The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: michael snellin on 18.08. 2012 17:16

Title: What pistons !!!!
Post by: michael snellin on 18.08. 2012 17:16
Hello,everyone, seem to have a problem now because iv,e bought this bike in bits, urgh, !! not a good idea,?? iv,e had or going to have the barrell,bored to +60 oversize, this is where it gets complicated, what do i fit low com,or high compression, and there seems to be a problem getting hold of flat top pistons,????? any help would be much appreciated, this what happens when you buy these things in bits, and have to source the parts.URGH !!. *sad2*
Title: Re: What pistons !!!!
Post by: Goldy on 18.08. 2012 18:55
Hi Michael. It depends what machine you have as there is no point fitting high compression pistons to a standard Gold Flash. Supreme list 60 thou pistons, have a word with them
http://www.suprememotorcycles.co.uk/product_details.php?ProdID=329
Title: Re: What pistons !!!!
Post by: A10Boy on 19.08. 2012 20:45
In my view [and our racing friends / roadburners will disagree], is that with today's rubbish petrol, the way to make a sweet, tractable and long lasting "road motor" is to go with the low comp pistons, especially in a Golden Flash as Goldy states.
Title: Re: What pistons !!!!
Post by: K1100 on 01.09. 2012 20:02
I'm thinly qualified to respond to this, but I have just bought a Golden Flash with a motor that was rebuilt in 2002 by SRM with 6.5:1 pistons. Acceleration is super up to 65 mph where there is the beginnings of a feeling of running out of energy although acceleration continues. I haven't been beyond 70. Between 45 and 65 the ability to maintain speed whatever the gradient is very enjoyable, and has made the 100 miles I have so far ridden very good fun. The previous owner toured Australia and NZ on the bike two-up so the low compression route gets my vote of support.

John
Title: Re: What pistons !!!!
Post by: t20racerman on 01.09. 2012 21:59
In my view [and our racing friends / roadburners will disagree], is that with today's rubbish petrol, the way to make a sweet, tractable and long lasting "road motor" is to go with the low comp pistons, especially in a Golden Flash as Goldy states.

Sort of..... If you have an iron head, or like to potter round at low revs, absolutely, without a doubt, fit low compression pistons.

If you are a petrol head speed freak with an ally head (on the bike, not you!) no, go for higher comp pistons. Mine is gas flowed, big cam, big carb, SRM conversion, polished rods, nitrided crank etc etc and goes like shit off a shovel with 9.1:1 pistons. However, the pinking is a real pain, and without a manual advance and retard, it would be unrideable. I've thought about the issue of pistons a lot lately and I reckon that 8.5:1 pistons would make a very good compromise, something I might change to in the future.
So really, the best advice is what kind lf rider are you? If you want speed and intend to thrash it, go for 8.5:1 or higher. If you are happy to keep it below 75mph and dont have the urge to beat that Ducati next to you from the lights, go for 7:1 pistons. Both routes have their merits - but only one route suits me!  *smile*

Title: Re: What pistons !!!!
Post by: KiwiGF on 02.09. 2012 01:39
JP Pistons in Australia also sell A10 pistons and I've bought flat tops from them, circa 7:1 comp.
Title: Re: What pistons !!!!
Post by: duTch on 02.09. 2012 12:06
Simon, I guess by now you'll  have a weight on those JP pistons, which also can be discussed in the bf/dyhi??

cheers
Title: Re: What pistons !!!!
Post by: KiwiGF on 03.09. 2012 09:02
Hi Dutch, JP emailed me the weights at 351 gr for the 6.5:1 and 343 gr for 7.25:1. They did not say whether this included rings, pin clips.

Anyway, below are the results of some measuring using a cheap electronic kitchen scale. I checked the scale by measuring baked bean cans of water and calculating the volume hence weight (1 liter water weighs 1000 grams) and I think the scale is accurate to within 2%, but the comparison between piston weights is probably more relevant anyway.

Desc                                                            Weight (grams)             Comment
========                                               =========              ===========
Bare Piston JP brand 7.25:1                      256                                 Bare means no pin, rings, clips
JP Pin                                                          78                                   Not tapered ID, 0.475' ID
JP set 3 rings                                              25
JP clips x 2                                                 1.5                      
Bare BSA piston flat +060 bore               261                                   its "partner" weighed 258 grams
Bare piston 8.5:1 std bore                        241                                  unknown brand
Bare hepolite 9:1 std bore                        251
Pin, tapered ID                                          78
Pin  unknown brand                                   89                                     Not tapered, 0.43 ID, HEAVY!
Title: Re: What pistons !!!!
Post by: duTch on 03.09. 2012 11:01
Ok Kiwi,
         
 I was surprised how accurate those cheap scales appear to be, having also done a few test weights. I bought a kitchen set from the 2$ shop(?) for $15, and two ebay gold weighing sets for 'bout the same total.
   
 My flat top +40 Hepolites are 359 & 361 grm, so that's about the same, pin I.D seems tapered,0.545 at ends.
 
I have 4 BSA 8.5 +20's (various pins),1@ 341g, 2@344g,1@355g(comp.w rings & pins).
 I'd been led to believe that JP's were 'heavy'(bit more than their guess 343<-360), but seems no more than what I'm using, funny thing with JP is domestic freight is same as international, so doesn't matter if I buy them from O/S.....?

 Anyway, Michael, I'd say if you just like to trek around and keep your license, the 7.25's sound the go, it's what I have and shall stick with. When it ran before, it got along just fine.

 Cheers for now, duTch
ps Kiwi am still working up to B/F
Title: Re: What pistons !!!!
Post by: Rocket Racer on 04.09. 2012 02:08
The nicest pistons I've come across are the early road rocket 8:1 but havent seen at +60

Personally have had too many issues with JP's -wouldnt fit them if someone paid me to. Nothing to do with their weight, didnt matter what clearances I ran they kept nipping up and one failed after  the shortest of test rides -suspect had a manufacturing fault as cracked.
Title: Re: What pistons !!!!
Post by: duTch on 04.09. 2012 11:38
Maybe tha's what my mate meant when he said "oih Geez, -they're heavy as buggery and....mmm...oih geeez...'...? basically wasn't encouraging, but I do like to support local industry....If it works.
 Had an email back from British Only Austria, re "Far-East Æ,'..?..they're 320 grms/ea,?109 ex vat
Any feedback on those??

 cheers
Title: Re: What pistons !!!!
Post by: chaterlea25 on 04.09. 2012 20:04
HI Dutch,
Re, "Far East " or new AE pistons, these were/are the ones supplied by Wassel
I posted re these some time ago,
I fitted a set to a customer bike, pistons are fine BUT the rings are CRAP!!!!
I also received a set of ""AE"" rings  recently the oil rings had a 2mm end gap when fitted to the bore  *sad2*
The supplier sent a new set without question, but still its a PITA

I believe Kiwi had problems with pistons he bought from British only???
?109 sounds a lot per piston?????

HTH
John
Title: Re: What pistons !!!!
Post by: duTch on 04.09. 2012 22:21
Thanks John,
                  I do remember reading posts about 'eastern' pistons and rings, it may have been Brian(?) who said he'd using some 'so far, so good'?
 At this stage i don't think I need some, but good to have advance research.

Cheers, duTch
Title: Re: What pistons !!!!
Post by: Rocket Racer on 04.09. 2012 22:45
after my (Norton - since sold) went through two sets of JP's, I put in some taiwanese pistons and they were fine. In my opinion far better!
Wiseco's from cake street classics are another option, could always fit a thicker head gasket or base gasket if compression higher than preferred.
I keep my eyes open for pistons at swap meets so have quietly built up a stash, but always need rings.
Title: Re: What pistons !!!!
Post by: trevinoz on 04.09. 2012 23:00
I have been using the Taiwanese 7.25:1 & GPM 8.3:1 pistons with no issues.

Trev.
Title: Re: What pistons !!!!
Post by: Duncan R on 05.09. 2012 13:46
I also have the Taiwanese pistons, no issues so far. I weighed them before fitting and matched perfect(can't remember the weight) I used my old gudgeon pins as they were a slightly better fit. The price quoted on the site is for the pair.

Cheers

Duncan
Title: Re: What pistons !!!!
Post by: KiwiGF on 06.09. 2012 08:09
Hi Chaterlea, yep I returned some pistons to BOA and got a full refund inc. the freight so no complaints there. I wish I had kept them now but the BOA rings and pins supplied were useless and the cost of buying those separately made the total price rather expensive. BOA could not replace just the duff rings and pins separately at the time.

The only reason I would not (now) give buying the pistons from BOA another go would be that they would not state the manufacturer of the pistons, so you cannot be sure you would get the same pistons as anyone else got!

I bought JP pistons instead, and at least for std bore flat tops I'm using I would say the JP's are not significantly heavier than the BOA pistons, but I am rather concerned about some of the comments made about JP's seizing and cracking etc - I just hope any quality issues JP have do not apply to std bore flat tops!
Title: Re: What pistons !!!!
Post by: chaterlea25 on 06.09. 2012 22:31
Hi Kiwi,
I have only hearsay evidence on the JP pistons for the A series  ???? ????
However I have fitted one to a BSA B33 and its running very well.
The JP's need more clearance than "other" brands
I have a JP piston which was a special manufacture item for the Blackburne 350 engine for my Chater Lea
A batch was commissioned some years ago by Martyn Adams but these have all gone
I  aquired another cylinder and was looking for a plus 20 or 30 piston for it, mucho $$$$$  *eek*
gone off track now havn't I  ????

Cheers
John
Title: Re: What pistons !!!!
Post by: KiwiGF on 07.09. 2012 02:34
Hi Chaterlea, I've a friend with a 55 GF rebuilt with JP's and it's been fine for 30k miles. The need for extra clearance I've heard of before, I guess the JP's must expand a bit more than other pistons, in any event this suited me as my barrel was sleeved to std with some seizure scratches and after those were honed out it gave a clearance of a bit over 005 according to the engineering company. I'll be extra careful running it in.

Regarding the alleged heaviness of JP's compared to other brands I've heard that some brands of piston (maybe JP's?) are all machined on the O.D. from the same "blank" (which is a cast blank in the case of JP's, I think) - this approach is to keep manufactureing costs down, the result of tht is that the oversizes of piston are inevitably heavier, due to the thicker walls.

Do you think there is any truth in that?
Title: Re: What pistons !!!!
Post by: gavinoz on 07.09. 2012 03:59
I've heard that some brands of piston (maybe JP's?) are all machined on the O.D. from the same "blank" (which is a cast blank in the case of JP's, I think)
This is the case, i was just speaking to JP r.e.8:1 pistons
gavinoz
Title: Re: What pistons !!!!
Post by: MG on 07.09. 2012 19:46
Hi Kiwi!
I got JPs in the A7 SS, with no negative things to report whatsoever! I fitted them with a clearance of 3.15 thou (8/100mm), for the larger bore A10 you might want a tad more, around 4 to 4.5 thou I'd say.
The need for larger clearance is not necessarily a sign of inferior material or greater thermal expansion, the main reason is that the JPs don't have slotted skirts for expansion control. The larger clearance might cause some more noise when cold (although I could never notice any undue noise, unless the pistons and bores are REALLY worn). But otoh a solid skirt piston is substantially stronger than its slotted counterpart. Slotted pistons in Norton Commandos are known to fall apart at high revs regularly, owing to the high piston velocity of the larger capacity engines.
At the risk of repeating myself: If you want something really really good and can afford them, go for a set of forged Wisecos from Cake Street Classics!
Cheers, Markus