has that ferule looking thing as 57-1645. A good call on your part. Thanks
11 735 Read More
British motorcycles in general Jump to new posts
National Motorcycle Museum and other UK sights Matthew in TO 2 hours ago
I’ve booked a trip to Britain this summer, including London, Birmingham, York, Leeds and Windsor. In London I want to visit the plaque denoting the AMC plant location (not far from my birthplace in Lewisham) and then to the plaque at the Meriden plant location and then a tour of the National Motorcycle Museum and the British Motor Museum

What other classic motorcycle points of interest should I see? Unfortunately Sammy Miller is too far south. I may look into the British Transport Museum, where I last visited with my grandfather in the 1980s (he was a retired London bus driver).

What about ?

0 17 Read More
Bsa gold star forum Jump to new posts
Re: New BB piston Kerry W 2 hours ago
The piston thread would be a good thread. Much data is missing from the relevant books.

Some of the broad-brush 'what fits with what' detail came to me years ago, by a process of elimination: when racing my old BB34 with the 1174 piston: it had a short rod and a cut-down cylinder (which leaked around the welded joint-below the fins but above the tappet cover-, regardless of what we did!). I had a piston problem at one point, right before an event and couldn't resolve it in time, so swapped top ends to a very nicely ported ZB33 setup, which came to us with a bunch of stuff. It had a shortened B33 cylinder, associated pushrods, pushrod tube and a couple of pistons. Swapping the top ned was simple and effective, though in the process I noticed the difference in pin height between the piston in the BB top and and the B33 - the latter used the CB-DBD height (i.e. the 'cut-down' job had been done properly - point of ID is 7 fins on the cylinder, not 9). Soon after that (early 1980's) I found what had been known since the mid-50's..the late B33's had a medium rod and the pile of 1174 pistons in a box suited it. Oddly, amongst the rest of the stuff was a ZB34 bottom end, with 350 flywheels and a medium rod..

The BB top end never went back together - the cylinder was a problem due oil leakage and the bike was a rocket with the iron top end, open megaphone and a 32mm MkI Concentric. It did get rather hot at 9:1, though the races were short. I put a compression plate in it, dropping it to about 8.7:1, and while it ran a little cooler, it did take the edge off it a little!
11 238 Read More
Hot links forum Jump to new posts
39 Royal Enfield Military Model C, California MTA franko 3 hours ago
0 14 Read More
Hot links forum Jump to new posts
71 Greeves Griffon Project, California MTA franko 3 hours ago
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Hot links forum Jump to new posts
78 Triumph T140E,Oregon $2,800 franko 4 hours ago
0 23 Read More
Hot links forum Jump to new posts
74 Triumph Trident, California $3,950 franko 4 hours ago
0 26 Read More
Hot links forum Jump to new posts
72 Triumph Tiger & Parts, Texas $5,000 franko 4 hours ago
0 11 Read More
Hot links forum Jump to new posts
51 ariel Square Four Parts, California $1,200 franko 4 hours ago
0 28 Read More
Triumph bulletin board Jump to new posts
Re: R vs. Std tappets Hillbilly bike 5 hours ago
On my dual engine 650 racer using a #330 Web Cams grind , .410 valve lift, 276 degrees duration at .050 lift, an R tappet added 4 degrees duration compared to a S tappet.I do believe it has an effect on performance aside from duration... I do not know if this would be the same with other cam profiles...
1 73 Read More
Bsa bulletin board Jump to new posts
Re: A50 Piston alternatives koncretekid 5 hours ago
As are B25's, TR25's, and Triples.
2 78 Read More
The triple forum Jump to new posts
Re: X-75 Resurrection GLJ Yesterday at 11:37 PM
Originally Posted by quinten
cant you just grab one of the pumps
Phillips screws with some vice grips and wiggle it out ?

I've grabbed the housing with channel locks and it won't budge.
26 1,573 Read More
Triumph bulletin board Jump to new posts
Re: Cali. crank balancing companies? MarksterTT Yesterday at 11:37 PM
Thanks dandruff, that was my thought also and the fact that I can hand deliver and talk to them face to face makes a big difference to me. Factor's been determined at 50% for an offset crank so that's what I'll go with. I checked out Marine crankshafts website and I have heard of them but I won't being driving that far so I hope Kenmonth becomes a go to shop for me as I have other cranks I'd like to have balanced also. Mark
2 164 Read More
Triumph bulletin board Jump to new posts
Re: freash rebuild do's and don'ts triton thrasher Yesterday at 11:09 PM
I check that the pump is putting oil into the pressure relief valve cavity. Only takes a few turns with the kickstart.

When you do start the engine, the oil light or indicator button should show oil pressure in a few seconds.
14 606 Read More
Triumph bulletin board Jump to new posts
Re: Triumph tuning information triton thrasher Yesterday at 09:53 PM
I quizzed the guy that bored and honed my 650 about grade of grit and he had to think about it, because he always used the same stones on old tractors, cars and bikes. He reckoned it was probably 130.

No laughter was involved and the engine compression tests at 170 psi with 7:1 pistons. I get the impression some owners are content with poorer ring seal than that.
10 372 Read More
Members bike projects Jump to new posts
Re: 1955 Triumph T110 Dave Swanson Yesterday at 08:20 PM
I picked up the fenders today. I couldn't happier with the result. Fantastic workmanship!

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54 12,244 Read More
Britbike remembrance forum Jump to new posts
Kevin Valentine, R.I.P. Irish Swede Yesterday at 07:25 PM
With great regret I learned today of the passing of Kevin Valentine, former chief judge of the Antique Motrorctcle Club of America

Although he was an expert on INDIAN motorcycles, he was a great techer to All AM
0 30 Read More
Bsa bulletin board Jump to new posts
Re: BSA rear fender with brackets. Adam M. Yesterday at 07:19 PM
Yes Gordo , Allan, thanks for the clarification, helped for sure.
Will be installing everything together with fender tomorrow to make sure everything fits properly.
18 424 Read More
Garage sale - The private buy & sell board Jump to new posts
Re: Norton Commando Front Brake Caliper: $40 Tridentman Yesterday at 07:03 PM
(***) = iqsdirectory
10 383 Read More
Bsa bulletin board Jump to new posts
Re: 67 68 BSA primary cover. kommando Yesterday at 06:58 PM
I have some M6 dowty washers I use on fork drain screws, must try them out on the same offending cover bolts on my B44 and see if that helps.
38 1,258 Read More
Norton bulletin board Jump to new posts
Re: Smoking Commando on idle rweb Yesterday at 05:13 PM
Usually when an engine does most of it's smoking at idle the problem is in the head.
Bad guides or seals, too much oil supplied to top end or oil drain back restricted.
12 338 Read More
Bsa bulletin board Jump to new posts
Re: Valve springs Phil in Germany Yesterday at 05:09 PM
I have checked the T140 workshop manual. It lists the valve springs as outer 41,3 mm (1 5/8") and inner with 38,9 mm (1 17/32").
Those figures are very close to Gavin's measurements so I am inclined to say the valve springs in question are originally for a T140.

As I stated before I have had no issues with those springs that were supplied by SRM.


7 340 Read More
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