The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: RGS man on 02.02. 2010 09:09

Title: rebore or not to rebore do I do it, thats the question!
Post by: RGS man on 02.02. 2010 09:09
RGS man
Ok I have stripped off the rocker box and head. the haed is now off to have the plug holes welded and redrilled etc, the thing should be mint when I get it back.
Now the thing is this. Do i take off the barrells (I havent done this before). there seemd to be very little movement in the pistons(which are incidentally +30). I have a feeling that after speaking to dealers etc that I should rebore and fit new pistons etc. The cost isnt the issue, its the fact that I'm scared of doing it in case I totally stuff it up. I will probably get pistons etc from Lyford Classics unless anybody has any other ideas. Its just fitting rings and getting them back in the barrells thats freaking me out.
Some imput would be great.
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: Mosin on 02.02. 2010 09:36

I did this a few months ago on my A7SS and, like you, I was scared of mucking something up, but with a lot of help and support from forum members I decided to tackle it. It was actually a doddle! In fact, getting the head back on and the pushrods aligned into the rockerbox was much more difficult and time consuming than refitting the barrels.

The only problem I experienced was that some of the piston rings were the wrong size, so I would suggest trying them ALL in the bore of the barrel first and checking that the gap is correct. Don't just assume that every ring in a packet is going to be the same size!

Other than that, everything is in the preperation:

When you come to refit the barrels, make sure that they are well suported on the top of the crank case with some bits of wood (it's worth taking the time to fashion a couple of bits that are exactly right for the purpose) Make sure that these don't scuff the new base gasket that you have fitted. Then make sure you use a pair of piston ring compressors which you have oiled slightly on the inside. They need to be tight enough to compress the rings but not so tight as they won't slide down as required. After this it's just a case of sliding the barrels down evenly over the pistons and once they are down removing the compressors. I have heard tell that this is a two-man job, but to be honest I did it myself with no difficulties whatsoever. I think that the trick lies in making sure that the pistons are really well supported underneath so they sit absolutely still during the process.

Be brave, like a lion, and let us know how you get on!


Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: Rusty nuts on 02.02. 2010 09:59
If she runs fine with good compression, no smoke & no piston slap I wouldn't bother unless you got money burning a hole in your pocket.
With the head off you can see the state of the bores with pistons at BDC.

Removing & replacing barrels/pistons is straightforward job requiring no special skill even if not mechanically minded.
I think you'll find replacing the head (pushrods) a trickier job.
Putting it back together is easier with two pairs of hands & piston ring clamps, but not necessary.
Fitting rings to piston without breaking is simple if done properly. I just use three pieces of card to ease on, remember to gap them first (mucho important).
If you do remove barrels pack a rag in the cranckase mouth as you lift barrels off & leave there until it's all going back on.

As a hamfisted teenager I managed to drop a circlip into the bowels of my Triumph & spent an hour outside in a thunderstorm with a magnet on a bit of string fishing around before retrieving it.
Lesson learned!

If you have a workshop manual (even a Haynes) you should be able to figure out the process & there's loads of help here on the forum if you get stuck.

Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: brackenfel on 02.02. 2010 17:24
Just my small contribution but, like others have suggested, if it were mine I'd  see what I'd got before planning too far ahead. You can probably see the state of the bores by getting the pistons to rise & fall. If there are no score marks or lip at the top & still wanted to be sure you could then lift the barrels, remove the existing piston rings and check them all in the bores for gap at both top & bottom of the piston travel.

You might fnd that you only need new rings. If you go this route take care when dressing (filing) the new ones to get the gap correct, it's very easy to take too much off & you can't put it back (as usual, don't ask how I know....!)

Good luck!
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: groily on 02.02. 2010 17:54
I'm with Rusty nuts and Adrian all the way. If it doesn't burn oil and the pistons don't slap around and the bores look undamaged, I'd leave well alone. No point in wasting money. Problem is that the 'Is it worn out?' question is about the hardest one to answer without seeing and feeling the part. One man's 'OK' is another's '(s)crap'.
If you start by thinking that a cylinder needs about 2-3 thou per inch of bore of clearance for the piston . . . and doesn't want an appreciable wear ridge at the top of the bore where the piston stops . . . that's where I begin, anyway.
Personally I err on the side of meanness to get as many miles out of every bit as possible (and I get it wrong sometimes). But there's nothing so very wrong with a bit of legitimate top-end wear provided the oil consumption is manageable. Well-run-in and dependable engines that have done lots of miles are pleasant things - and far more fun than tight newly-built ones where you're always worrying about clearances, heat build-up and all the rest of it.
Perfection is philosophically fine but fleetingly ephemeral . . .  
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: Goldy on 02.02. 2010 20:00
Myself I do not like piston ring clamps because once you have pushed it all into the barrel you do not know what has happened. If you have broken a ring you would not know. I prefer to support the barrel and enter one piston in to the barrel, and then compress the first ring with two small electrical screwdrivers. It's easy once you get the idea and you can see the ring go in. Turn the engine slightly to raise the piston and work through the rings in turn. Also dont forget you will need to warm the pistons to remove and re fit the gudgeon pins. All the best.
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: rocket man on 02.02. 2010 20:30
hi RGS MAN ive fitted new rings what i did was i tied some very strong rope around the barrels and tied it to the top tube
and pushed each piston ring in slowly with fingers spaced them out so the gaps dont all line up until both pistons where in
job done  *smile*
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: BSA_54A10 on 06.02. 2010 10:23
Just to throw a spanner into the works.
A old racer in the club showed me how to support the barrels and turn the crank to fit pistons.
Mush easier than doing a contortionist trick while holding a 15 kg iron casting 3" above the crankcases
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: jfligg on 06.02. 2010 12:32
Hello RGS man
  Heres my 2 cents.  In my opinion the only way to be sure is to measure the bores.  Measure cylinder wall clearance,  Bore tapper and bore out of roundness.  If you don't have the tools take it to a machine shop.  hat way there is no guessing.  Jeff
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: chaterlea25 on 06.02. 2010 23:28
Hi RGS man,
Lift the cylinders and inspect the pistons, rings, conrods, bigends for "rock"
and check the cam and followers for wear,
Wear on the cam and followers is the first sign of bottom end trouble *eek* *eek*

If all is well you will have away more confidence in your motor *smile* *smile*
A few minutes will lift the cylinders and it should not take too much time to check out and reassemble
John O R

Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: A10Boy on 15.02. 2010 19:41
So how would you know if you had broken a ring on assembly? Presumably the broken bit would fall out if it was trapped and snapped off. I'm guessing the end of a broken ring would score the bore and that would be visible when turning it over prior to fitting the head

Would it make a noise? anyone ever actually broken a ring where the piece didnt snap off and fall on the floor?

Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: muskrat on 16.02. 2010 09:34
g'day Andy,
                Yep, didn't notice for a few laps, then started smoking. No noises but down on compression. 2nd ring was broken and scored the bore. But on the other hand I have seen motors with broken rings run for quite some time and only discovered at rebuild. The worst for me was a 3 piece oil ring that had the last piece come out of the groove on assembly. Took just 2 minutes to destroy the bore.
 If one breaks opposite the gap it will most likely stay put and not be noticed for some time. If it breaks into small pieces a bit might dislodge and wedge between piston and bore. This will be evident much more quickly.
 I use a piece of poly pipe ( plastic drain pipe) about 1 1/2" long and cut through to be just smaller than the piston and clamped with a hose clamp till its tight then back off till it just spins on piston. Cheap to make and works for me every time.
Title: Re: rebore or not to rebore do I do it, thats the question!
Post by: A10Boy on 16.02. 2010 14:07
I am always concerned with the risk of breaking a ring, so after fitting the barrels and before the head goes on, I always turn it over carefully listening and feeling for any horrible scraping as i turn the end of the crank.

On my last rebuild, I must have kicked it over 200 times before fitting the head, just to make sure there was nothing nasty going on with the bores.

Its one of those things like when you go on holiday, you wonder if you remembered to turn the telly off! You can take pics of the circlips after fitting so you can remind yourself yes you actually did fit them, but you cant see the rings.