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Pre-Unit Singles Pistons
#795988 01/19/20 4:54 am
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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 of bottom land to top of top land:

[Linked Image]

[Linked Image]

[Linked Image]

Last edited by Magnetoman; 01/22/20 3:21 pm. Reason: oops, I recorded the two top grooves as 0.62" and 0.67" on the bottom figure, when they are a somewhat narrower 0.062" and 0.067"
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Re: Pre-Unit Singles Pistons
Magnetoman #796017 01/19/20 5:55 pm
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MM: Should be enlightening for those dealing with these 70 year old engines. As I look through the pistons I have I find some interesting castings inside the piston. Part numbers and AM, part number and 2793 AM, BSA and part number, part number and AM 405. Sometimes the casting part number is not the same as the stamped in number shown on the crown.

The stamped in crown numbers are not the same as the numbers shown in parts listings, sometimes 1 off and other times more off. The number sets are not consistent with the model of engine. All piston numbers start with 65 so I will ignore that for simplicity. The ZB/BB engines have 1200, 1300. 1400, 1500, 2300 and 2500 series pistons (ie 1244 etc). The short rod big fin engines have 2200, 2300, 2500 and 0900 pistons. What was the system?

Did BSA make their own pistons? Some I have are stamped with the three rifle logo. Does that have any meaning?

Gordo

Here is the top a piston with the rifles and part number and HF. The 3/32 is the size of the compression rings (one of two sizes) and the .830 is the distance from the top of the pin to the deck.

click to enlarge

[Linked Image from i.postimg.cc]


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Re: Pre-Unit Singles Pistons
Magnetoman #796022 01/19/20 6:23 pm
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Quote
Did BSA make their own pistons?


Yes they did make their own, diamond turned as well. Stopped making them around the mid 60's and then went to Hepolite.

Re: Pre-Unit Singles Pistons
Magnetoman #796033 01/19/20 8:34 pm
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500 cc Pistons

Offline Gordo sent measurements on five ZB.GS/BB.GS pistons, with the distance from the top of the pin to the deck ranging from a low of 0.820" to a high of 0.853". I measured a representative sample of five of my DBD pistons (2 Hepolite, 2 M-C, and 1 unknown), plus the Wisco that Phil Pearson supplied with my crankshaft, and found the same distance ranged from a low of 0.875" to a high of 0.919" (the Wiseco is 0.898"). These are suspiciously close to being the same as Gordo's, but taking both of our measurements at face value Gordo's would have a ZB.GS/BB.GS piston 0.022"-0.070" lower in the bore at TDC than a DBD piston. There's certainly enough material in the pistons that shaving ~0.07" from an annular ring on the deck wouldn't be a problem, which seems to mean that a DBD piston could be used in a BB.GS (nb. I would do a test fitting to see how much, if any, material needed to be removed from a given DBD piston). Has anyone made such a substitution?

The next photograph shows the importance of the proper location of the gudgeon pin with respect to the deck (the piston on the left is sitting on a â…›" spacer to have the holes at the same height).

[Linked Image]

If the piston on the right were used in a Gold Star it would try to punch its way through the head.

Also of interest is that the weight of the five bare pistons I measured ranged from 354 g to 425 g (Wiseco = 375.5 g). This 71 g difference would have a significant affect on the balance factor if not compensated for.

Cylinder Heights

Since the height of the cylinder is directly related to the length of the rod and the position of the gudgeon pin in the piston, it's important to have that data as well for both the 350 cc and 500 cc Gold Stars. For those of you with spare cylinders on the shelf, while you're in the garage measuring pistons, please also fill in the tables below for the cylinder height (from base of muff to sealing flange on sleeve)

350 cc
Early ZB.GS: 5.781"
Late ZB.GS and BB.GS:
CB and DB:

500 cc
ZB.GS and BB.GS: 5.830" +/-0.005"
CB, DB, and DBD: 4.920" +/-0.005" (i.e. shorter than a ZB.GS/BB.GS by exactly the difference in their rod lengths of 7-3/8" and 6-15/32", respectively)

The next photograph shows my measurement of a BB.GS cylinder with the sealing flange sitting on a flat surface (nb I revised the figure in light of the information in the next two posts).

[Linked Image]

350 cc Gold Stars

The rod went through three lengths for 350 cc Gold Stars:

Early ZB.GS: 7-3/8" [the same as on 500 cc ZB.GS and BB.GS]
Late ZB.GS (from engine no. 6001)* and BB.GS: 6-7/8" [this length wasn't used on the 500 cc]
CB and DB: 6-15/32" [the same as on 500 cc CB/DB/DBD]

*The late ZB.GS was a short-lived transitional model that was essentially a BB.GS engine in a ZB.GS frame. Only approximately 109 late ZB32.GS and and 66 late ZB34.GS were produced.

Roy Bacon's 'BSA Singles Restoration' book claims one 350 cc piston (part no. 65-2254) was used on both the CB and DB. However, I believe this is in error. The 1954 Spares catalog shows this 8:1 piston for the CB Clubmans and Road Racing models, and it is also shown for the CB Road Trim and Touring, but not for any model DB, in the 'Spares Supplement for 1954 and 1955 Models Gold Star'. As far as I can tell, each of the three rod lengths used in 350 cc Gold Stars had its own unique set of pistons.

Last edited by Magnetoman; 01/20/20 10:44 pm. Reason: added 350 ZB.GS & DBD cylinder measurements
Re: Pre-Unit Singles Pistons
Magnetoman #796038 01/19/20 10:21 pm
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MM: Some cylinder measurements

Late ZB34GS (looks like a BB) 5.833 inches

BB34GS 5.836 inches

Early ZB34 GS 5.826 inches

ZB32GS (NOS original) 5.781 inches

Of course for the first three I have no idea if they have been altered.

Gordo


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Re: Pre-Unit Singles Pistons
Gordo in Comox #796043 01/19/20 11:25 pm
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Originally Posted by Gordo in Comox
Late ZB34GS (looks like a BB) 5.833 inches
BB34GS 5.836 inches
Early ZB34 GS 5.826 inches
I wasn't happy with my previous measurement so ignore it (I've corrected it in my previous post). This time I deployed an actual height gauge that I first checked with a 4" gauge block and found it within the 0.001" resolution of the vernier. The liner in this cylinder is damaged and has to be replaced so I had no qualms about lightly hitting the sealing surface with a fine file to eliminate several raised burrs prior to this second attempt. With it sitting on that mating surface on a surface plate, and working my way around the base as much as I could given the fins on one side interfered with the base of the gauge, readings were between 5.827" and 5.831". So call it 5.829" +/-0.002", as compared with an average of 5.832" +/- ~0.005" for Gordo's. This has to count as perfect agreement, especially since it's hard to imagine original quality control at the factory was better than ~0.005" anyway.[*]

[Linked Image]

What we need now is for someone to post a careful measurement of a CB, DB, or DBD cylinder.

[*]BSA designers worked with fractions, not decimals, so it's perhaps not coincidental that 5-53/64" = 5.828"

Last edited by Magnetoman; 01/19/20 11:57 pm. Reason: added [*]
Re: Pre-Unit Singles Pistons
Magnetoman #796091 01/20/20 6:06 pm
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By measuring the cylinder with this method you are assuming the sealing flange is the same on all liners. The replacement liners are machined to a nominal dimension and I believe, and could be wrong, that they were meant to be finished machined when installing, this includes the sealing flange. It may be only a few thousands of an inch but this is why a pealable head gasket was used.


Bill B...


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Re: Pre-Unit Singles Pistons
Magnetoman #796098 01/20/20 7:18 pm
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Bill: I do not disagree with your comment but by measuring it this way we can calculate how high up the various pistons will come in relation to the top of the sleeve. The starting point will be the center line through the crank but the common point will be the mouth of the cases where the cylinder sits. Once the the distance from the crank center line to the case mouth is measured one can calculate how each piston will be positioned at the top of the stroke without trying them out.

Our measure is the distance from the top of the lip to the base that sits on the mouth because it is the lip that the cylinder is resting on when measured upside down on a flat surface.

I went through this process when I realized that the BB32GS piston I had planned to use was to high for what turned out to be a die cast ZB32GS cylinder meant for the long rod engine. I now have a ZB32GS piston for that engine.

Gordo

Last edited by Gordo in Comox; 01/20/20 8:01 pm. Reason: added info

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Re: Pre-Unit Singles Pistons
Gordo in Comox #796101 01/20/20 7:39 pm
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Originally Posted by Boomer
By measuring the cylinder with this method you are assuming the sealing flange is the same on all liners.
Originally Posted by Gordo in Comox
by measuring it this way we can calculate how high up the various pistons will come in relation to the top of the sleeve.
Measuring it this way also assumes the sleeve is pressed fully into the muff and/or there are no slightly raised areas on the Al casting under the sealing flange. However, as Gordo said, for the purposes of identifying what pistons will work I believe this dimension is the best to have for using in combination with measurements of the piston. I know Bill, Gordo, and many others know the following but, for completeness, and for illustrating the words with pictures:

The first photograph shows the piston in my 1928 Ariel, whose head seals to the flange on top of the sleeve in the same way as in a Gold Star.

[Linked Image]

Although the crown of the Ariel piston rises above the top of the flange, what's important is that the circumference doesn't make contact with the head, which means the deck can rise no higher than the top of the flange. However, as the second photograph shows, the Ariel doesn't quite have this restriction since the flange on the sleeve is 0.14" wide while the surface on the head that seals against it is only 0.10" wide.

[Linked Image]

The above photograph shows that, thanks to the narrower sealing surface on the head, an Ariel piston could rise above the flange as far as the top of the first ring before making contact. In fact, it only rises as far as the deck, but in principle it could rise to the first ring. However, tolerances are tighter on a Gold Star, and the sealing surfaces on the sleeve and head are approximately the same ~0.2" width. As a result, as the next photograph shows, the deck of a Gold Star piston makes contact with the sealing surface of the head.

[Linked Image]

What the previous photograph shows is the pin-to-deck distance is of critical importance on a Gold Star. There has to be enough clearance that the deck rises no higher than the top of the flange at 8000 rpm and when the piston expands from the heat. If an aftermarket sleeve were available whose flange was, say, 0.020" thinner than stock, or 0.020" skimmed from the base or top of the muff before installing a new sleeve, that would lower the sealing surface by the same amount and potentially allow the piston to make contact with the head.






Last edited by Magnetoman; 01/20/20 7:50 pm. Reason: added a sentence at the end
Re: Pre-Unit Singles Pistons
Magnetoman #796136 01/20/20 10:49 pm
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There is a piston chart in the BMS book that has the numbers for the 'piston only' and the 'piston complete' for some pistons, ie 65-1532 and 65-1537 respectively. This chart gives the compression ratio for this piston as 9.0. With a Compression Plate (base plate) of 1/16 inch it drops to 8.4. This would imply that a cylinder height increase of 1/16 inch over 'standard' would have the same result for that piston. Another piston with a 1/32 inch plate drops the ratio from 7.5 to 7.2. However if the cylinder is shorter than 'standard' there would be similar increases in the C:R I would expect.

Gordo


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Re: Pre-Unit Singles Pistons
Magnetoman #796142 01/20/20 11:51 pm
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MM: here the sheet on piston 65-1265 and a photo to show what it looks like in the flesh.

Gordo

Click twice to enlarge

[Linked Image from i.postimg.cc]


[Linked Image from i.postimg.cc]


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Re: Pre-Unit Singles Pistons
Magnetoman #796143 01/21/20 12:15 am
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Originally Posted by Gordo in Comox
...This would imply that a cylinder height increase of 1/16 inch over 'standard' would have the same result for that piston...
Not quite. Consider the extreme cases of a flat-top vs. a cone-top piston. In the former case raising it by 1/16" would decrease the volume in the head by π r2 × 1/16". In the latter case by only 1/3 of that. Of course, the actual tops are more complicated than either of these extremes, also with valve cutouts, but this example shows that the shape of the crown has a significant effect on the reduction in CR.

I have 17 pistons in a box, but because of carbon the tops of many of the markings aren't visible. But, of the ones that are visible, none indicate the CR and many have nothing more than the oversize stamped on them. Although I can tell by looking that the piston on the left has a higher CR than the one on the right, even if I knew the CR of the one on the right (which I do; stamped on it is 65-2265 so it's 8.5:1) it would require a measurement of the volume of metal above the deck of the other one to calculate the CR.

[Linked Image]

My hope is that the basic "design" of the tops will be close enough to the same for all of them so that a measurement of the height above the deck (given by C—D in the drawing I posted yesterday) will be enough to determine the CR with reasonable accuracy. However, for that to happen, we'll need data on as many pistons as possible. Otherwise, it will require a test assembly of the engine and measurement with a burrette. Or, dropping an unknown piston upside-down into an oil-filled head and then measuring how much is left in the combustion chamber after the rest has been squeezed out onto the workbench.

Originally Posted by Gordo in Comox
here the sheet on piston 65-1265 and a photo to show what it looks like in the flesh.
Fantastic. To the best of my knowledge information like this isn't available anywhere, but it's essential to have for anyone who wants to assemble any BSA single from parts, rebuiild any worn-out BSA single, or sort through a pile of pistons looking for the right one. That is, unless they have a source for NOS pistons in all possible CRs and over-sizes. Unfortunately, those sources are few and far between. So, everyone, please make those measurements and post that data. Not just Gold Star pistons, all pre-unit BSA pistons. Even if you don't know what machine a piston came from, or don't own a scale, whatever data you can provide still will be valuable.

Although I now know a DBD piston will work in my BB.GS, I also know the smaller size means a lighter weight so I would have to rebalance the crankshaft by drilling holes in it.[*] I hate to make permanent modifications if they can be avoided so I'll still be looking for a genuine ZB.GS/BB.GS piston.

[*] Gordo's ZB.GS/BB.GS complete piston assy. weighs 513 g as compared with Wiseco's DBD 487 g. I'll have to find the balance factor for a BB.GS, but assuming it's the same 58% as for a DBD it means 15 g would have to be drilled from the flywheel to compensate for the 27 g difference in weight in order to use that lighter DBD piston in my BB.GS.

Last edited by Magnetoman; 01/21/20 3:34 am. Reason: fixed the [*] in light of Gordo's correction
Re: Pre-Unit Singles Pistons
Magnetoman #796151 01/21/20 12:34 am
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My logic was that a 1/16 inch base plate in effect raises the head so that it is 1/16 inch further away from the piston creating a larger effective combustion chamber. A cylinder that is 1/16 inch higher does exactly the same thing by moving the head away from the piston.

I agree though that a different piston of with a different C:R could have a different result.

Gordo


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Re: Pre-Unit Singles Pistons
Magnetoman #796168 01/21/20 3:25 am
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MM If you follow down my sheet above I have only given you the all up weight of piston, circlips, pin and rings for the 513 grams. I have entered n/a for the bare piston because I do not want to take the rings off or dig out a circlip so as to get the pin out. A BSA pin is about 87 grams and a GPM pin is 78 grams.

Gordo


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Re: Pre-Unit Singles Pistons
Gordo in Comox #796219 01/21/20 8:03 pm
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Originally Posted by Gordo in Comox
I have only given you the all up weight of piston, circlips, pin and rings for the 513 grams...
Thanks for catching my mistake. I've corrected it in my earlier post.

Thanks to an offline exchange with Boomer I turned to p. 31 of Golland where he states that starting in 1954 (with the BB and swinging arm frame that was new from 1953) the 500 cc balance factor was 58% (65% for the oval flywheels of the CB). Goland's wording could be a bit clearer but I interpret it to mean the original 1953 BB used 55% but it changed to 58% for the 1954 BB and for the subsequent DB and DBD.

Gordo's data established that the pin-to-deck distance was the same on the ZB.GS/BB.GS as on the later CB/DB/DBD, as opposed to the larger distance on the B33's 11744 piston, so "mechanically" a DBD piston would fit in a BB.GS. He also gave the weight of the total piston assembly as 513 g whereas, for example, my Wiseco DBD piston is 26 g lighter at 487 g.

With this in mind, and on the issue of substituting a "modern" DBD piston for a hard-to-find ZB.GS/BB.GS piston, BSA Dealer Parts and Service Bulletin No. 51 from 1 October 1960 gives the following formula for balancing a Gold Star crankshaft:

[Balance Weight + little end weight] / [total weight of piston assembly + little end weight] = 58%

where the Balance Weight is the amount that has to be hung from the little end when balancing the crank to achieve this balance factor.

I made a quick measurement of the little end weight by holding the big end with a pencil (i.e. don't rely on the following to better than a few grams) and found 180 g, so with Gordo's piston:

[Balance Weight + 180 g] / [513 g + 180 g] = 0.58

so

Balance Weight = 221.9 g.

If instead the Wiseco piston were substituted without altering the flywheels to compensate, the balance factor would change to:

[221.9 g + 180 g]/ 487 g + 180 g] = 60.3%

Another way to look at this estimate is the balance factor changes by ~2.3% for a 27 g change in piston weight, or 0.085%/gram. If, say, a maximum 0.5% change from the 58% value were acceptable, the maximum difference in weight of the piston would be 5.9 g. That said, I don't know how much different a Gold Star would feel with a, say, 60% balance factor rather than 58%.

Another estimate is relevant, that of the effect of an oversize piston on the balance factor. Assume a piston assembly of OD 85.0 mm and height 3.00" (76.2 mm) weighs exactly 513.0 grams. Ignoring the holes for the pin, a 1 mm (+0.040") overbore piston has an additional volume of Al of ~π/4 × {8.62 – 8.52} cm2 × 7.62 cm = 10.23 cm3. Al has a density 2.7 g/cm3 so the oversize piston would weigh ~27.6 g more than the stock one and would change the balance factor to ~55.7%.

What the above paragraph shows is that simply installing an oversize piston will have a significant effect on the balance factor. So, how many people even weigh their current and replacement pistons, let alone completely disassemble their engine and rebalance the crankshaft when installing a different piston? Of course, a complicating factor is many engines today won't even have their stock pistons in them so a significant problem is there's no way to even know what the original pistons weighed to compare with a replacement... oh, wait, yes there is. If people respond to this thread with measurements on a variety of pistons we'll all have that important information available.

[Linked Image]


Last edited by Magnetoman; 01/21/20 8:32 pm. Reason: added data for 65-2261 piston
Re: Pre-Unit Singles Pistons
Magnetoman #796249 01/22/20 1:58 am
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Back when thse were being made, imperial ring dimensions were as follows
Ring width in 1/16" incriments to 1/4"
Radial depths in 1/10"
And diameter in either inches or mm.

Now not meaning to be picky but the measurements above look a little off
.010" side clearance on th fire ring looks a bit wort
.0015 clearance on the 2nd ring looks a bit tight
and .030 side clearance on the scraper looks a tad loose

or do goldies stray from the standard 1/16 - 1/16 -1/8 ring sizes ?

This brings back memories of wadeing through ring catalogues for hours trying to find oversized rings to keep this paupers bike on the road for as lons as possible before a strip down & rebore.

Last edited by BSA_WM20; 01/22/20 8:32 am. Reason: drugs affecting my math

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Re: Pre-Unit Singles Pistons
BSA_WM20 #796251 01/22/20 2:10 am
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Originally Posted by BSA_WM20
Now not meaning to be picky but the measurements above look a little off
I have a different theory for the measurements being "off." Quoting something in the first post in this 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." This thread isn't about fabricating new pistons, but making quick and easy measurements with calipers -- not micrometers -- on used and abused pistons that are good enough to use in the future to identify other pistons.

Re: Pre-Unit Singles Pistons
Magnetoman #796262 01/22/20 8:08 am
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So what is wrong with listing them as 1/16" 1/16" & 1/8"
No presumption about high accuracy with those numbers .
I think that most of us know .0625 is 1/16 and if not it is on the back of most engineering rules;
can see that the height from the pin to the bottom of the fire fing is very impportant as you don't want it springing out & catching on the liner but for the sake of simplicity the 2nd ring & oil scraper are not all that important
Simple is good, I like simple & simple likes me.

Although I did notice that the right piston in post #796033 seemed to have large rings for a motorcycle piston.

Please don't get me wrong, this is the sort of information that I have been trying to compile for decades for other BSA models as I don't have a GS & doubt I ever will and they seemed to be rght royal nightmare.
And that is before you get to all of the racers who made their own pistons,
OR EU pistons that decided to use mm ring sizes


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Trevor
Re: Pre-Unit Singles Pistons
BSA_WM20 #796287 01/22/20 3:22 pm
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Originally Posted by BSA_WM20
So what is wrong with listing them as 1/16" 1/16" & 1/8"
I'm afraid you're still missing the point. A fundamental principle of experimental science is, to the extent possible, you record data independent of any theory you might have for that data. Only later do you see if the data fits your theory, or if you have to modify your theory in light of the data. Further, even for a brand new piston the slots wouldn't be 0.0625" and 01250", they'd be a few thou. wider than that, so 1/16" and 1/8" are incorrect values. That's what's wrong with recording them as 1/16" and 1/8". It couldn't be simpler: record what you measure with your calipers, not what you think you should have measured.

Originally Posted by BSA_WM20
I don't have a GS & doubt I ever will and they seemed to be rght royal nightmare.
?? They're no more or less of a nightmare than any other BSA model of the same vintage.

Last edited by Magnetoman; 01/22/20 3:56 pm.
Re: Pre-Unit Singles Pistons
Magnetoman #796294 01/22/20 4:56 pm
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From my point of view we are on the right track. The more we highlight the pitfalls of sourcing the correct GS bits the better.

One list I have of the long conrod pistons (ZB/BB 34 GS) for the most part lists the compression ring size as either 3/32 or 1/16 in. A few have no size shown. This info alerted me to ring size differences. A forum member had posted this page many years ago and I only came across it while searching the internet for a ZB32GS piston. I contacted this member through the Forum and obtained the page for the 350cc engine. This was a big help in my search.

If all of these references or links to them were listed here we would have a very valuable resource.

Gordo

Last edited by Gordo in Comox; 01/22/20 11:53 pm. Reason: expanded info

The roadside repairs make for the best post ride stories.
Re: Pre-Unit Singles Pistons
Magnetoman #796330 01/22/20 9:53 pm
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Hi All,

Quote
or do goldies stray from the standard 1/16 - 1/16 -1/8 ring sizes ?


Quote
Although I did notice that the right piston in post #796033 seemed to have large rings for a motorcycle piston.


Apparently yes, the oil control ring on a NOS original BSA 65-908 I have measures 4mm (0.1575)

I have sent MM the details of this piston by email

John

Re: Pre-Unit Singles Pistons
chaterlea25 #796350 01/23/20 1:59 am
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Originally Posted by chaterlea25
Hi All,

Quote
or do goldies stray from the standard 1/16 - 1/16 -1/8 ring sizes ?


Quote
Although I did notice that the right piston in post #796033 seemed to have large rings for a motorcycle piston.


Apparently yes, the oil control ring on a NOS original BSA 65-908 I have measures 4mm (0.1575)

I have sent MM the details of this piston by email

John


So were the compression rings also metric ?


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Trevor
Re: Pre-Unit Singles Pistons
Magnetoman #796429 01/23/20 10:06 pm
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Hi Trevor
The compression rings measure 0.062in. (1/16)

John

Re: Pre-Unit Singles Pistons
Magnetoman #796502 01/25/20 12:00 am
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Originally Posted by Magnetoman
Originally Posted by BSA_WM20
I don't have a GS & doubt I ever will and they seemed to be rght royal nightmare.
?? They're no more or less of a nightmare than any other BSA model of the same vintage.
Isn't that why you started this thread? The post-War Gold Star models went through more revisions than the others, and had more compression ratio options in any given year.

As per the first post:
"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. "

That's why collating this information is so useful.


A65s are a doddle compared to that.

Re: Pre-Unit Singles Pistons
Shane in Oz #796510 01/25/20 2:31 am
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Originally Posted by Shane in Oz
Isn't that why you started this thread?
I started it because they're a headache, not a nightmare.. A Rocket 3, now that is a nightmare...

I'll be back working on the piston issue this weekend. Meanwhile, I've been at the Las Vegas auctions this week, not to buy but to meet collectors. However, it's given me the chance to look at several Gold Stars as if I were going to buy them. They're going through ~350 bikes/day, with the actual auction taking place in a room where I measured the sound level to be 91 dB (prolonged exposure to greater than 85 dB can cause permanent damage), so I haven't seen very many sales in person. Two Gold Stars that I know sold today (there might be more) both had good numbers on the engines and gearboxes (paint too thick, and room lighting too dim, to make out much with the frame numbers), but two that will come up tomorrow (again, there might be more) are less than meets the eye. As well as less than meets the catalog's claims. The same with a 1959 Spitfire Scrambler whose engine has 1957 numbers on it.

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