Author Topic: A10'1956 (modifying compression ratio)  (Read 6073 times)

Offline 1KCBC

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A10'1956 (modifying compression ratio)
« on: 12.11. 2007 09:43 »
Hi all,If I have an A10'1956 engine and I would like to modify it to 9:1 compression ratio.Can I do it and
what is the part that I have to replace? Thanks.

Jaran.
1951 A10GF,(looking for  A10 swinging-arm)

Offline Pollock

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Re: A10'1956
« Reply #1 on: 12.11. 2007 12:33 »
I believe 70mm pistons on Golden Flashes of that era came with 9-1 compression. What compression do you believe you are at? Few ways to increase compression. Take matterial off bottom of Jugs. Take matterial off top of Jugs. Or the more common way is shave the heads. Also, you can use A7 flywheels and standard piston and make a stroker. Another way to boost compression.

Now, If you are at say 10.5 to one and want to to decompress to 9 to 1 you can buy pistons that don't have as high as dome or longer distance from top of piston to pin. There was a fellor on the Horse Tech board called MR. Wild who new all this stuff on british bikes and shared. I haven't see his posts lately.  If you look you will find harley sites that speak of rising and lowering compression.

Hope my rambles have helped you.

Pollock

Online RichardL

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Re: A10'1956
« Reply #2 on: 12.11. 2007 13:06 »
Jaran,

With due respect to Pollock, I don't believe you need to, nor should, change the height of your barrels or shave the cyclinder head (unless just skimming to make them truly flat). You should just need to get the pistons with the gently domed tops. (The pistons with big domed tops that come to a peak are the 10:1 variety.) BSA also increased the throat size of the carburetor for this mod, but you probabaly won't have a problem with the one you have.

As I always point out, I am not a professional or even the most knowledgable amateur on the forum, so, for definitive information, try calling someone like SRM Engineering.   

Richard
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Re: A10'1956
« Reply #3 on: 12.11. 2007 20:17 »
Jaran, you are probably better going to 8:1 or 7.25:1 with modern fuels with low octane. Also thin flange barrels don't like high compression and tend to break around the flange.  Trev.

G/F DAVE

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Re: A10'1956
« Reply #4 on: 12.11. 2007 22:20 »
I would not recommend raising compression ratio unless you have a alloy head fitted as per rgs & super rocket & thick flange barrels.As for valve seats I have run a A10 fitted with cast head for 18 yrs on unleaded petrol with no problems & normal tappet adjustment.The valve seats seem fine when I have had to rebuild the head & only need light regrind to perfect a seat. High comp ratios work well on triumph motors but not well on ordinary road going A10,s (I have had a rod thru crankcases on a A10 at 110mph locked rear wheel)  *eek*not recommended but was a quick bike though.You have to remember these motors are 45-50yrs old & as such need care when they are rebuilt & used unless you have a good supply of spares. DAVE......

Offline a10gf

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Re: A10'1956
« Reply #5 on: 12.11. 2007 23:31 »
My personal opinion, go after reliability and good looks, not power, and enjoy the work of keeping the bike in good shape. Power up > reliability down, in most cases. Guaranteed extra wear on all parts, engine, gearbox, brakes.

(Get a modern bike for speed. The roadholding and (lack of) brakes at 90+ mph is not something I'd like to have to test in an emergency situation on an A10, or any other classic)

e

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Online groily

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Re: A10'1956
« Reply #6 on: 13.11. 2007 07:01 »
So agree with G/F Dave and A10GF - had similar disasters experimenting unsuitably with hc pistons etc years ago (thin flange barrels parting company all round, etc etc), and couldn't agree more about going for reliable. I've swapped acceleration for stress-free cruising ability with an extra tooth on the rear sprocket on my recently-acquired A10 (I really recommend that and it will just about keep up with modern traffic on older-style A roads), but if I want to go in a real hurry I push the button and a Yamaha stirs itself into modern foolproof life. Although they sounded great when they worked, I don't remember highly tuned A10s holding together like a Triumph could either (to my chagrin). Maybe with modern parts and modern oils, new con-rods and a shed-load of associated expense it's easier than it was when some of us were young. But 8-ish to 1 would seem to me to be about the comfortable compromise to provide an all round pleasant bike. Very middle-aged approach I know - god how boring! Groily
Bill

Online RichardL

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Re: A10'1956
« Reply #7 on: 13.11. 2007 11:32 »
Gentlemen,

I suppose if Jaran had asked, "Should I change to 9:1?" instead of, "Can I do it and
what is the part that I have to replace?" I would have kept out of it and left the issue to more experienced folks, as yourselves.

Honestly, I appreciate your comments and must now choose for myself if I want to stay with 9:1 in my own current rebuild. MY '55 A10 has thick flange and alloy head, as I believe Jaran's must. (Jaran, how about commenting here regarding your bike's head and barrels. Thick flange in 12.2 mm.) The reason for my rebuild is a thrashed left-side big-end bearing with signs of oil starvation, so I don't think it was a matter of being at 9:1 vs. 8:1. It seems to me that the crank might have been bouncing left and right from a too-loose cush nut, leading to enough shift in the bearing shell to cover the little oil hole. Another possibility is that the piston was so loose in the barrel that it caused the shift. So you ask, "Okay Richard, why did you let your piston be so loose in the barrel? Answer - I was being cheap and I wasn't confident in the beginning that I would get the engine running. So, I just honed away rust until I had a cylinder wall, figuring that a top-end rebuild was in the offing and doable, if necessary. When the finished engine started on the second or third kick, with no obvious slap, I figured I was in the clear.  However, to hedge my bets, I bought a set of 9:1 pistons in the next size up in order to be ready for the rebuild, when necessary. I guess I'll give some more thought as to whether or not to use them.

Regarding valve seats, Dave, that was very interesting information. Are you recommending against lead substitute for any particular reason? My riding miles are not enough ot make it a cost or inconvenience factor.

Well, that's enough for now. I think my insomnia is subsiding.

Regards,

Richard

 
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Offline Pollock

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Re: A10'1956
« Reply #8 on: 13.11. 2007 13:29 »
No offence taken. I was just giving some information. Good one on the domed pistons. Missed the obvious. I think you will find calling SRM the only way they will suggest doing anything is sending it to there machine shop.  I grew up on a farm wrenching tractors. My old man worked in a hot rod garage for a while. Shaving heads, is an old school car technique from the earliest times. Taking material from the top of bottom of the jugs is a motor cycle thing because jugs are normally less valuable then the aluminum heads.
Many get offended when you modify a classic old part but I assure you there are plenty of parts around for everyone to enjoy in there own way.
I guess I should put a disclaimer in my information also...I usually just shoot from the hip.  Doing a rebuild or tuning for performance means different things to different people. I hear people saying they are doing a rebuild and the engine was sent to so and so. The sheet metal was sent to another. The took the bike to another to upgrade the ignition.  I am not throwing stones but when I rebuild I get my hands in there ...rebuild the engine, Time it, tune it ect.

 And I don't know anyones level of skill but my own..And that is questionable. I have already had a shaved head I was trying to save crack on me. Long story but someone did a homestyle shave of a head.  I took it to the machine shop and had them get them level ...but there just wan't enough left on the old harley heads. The cracked from the bolt holes to the ends when they got hot.

Pollock
BTW my information is given for educational purposes only. Please consult a professional other then this load mouth.
Peace
Jaran,

With due respect to Pollock, I don't believe you need to, nor should, change the height of your barrels or shave the cyclinder head (unless just skimming to make them truly flat). You should just need to get the pistons with the gently domed tops. (The pistons with big domed tops that come to a peak are the 10:1 variety.) As I always point out, I am not a professional or even the most knowledgable amateur on the forum, so, for definitive information, try calling someone like SRM Engineering.   

Richard

Online RichardL

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Re: A10'1956
« Reply #9 on: 13.11. 2007 15:17 »
Pollock,

Thanks for the reply. I understood you completely when you were descibing the barrel/head mods and know that in certain cases they would apply. As for SRM, they have been very kind to me with information. Granted, I am a parts customer, but that hasn't mattered in most cases. If I lived in the U.K instead of the U.S I would readily want them to do my machine work, but shipping costs are prohibitive for the likes of me. You have probabaly seem my bike as "Richard's 55 A10) (top of the bike pictures list right now with a few in-the-works photos also). Reading there you will see that I also did my whole restoration except paint,  wheel lacing and chrome. Though, from one of the pictures, you will see that I tried to lace my own wheels.

Considereing your dad's background, I see where you get some of the ideas. My dad was worked on submarines in the '20s and later became an electrical and refrigeration contractor, so, that's where I get mine.

Richard
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Online groily

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Re: A10'1956
« Reply #10 on: 13.11. 2007 17:26 »
What a great thread. Similar dad's and similar bikes! My old man mended aeroplanes in the navy most of his life - that's where my interest - but sadly with complete lack of his professional skills - comes from!

Re the cause of your lh big end prob Richard/Manosound, not sure about the diagnoses . . . the cush nut shouldn't control the crank end float should it? And nor ought a knackered piston. If the crank's wobbling around I think it's a matter of setting it up with the right shims and making sure the timing side bush is in good shape and the drive side bearing ditto. But even if it the crank moves a bit more than it should, the rod should stay in the same place relative to the journal it's on! (How far can it go walkies?) A wobbly piston might wobble in its 'ole, but it shouldn't make much happen down below, and certainly not cause the bearing shells to up sticks left or right. I would rather suspect the crankshaft oilways/sludge traps and the oil pump itself (plus look at the drilling from pump to the drive side main bush). The lh end is furthest away and will suffer first from any shortage of the slimy stuff. I very well remember having a left hand big end seize light years ago cos of a dodgy oil pump worm gear which went awol and caused a monumental catastrophe - in fact it was the thought of the cost, even then, of trying to fix it without any money (I'd used up all my stock of tired bottom ends) that made me give the bike away! Groily.
Bill

Offline a10gf

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Re: A10'1956 (modifying compression ratio)
« Reply #11 on: 13.11. 2007 18:52 »
Great posts. Looks like Jaran got a thick book's amount of info to evaluate ! Tell us what you choose to do.

e.

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Online trevinoz

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Re: A10'1956 (modifying compression ratio)
« Reply #12 on: 17.11. 2007 20:37 »
Manosound, I think you will find Flashes used thin flange barrels and cranks until '57. Rockets had thick flange and large journal cranks from at least '56. Someone may know whether Rockets had the heavy duty items prior to '56.

Online RichardL

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Re: A10'1956 (modifying compression ratio)
« Reply #13 on: 18.11. 2007 02:30 »
Trevinoz,

I think BSA was somewhat schizophrenic with mixing parts and blurring the differences on various models for the '55 model year. My crankcase number is CA10 4930, which would be interpreted as a '55 Golden Flash, but the case is also stamped with HHC and my barrels are 67-1210 (with the 7/16"-thick flange, which I am rather certain is the thick one; correct me if I am wrong). These barrels are listed in my parts book as Road Rocket. Now, I thought I had other text backing up this point, but I cant find it. Instead, I'm showing, below, an excerpt from British Only's catalog showing that my barrels were available in '54:

01-67-1210   CYLINDER,A10,A10RR 1954-1962

Should we assume this is accurate?

Since I've only owned the motorcycle a short time (since 1973, LOL) I can't really say what happened to the bike before I got it and what model it was designated when it was new. One hypothesis I have is that it was a dual-duty street/scrambles bike built for the U.S. market, as it has a scrambles gearbox. On the other hand, there are a variety of non-original parts on this bike, so it really is a guessing game. If I wanted to take the time right now to browse some of the brochures on line perhaps I could put the story together better. Also, I know there are folks out there who could answer this definitively, but I'm not one of them.

     


 
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Online a101960

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Re: A10'1956 (modifying compression ratio)
« Reply #14 on: 18.11. 2007 10:17 »
Manosound, you have opened a whole can of worms here! Leaving aside the issue of expediency whereby over the years previous owners may well have substituted whatever parts that might have been available in order to get the bike back on the road from all kinds of sources, in the sixties in the U.K. the cafe racer cult  saw many people mixing and matching many parts in order to give their bike street cred.
It is well known for example, that the RGS was originally spawned by Eddie Dowe of Banbury building bikes to customers specifications, that is to say converting Super Rockets with frames to Gold Star frame and cycle part specifications. Exactly what the absolute truth is, is a matter of speculation. If you have an RGS with an Oxfordshire registration number (UD or BW) then there is a good chance that it is an Eddie Dowe conversion which would not conform to any BSA catalogued model. So, this raises the question of weather or not an Eddie Dowe RGS is a genuine RGS or not. Genuine BSA RGS's had a unique frame number sequence (GA10), but the Eddie Dowe models would have had a standard BSA Super Rocket frame number. All RGS engines shared the same engine number sequence with the Super Rocket. Nobody would question weather an Eddie Dowe RGS is the real thing or not, because in many respects it could be argued that the Eddie Dowe conversions were the only real real RGS's ever made, because BSA never produced an RGS until after Eddie had demonstrated that there was a market for such a bike. The RGS is a good example of how relying on catalogue specifications can lead to all kinds of confusion. Even when BSA produced their own RGS the list of options was endless, and therefore there were many permutations of what customers could order. It's all in the detail, and even on more mundane models, BSA in common with all manufacturers would use what ever parts were available to keep the production line moving. There may well have been a base specification, but the reality is that alternative parts would be substituted as and when the need arose to facilitate production line requirements. It is next to impossible to state with absolute certainty that all bikes conformed to catalogue specification. The BSA Super Rocket was fitted with two different camshafts for example, from 1958 until 1961 it would have been a 67-356, and from 1961 to 1963 a 67-357 so during 1961 you could have had either one fitted. An Anoraks nightmare I would suggest!