Advice needed - engine tightening up - help - tips

[A10Boy]

Its only my opinion, but I wouldn't put any additive in the oil which would prevent the rings bedding in. If the rings don't bed in properly they "glaze" the bores, it will always be tight and will always use oil.

The sensible thing to do for a long term solution in my view is take the head and barrel off and check the ring gaps in the bores. Maybe the dude who rebuilt it forgot to "gap" the rings and when it heats up, it tightens.

If you haven't done this before, you take the rings off the piston and push them one at a time into their respective bore and push it down an inch or so with the piston to set the ring level in the bore, then check the end gap with a feeler gauge. You should have a gap of around 13 thou for the compression rings and 11 thou for the oil scraper.

If they are any tighter than that check the piston clearance you should have about 4 thou piston to bore clearance - if you have, its bored to the right size, so then you can set the ring gaps as above, you do this carefully with a smooth file on the end of the rings while held gently in the hand.

If there is too little piston bore clearance you will need to take the pistons and barrel to a re-borer and have them honed to size.

While its apart, look for seize marks on the pistons, if there aren't any, its not the pistons seizing - but it could still be the rings.

If you've never done anything like that before, I would recommend you get experienced assistance as the rings can break very easily.

If you already knew all that sorry this has turned into another epic.. I'm going for a lager.

[MikeN]

Hi Mike,

As I metioned, it has a new concentrc carb. I have not experienced any pinking but I think I will have a look and see if I can increase the fuel flow from the main jet. It could well be that at lower revs, the mix is OK but as I start to increase speed, it becomes to weak? If there are any other views or advice on the concentric carb, very welcome.

Nigel

 Nigel,
My bike too was fitted with a new (monobloc) carb.But raising the needle cured the problem. It must be worth trying this before a strip-down.
Mike

[nigeldtr]

Andy, Mike,
Thanks for the reply. For my own peice of mind, I will probably have to take the head and barrels off as the memories of pistons scratching away in damaged bores is still all to fresh in my mind even after 30 years! Equally, I would probably never feel confident enough to open her up incase of the sudden, screech, bang and clatter of bits of metal skipping down the road in front of me as I slither to a halt!!

My big problem is time, I bought he A10 to ride and enjoy, not repair, so firstly I will see if I can return it and have a look for something else hopefully more usable. If this fails then I shall have to invest in some tools and try and find some time.


Thanks for the advice on the ring gaps and if I do take the top off,  will I check these out - hope you enjoyed the beer!

Many thanks

Nigel

[A10Boy]

I fully sympathise with your situation, it?s a b?stard when this sort of thing happens.

I hope adjusting the carb is all thats needed. Good luck.

[trevinoz]

Andy, piston clearance depends on whether they are split skirt or solid. I wouldn't run solid pistons that tight.
Probably 4.5 - 5 thou.
Trev.

[A10Boy]

I thought it looked a bit tight, those are the figures from the haynes Bible. Had a quick look in Roy Bacon but couldn't find it in there. I guess 4 thou would be good.

[Brian]

Here is a extract from a BSA manual.

[nigeldtr]

Guys,

Just reading through  the BSA instruction manual (Nov 1950) which came with my bike and to my delight it talks about rings and clearances - I wonder how many "modern" manufacturers would dare to do this, and it says, .010 to 015 for the upper rings and .009 to .013 for the scrapper ring - page 54 section 6.

Nothing like a bit of conflicting information to keep things lively 

Regards

Nigel

[trevinoz]

The service sheet that Brian posted shows the RGS clearance as .001" - .0025".
This is ridiculous for a solid skirt piston.
When I was a young bloke 30 odd years ago I had my RGS barrel bored to this clearance, after all it is in the factory book.
The engine did as Nigel's did, loss of power and tending to nip up.
When I finally pulled it down I had scuffed pistons as well as a tight bore. A fair bit of honing and polishing of the pistons and all was good.
Trev.

[RichardL]

I can't believe 0.001" sould have been spec'd for any aluminum piston split skirt or otherwise, in a cast iron cylinder. Coefficients of expansion are very different and I believe that was know before WWII.

[Brian]

There are a couple of clearance recommendations in that list that are suspect if you look through it. When I did my 61' (iron head flash) top end I specified .0045". I used genuine BSA 8.5-1 solid skirt pistons and havent had a problem and I dont run my engines in, well maybe to the end of the street and back.

[nigeldtr]

If I can, I shall take the head and barrels off in the next couple of days and see what is going on. I strongly suspect that I wil have to do as you did Trev, have the bores honed out a bit and see if I can get away with cleaning up the pistons - bit if fine wet and dry I surpose?

I play a lot with steam engines and a driver once said to me, concerning  a very clunky old loco, it is better to hear the bearings clunking then smell them!! (apparently they used to put some "stuff" around the bearings that when they got too hot,  it melted and really stunk and could be smelt in the cab even at speed).

[A10Boy]

That's a true story, they used Garlic oil.

[trevinoz]

Nigel, when you get it apart, measure pistons and bore. If you don't have the equipment, get an engineering shop to do it. Probably a good ides to get them to hone the bore to correct clearance.
If your pistons have marks on them a fine abrasive paper should do the job.
Worst case will probably be grooves down the pistons and the piston skirts collapsed.
Mind you , this is if you have a tight bore. It could be something else entirely.
Trev.

[Josh Cox]

If I try to go above about 55 mph for any length of time (motorway or dual carriage way) the engine tightens up and stops - like someone quickly increasing pressure on the rear brake. When this happened, I quickly pulled in the clutch and coasted to a halt. I found that she turned over straight away and fired up and ran smoothly on tick over

The above is from the original post, I am no expert, but;

If the engine is "tightening up", like some one locking on the rear brake, then stops, then can be started straight away with no problems.

It is not consistent with too close engine tolerances, or any other mechanical part of the bike,  for example if the rings were too tigh in the bore, it would have to take atleast 10 inutes for them to cool and subsequently contract to allow movement.

Sounds to me more like a fuel restriction or in the very distant possibilty a wiring/switch issue, I'd start with the carby and fuel lines, turn on both fuel taps, its quicker, easier and will cost very little.

If it was the magneto condensor it would probably take a while for it to cool.