The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: bsa-bill on 01.05. 2010 19:39

Title: Nipping up
Post by: bsa-bill on 01.05. 2010 19:39
HI folks - a friends rebuilt A10 ( 57 I think) tightened up and the other day, locked the rear wheel, pulling the clutch in he got it pulled into the road side he then was able to kick it over.
Prior to this he has been having trouble with what sounds like a single tappet noise, this occurs at a specific rev range above and below this it is quite quiet
Other things we thought it could be were a broken ring, small end, lose valve guide.

Is it possible after a partial seizure for the engine to free in such a short time

He does think that the engine has been running hot and has lifted the needle a notch, it also blew a head gasket a month or so back

Any suggestions , I'll pass them on

All the best - Bill
Title: Re: Nipping up
Post by: muskrat on 01.05. 2010 20:37
G'day Bill,
             if the nip up is due to over heating they don't take long to free. coasting for 100 yards with the clutch in will do it.
Sounds like a loose guide and might be due to a weak mixture at that rev range/throttle opening creating too much heat allowing the guide to move. I have had it happen. Try a richer slide and or main jet depending on where the troublesome spot is. It may also have played a part in the head gasket leak.
Title: Re: Nipping up
Post by: MG on 01.05. 2010 20:46
Hi Bill,

just as a matter of interest: Did the rebuild include boring the cylinder and new pistons? If so, what make and what clearance?
Somehow this story sounds familiar to me.......

Cheers, Markus
Title: Re: Nipping up
Post by: bsa-bill on 01.05. 2010 21:05
Yes Markus full rebuild, rebore and pistons, also new guides.
Don't know make of Pistons but will find out
Title: Re: Nipping up
Post by: Triton Thrasher on 01.05. 2010 22:02
Seizing after a rebore points to insufficient piston/bore clearance.
Title: Re: Nipping up
Post by: Brian on 02.05. 2010 07:05
Like it or not he needs to pull the top off the motor and inspect it. There is a good chance the seizure has trapped the rings in the piston.

Like Triton Thrasher says the clearance may not be sufficient. He needs about .004". Also depending on the make of the pistons check the ovality, they should be about .005" narrower across the gudgeon pin side.

If it has scored up the piston and bore which is most likely has then a light hone of the cylinder and a bit of work on the piston will repair it, at worst a new set of rings will be needed.
Title: Re: Nipping up
Post by: bsa-bill on 24.05. 2010 20:09
Update - Drive-side pot needs attention probably honing will suffice
drive-side piston needs replaced according to the shop as the top ring was seized in its groove

I saw the bike today as he has it stripped down and my gaze was drawn to the timing-side piston that is still in situ, it struck me as a high compression (for a Flash)
it looks the same as the 9.5:1 pistons I have installed in my project (Flash with Alloy head,357 cam).
The pistons are Italian made and bought to match the pistons that were in the bike when he bought it, and they do match as he showed me the old ones, they are marked BSA 67-1604, have a wedged top like a squish band  and have recesses  for the valves, I think these are wrong for the bike and are probably for a Rocket with alloy head, I'm guessing the are at least 8.5 and probably higher and may well have contributed to the engine overheating.

Can anybody confirm the compression ratio for these pistons and there intended use.

Many thanks - Bill
Title: Re: Nipping up
Post by: MG on 24.05. 2010 20:57
I was almost certain you would tell us it were Italian pistons (GPM I assume).

Have a look at the last post in this thread I wrote:,1966.0.html (,1966.0.html)

The GPMs I had were 8.5:1 and seized twice.
Now I'm using 9:1 Wisecos (that's the only CR available with these) which work without any problems with the iron head, although I've really ridden much harder than with the GPMs (was still running her in carefully then).

I'll never buy a set of that Italian yeast piston crap again, that's for sure.
Title: Re: Nipping up
Post by: Brian on 24.05. 2010 23:24
Bill without seeing the piston its hard to make a comment but having the ring trapped in the groove is no reason to replace the piston. If the piston is badly damaged then the piston will need replacing but usually you can clean them up. It takes some work with a small file to repair the groove and make sure the new rings are free in the grooves.

The pistons you describe do sound like 8.5 to 1. I dont consider the extra compression would cause it to nip up. Lack of clearance is almost certain to be the cause. It really does pay to find a machine shop that has experience with older engines to do your rebores. Most modern shops that deal with modern engines cant get their head around the clearances required on our older stuff and almost certainly will go for closer tolerances that needed.

The jury is still out as regards GPM pistons. I have a friend who uses them exclusivly in his A10's and has never had a problem but there are others like MG who have had problems. I have always thought they were well made but too heavy (like JP's) but I have not used a set in a A so cant talk from experience. I have used them in singles without a problem.

Unless your piston is badly damaged I would clean it up, fit new rings, give both bores a hone and make sure you have about .004" and put it back together. I would be confident it wouldnt give you any more trouble.
Title: Re: Nipping up
Post by: chaterlea25 on 25.05. 2010 00:02
HI All,
At the moment I am rebuilding a Super Rocket engine, the owner is well into his retirement so we decided to detune the engine a little, 7 to 1 pistons and a 356 cam,
I am presuming the pistons are GPM, they arrived with out boxes?
When I measured them up I could detect a difference between the pair!!
I mentioned this to the machine shop who did the bore and hone and they confirmed my measurements
there is a difference of about 3/4 of a thou between the two *ex* *ex* *ex* *ex*
So if the guy only measured one piston its feasable that the other would be too tight *sad2* *sad2*

John O R
Title: Re: Nipping up
Post by: Brian on 25.05. 2010 00:19
That is actualy fairly common, a good machine shop always matches each piston to the bore. Mind you like lots of things these days finding a "good" machine shop can be nearly impossible. A lot of them dont have any experience with older engines and some think they know it all and wont listen to what you tell them. I have had a few "heated" discussions with various machine shops over the years. Luckily I now have one that knows what they are doing, mainly because he is a older fellow who grew up working on engines build in the era of our bikes.

Just one short story about so called qualified machinists. About 30 years ago I took a C10 BSA (250cc SV) barrel and piston to get bored, I told him I wanted .004" clearance, he said no worries as he was a expert at doing rebores. I went and picked it up when they rang and it was obvious something was horribly wrong but he said he had given it exactly .004" as I had requested. The idiot had given it .004" on both sides of the piston, total of .008" clearance. Trouble was that was its last bore so I had to get it sleeved and start again, the whole exercise cost me more than I had paid for the bike. *angry*
Title: Re: Nipping up
Post by: trevinoz on 25.05. 2010 01:02
          I know you had a bad experience with GPM pistons but I have used them in my RGS and Atlas and I am very happy with them.
Maybe you got Friday production pistons!
John O R,
                  GPM pistons usually come in a well identifiable box.
Title: Re: Nipping up
Post by: MG on 25.05. 2010 08:16

That could of course be the case, nevertheless, the question is how many "Friday productions" are around.

I found this site here, and this is pretty much what several Vespa/Lambretta guys here have told me about the GPMs: (

Like you, many others use them without problems, however, from my personal experience I can not recommend using them with quiet conscience.

btw: In German we call it "Monday production", as this is the first working day after people were out drinking during the weekend, thus resulting in lower quality products. Funny it is Friday with you guys.  *smile*

Cheers, Markus
Title: Re: Nipping up
Post by: muskrat on 25.05. 2010 08:30
MG, no one works Monday, too hung over. Friday thinking about getting on it !!!
 When I finished my apprenticeship I had to do 3 months extra to make up the Mondays !!! *eek*
Title: Re: Nipping up
Post by: MG on 25.05. 2010 10:19
Maybe I should consider emigrating to OZ.....
 *smile* *beer*
Title: Re: Nipping up
Post by: A10Boy on 25.05. 2010 21:11
I have quickly read thru the thread and didn't see any mention of "gapping" the rings. Remind your friend that he should have about 10 thou end gaps. The old rule of thumb was 3 thou per inch across the piston.

Also, I'm not in favour of high compression pistons in iron engines, it can cause a hot engine.

Also, the engine should be "run in" on mineral oil not synthetic as they don't bed in properly.
Title: Re: Nipping up
Post by: bsa-bill on 25.05. 2010 21:37
Thanks all - spoke to Bob this afternoon (pointed him in our forum direction so maybe he has already read this)
The bore should be ok with a hone but he will need a piston.
He was also told by the shop the rings should not need gapped, I know I have been led to believe this too but always check and found while sometimes some of them are correct others need a rub with a file, may well be true for modern stuff with tighter tolerances.
Title: Re: Nipping up
Post by: Brian on 25.05. 2010 23:46
When you say the shop told him the end gap didnt need to be checked do you mean the shop where he bought the pistons or the machine shop that did the rebore. Either way I wouldnt use that business again.
Title: Re: Nipping up
Post by: A10Boy on 26.05. 2010 10:14

If the bores are at the right tollerance, then the advice that the rings dont need gapping has almost certainly caused this siezure.

Your friend should go back to whoever gave him that duff advice and tell them that they are responsible for the siezure and if it was the piston supplier demand a replacement free of charge. How can a piston supplier know what the bores are, and how can a cylinder borer know what the ring gap will be unless hes checked them after boring?

EVERY component used on an engine build should be checked for tollerance otherwise this kind of thing is bound to happen.
Title: Re: Nipping up
Post by: bsa-bill on 26.05. 2010 10:36
Good point Brian and A10 boy I'm not sure who it was told him, he was looking for the receipt for the pistons but has lost it so not sure where he got them but I have had two rebores done by the local shop without problem and they never mentioned rings so I just assumed it wise to check and gap as necessary, but I do know tractor pistons our workshop have installed came ready to drop in ( having said that we did have a combine engine pick up after a rebuild but that was due to wrong assemble, offset on rods IIRC)
The shop we use is the only one for some distance ( yep distance here in UK is not the same a distance US or OZ I know).
To be fair they are generally pretty good and mostly do good work and will put right anything they get wrong, thing is lots of the engineers that know about our bikes and old engines will be retiring, no disrespect to present day engineers but mostly they are fitters, plug in their laptop and replace the part it tells them to, just like many industries these days - reduce the skill level reduce the labour costs.

rant over (good to have one a day I find)
Title: Re: Nipping up
Post by: A10Boy on 27.05. 2010 17:03
I suppose the lesson here is not to believe what these people tell you. As I said, EVERY component used on an engine build should be checked for tollerance otherwise this kind of thing is bound to happen. I cant imagine putting pistons in any engine without checking the ring gaps, it just dont seem right.

PS a few years ago, I was part of a team restoring a Merchant Navy steam loco. When we fitted the pistons which were 22 inches across [from memory], I did the 3 thou/inch calc and gapped the rings accordingly - the biggest pistons I'm ever likely to fit.  *smile*