The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: tinu on 27.08. 2019 22:03

Title: Possible cause for seizing
Post by: tinu on 27.08. 2019 22:03
Good evening lads,
I have had a seizing engine early this year, but I didn't find the time to take off the head and the cylinder to have a look at the damage. Now there is some spare time (what a wonder) and the damage was very obvious (look at the pictures). I found a guy close to me who makes the bores fitting new +40 pistons. He thought of insufficient oil feed to the piston(s). As you may see, there are bad scars on the rhs piston, much less on the lhs one.
Is there a possibility, without further disassembly, to check oil feed etc.? What else should I think of...?
The engine was rebuilt by the owner before the previous owner (run 5k since then) and I do now almost nothing about his capability on engines... except he was a motorcycle mechanic. The bike ran smooth and nice, no smoke, no suspect noise (except last run round the block I aborted quickly).
Title: Re: Possible cause for seizing
Post by: trevinoz on 27.08. 2019 22:08
Not enough piston to cylinder clearance.
Title: Re: Possible cause for seizing
Post by: ironhead on 27.08. 2019 22:13
Hi Tinu.
Seizure on the sides of pistons can be 1 of 3 things.
1- bent rods.
2- bores not bored square to cylinder base flange.
3- crankcase deck not square to crankshaft.
 Most likely the rods are bent (they bend easily especially if gudgeon pins are hammered in   *problem* )
Title: Re: Possible cause for seizing
Post by: Slymo on 27.08. 2019 22:30
Or as previously mentioned lack of initial clearance or insufficient oil. I personally have issues with other peoples rebuilds. I have had several bikes that had potentially fatal flaws from previous setups. One Royal Enfield had its big end replaced with an expensive roller bearing setup but the old white metal from the previous failure had not been removed from the oilway. Another bsa had an oilway blanking plug left out of the crankshaft. If the bike is sold in a not running or not running well condition I tend to pull them down to component parts.
Title: Re: Possible cause for seizing
Post by: Triton Thrasher on 27.08. 2019 22:46
Not enough piston to cylinder clearance.

Yep, the classic four corner seizure.  Can be caused by inducing pinking by labouring the engine, but on a recent rebore with no piston crown damage, it’s almost certainly lack of clearance caused by an ignorant engineer.

Nothing complicated like oil or bent rods involved.
Title: Re: Possible cause for seizing
Post by: coater87 on 28.08. 2019 01:12
 Flat top pistons, is that a compression plate or are my eyes deceiving me?

 Lee
Title: Re: Possible cause for seizing
Post by: tinu on 28.08. 2019 08:19
Thanks a lot for all your answers!
Can a bent rod be checked when still in the crankcase...?
My first idea was the piston clearance, when I read old threads in the forum. The guy at machining shop told me that the clearance is already included in the measurement of the piston. That means the +40-piston mesures only 70 mm plus 28 thou (40 thou minus 6 thou clearance on both sides), ist that correct?
@Lee, you're right, it has a compression plate...
Title: Re: Possible cause for seizing
Post by: BSA_54A10 on 28.08. 2019 08:19
take the pistons off and then photograph them sitting on a table with good light .
To me looks like too little bore clearance.
Pistons could very well clean up enough to go back in with a hone
Also some photos of the inside of the pistons to verify there was oil up there
And with pistons off thread a length of dowel through the little end eyes to check for truth.
They have to come off anyway so while it is apart costs nothing to check
Title: Re: Possible cause for seizing
Post by: Triton Thrasher on 28.08. 2019 08:24
I don’t believe there was 6 thou piston skirt clearance.

Title: Re: Possible cause for seizing
Post by: Swarfcut on 28.08. 2019 08:48
Hi tinu, just reading your post again and again to make some sense out of it.

Looks as if the bike has done some 5K  Km with no problems on plus 20 flat top pistons with a  compression plate on the cylinder base. Then you mention plus 40's.  Does this mean you have had it rebored to  plus 40, but need to be sure the oil supply is OK?

 If the plus 20's seized soon after being put in, then the cause is most likely a too tight bore/piston clearance. If it suddenly happened, after some uneventful regular use, then something is wrong.
 If you still have the barrel on plus 20, get the bore honed, clean up the pistons, make sure the rings are free and try again. It may rattle a bit, but you know why, and it is worth doing before spending again. Of course the perfectionists will be in horror, but they are not spending the money.

 Without stripping the motor, the oil pump output can be checked by removing the pressure relief valve....you should get a good  dribble of  oil when you rotate the crankshaft. You can also check for any lift in the big ends, as an indication of any bearing problem due to lack of oil.

 A couple of additional posts were added  while I was typing this.  Bent rods are unlikely if the piston damage is more or less consistent on both pistons. Straight rod across the small end bushes, as suggested. Heating pistons with a hot air gun means the pins will push out with very little force.

 Piston manufactures state the nominal bore, plus the oversize, and this is the size the finished, honed bore should be. The piston is manufactured to a size which takes into account this running clearance. Some machinists discard this information, measure the piston and add on a clearance from their experience, usually with water cooled engines which run to a tighter bore than air cooled motors. Some are not too happy to be told they got it wrong, so folks don't complain and just go somewhere else, so the guy continues to bugger pistons and bores for years, unaware of the misery he has caused.
 Clearance is the diameter of the piston measured at the pin height, at 90 degrees to the pin, compared to the bore diameter. Pistons are slightly oval, being narrower each side, compared to thrust faces, so the measurement is the thrust faces, front to back, not the pin end sides. The piston crown above the top ring is narrower than the skirt, which is why you can sometimes see the top ring when you look down the bore and foolishly convince yourself it's worn out and needs a rebore! The piston running clearance is in the BSA published data for your model year.

 Swarfy.
Title: Re: Possible cause for seizing
Post by: Greybeard on 28.08. 2019 10:25
The piston crown above the top ring is narrower than the skirt, which is why you can sometimes see the top ring when you look down the bore and foolishly convince yourself it's worn out and needs a rebore!
That is great advice  *smile*
Title: Re: Possible cause for seizing
Post by: tinu on 28.08. 2019 10:48
Hi Swarfy
As you read right I did 5k km with no problem (smooth riding, no racing) with +20 pistons and compression plate. Sometimes there was little pinging on steep hills. The seizing happened on top of a mountain pass (Susten, 2'260 m above sea level) on a straight street running aprox. 80 km/h.
I had a close look at the bore with the machining guy. The scars where too deep, he said they will still be there after just a hone. So I bought a set of new hepolite flat top +40 pistons...
There was an equal amount of oil inside both of the pistons, but when I removed the gudgeon pin, the hole was damaged. I'll have a quick look at the shop to make some more pictures from the piston and the bores. And I'll talk again to the guy about clearances.  *doubt*
I'll keep you updated...
Title: Re: Possible cause for seizing
Post by: Triton Thrasher on 28.08. 2019 11:17
Generally, if it can do 5000 km it should really run forever.

Never let it pink though. Change to a lower gear.
Title: Re: Possible cause for seizing
Post by: Swarfcut on 28.08. 2019 12:42
 tinu, Just had a look at the Susten Pass, Switzerland. Thanks to Google. Looks a truly spectacular road to ride, it is listed in the category "World's most Dangerous Roads"

 I think it's a case of the motor working hard, with  perhaps a bit too much advance for the fuel being used, despite the compression plate, and the mixture on the weak side for the altitude. Overheated, seized and nipped up before too much damage done. TT is right, never let it pink, back off, change down, even add a bit of choke to richen up and aid cooling. Looked to be a reasonable and reliable build until challenged with something out of the ordinary for most of us, maybe everday riding for you.

 New stock Hepolite Pistons are not the same as the ones we trust from UK maker AE (now no longer in business as such after numerous takeovers). The new style packaging features simulated greasy finger prints as if they can't be bothered to use clean hands to pack them, and being distributed by W*SS**l is your guarantee of hassle. There are numerous threads on the forum about the quality of the various commonly available rings and pistons. Go by their suggested running clearance, as the modern materials used are unlikely to match the expansion rates of original pistons, and makes published data from way back redundant.

 Your new pins should just slide into the small end bushes, make sure the bushes are OK and the oil hole matches the one in the rod, as the bushes sometimes creep on the small end eye. Was the damaged hole on the pin boss of the piston, or the rod? Have a good look at the base of the clutch (Drive) side con rod. There should be a small bleed hole that points towards the central flywheel.

 Swarfy.
Title: Re: Possible cause for seizing
Post by: muskrat on 28.08. 2019 22:35
G'day tinu.
As mentioned by others most likely not enough piston clearance, hard work going up the mountain and pinging. If the ignition timing or carburation is a little biased to one side that side will heat up more than the other.
To check little end bush's I have a shaft the same size of the pin about 10 inches long. It should pass through both rods.
My last nip up on the A7 plunger had similar damage. (purists look away  *eek*) I just cleaned up the piston, give the bore a light hone and put the bad piston in the good bore and the good piston in the bad bore with new rings. Done over 3000 trouble free miles since.
With those flat tops and a compression plate you could run the bike on kero  *roll*
Cheers
Title: Re: Possible cause for seizing
Post by: ironhead on 29.08. 2019 00:02
To check little end bush's I have a shaft the same size of the pin about 10 inches long. It should pass through both rods.


Hi Tinu,  As Musky's suggestion above.   This pin must be the same size as the gudgeon pins ( not a loose or rattle fit)

If it slides through smoothly ,both rods should be straight. After this I always set 2 parallels, 1 on each side of the crankcase flange & rotate crank forward gently until the checking pin(mandrel) lightly rests on the parallels. If both touch the same, all good. Then I do the same rotating the crank backwards. If not , the case flange is not parallel to the crankshaft. If good rotating one way & not the other, there is a crank problem.

  These are 2 ways of quickly checking if everything usable. Looking again at your right piston, there is heavy scuffing from the top ring to crown on the right side & none or very little on the left. The piston has been running heavily to the right side of the cylinder wall ( bent rod. Mis aligned  bush) 
The left side piston looks like as others have said to be a normal heat / clearance  seizure, there wear & markings look similar all the way round.
You mentioned a damaged bush , was it on the right side?
Hope this helps.
Ironhead.
Title: Re: Possible cause for seizing
Post by: tinu on 29.08. 2019 08:38
I have made a few piccies yesterday: both pistons from all sides plus one from the damaged pin hole.
I will check the rods and make the parallel test asap.
Musky, I think I don't reuse that Piston again... *eek*
Left hand side in this post.
Cheers tinu
Title: Re: Possible cause for seizing
Post by: tinu on 29.08. 2019 08:38
Right hand side...
Title: Re: Possible cause for seizing
Post by: tinu on 29.08. 2019 08:40
Damaged gudgeon pin hole...
Title: Re: Possible cause for seizing
Post by: BSA_54A10 on 29.08. 2019 09:33
Thanks for the photos

Because we all interpret things differently. This is what & see and why
1) The outside tops of the pistons were clean while the underside is heavily oxidized and shows burned oil , =  both cylinders way too hot ( lean burn)
2) burnt oil between the rings adds to the above & indicates that the cylinders were in fact getting oil
3) oil under pistons again shows plenty of oil in the bores
4) Both pistons showing more damage on one side than the other,
a)  if this was the inside of both pistons = overheating  because the outside cools better than the inside
b) if it is the same side on both pistons = crank not true to the bore ( usually because bore is not true to crankcase mouth or the bores are off set with respect to the journals )
Car cylinders are generally indexed off the head but motorcycle cylinders must be indexed off the base flange.
5) no scuffing on the thrust face so the clearance looks to be OK

What do the little ends look like ?
nice & silver or stained brown ?
Title: Re: Possible cause for seizing
Post by: ironhead on 29.08. 2019 10:18
Hi Tinu.
Could you post some pics of the top part of the rods?
Title: Re: Possible cause for seizing
Post by: tinu on 29.08. 2019 12:27
Short lunch at home, take some pictures, back to work...
Title: Re: Possible cause for seizing
Post by: ironhead on 29.08. 2019 23:28
Hi Tinu.
Many problems there, A few things that stand out at a quick look to me are
* Right hand rod has bash marks around the hole.
* Same rod has been running hot.
* Left bush hasn't been reamed properly ( not supporting the bush full length)
* Rods have a lot of schrapnel damage.
This alone would have me pull the engine completely down & start from scratch.

A couple of other things, one of your cam lobes has wear marks. Also looks like the crankcase flange may have been machined at some stage. ( never seen those stamp marks on any cases I've seen over here but may be different overthere) . A few studs look like there starting to pull as well.
Others will probably see other things but there is a start.
Have you tried the mandrel tests yet?
Ironhead.
Title: Re: Possible cause for seizing
Post by: BSA_54A10 on 30.08. 2019 05:53
Thanks for the pickies.
Basically doubleing up on Iron Heads posts
Pull it back apart
Some one has bashed the RH gudgeon pin with a 10lb hammer without aligning the piston properly resulting in that nice big divot.
IT will have to be removed or the rod is likely to let go along the dent line
In fact both rods will need a clean up. All those nicks have to go or you are riding a time bomb.
Neither little end bush looks good, I would replace both and ream them to the pins.
If you don't have a reamer most engine rebuilders will do it for you .
Car pins are tight on the rod & loose on the piston.
Bike engines are tight on the piston & loose on the rod.
With those divots in both rods I seriously doubt either one will be strait.
They can take a pounding up & down but sideways they bend really easy.

Not good news but neither is a big blow up.
Anyway is it coming into winter up there so shed time .
Title: Re: Possible cause for seizing
Post by: Swarfcut on 30.08. 2019 08:44
tinu, You have come, as we say, to a fork in the road.  The bike ran OK for you up until being subject to some really hard work, overheated and seized. So you could just put in your new pistons and things would be just fine. Maybe.....

 Looking at what you have, for any serious fast riding or long distance reliability I would be inclined to strip it to the last nut and bolt. This engine has signs of heavy handed abuse in the past, such as bruising to the small end eyes, burred crankcase join where the breather passage runs. Previous posts mention important tell tale signs that all is not as good as it could be. All signs that whoever built it was not as good a builder as you would like...smacking pistons with a big hammer is not a recommended workshop practice.  Stripping it will allow you to check every part and replace or refurbish as necessary.

 The camshaft is also showing signs of wear, so easily replaced or  gently stoned smooth, depending on your budget or future plans, while all is apart. Cam followers will also be wearing, so again, check here. The crankshaft sludge trap can also be checked, this is a well known failing.

 The con rods need to be checked by your machinist. If they are bent, replacement is the only sensible option. Big end eyes need checking for ovality, and if they are found to be at fault, they can usually be reworked. Have the small end bushes replaced and accurately sized to match the new piston pins. A good firm hand push fitting at room temperature is about right. With the pistons warmed up with a heat gun, the pins will slide into place, needing just a little hand pressure. No need for violence or big hammers. Have the rods polished to remove nicks and burrs, and matched for weight.

 Check the crank cases are a matched pair. With the engine removed you should find matching numbers stamped on the front faces of the cases, below the dynamo mounting, hidden by the front engine mounting plates. Mis-matched cases are not the end of the world, but could possibly be a factor in your problems.

 Compared to modern machines, this engine is relatively simple to work on, but requires attention to detail, extensively covered by the thousands of posts on this forum.

 At the end of the day it is your choice, depending on your budget and how much reliable use you want from the bike.

 Swarfy.
Title: Re: Possible cause for seizing
Post by: tinu on 30.08. 2019 09:05
Thank you guys for all your advice given! I was expecting to abuse my family and work all night in the shed one time...  *work*
Although I am not racing with my A10, there are various steep hills in our country that are a joy to ride. Therefore I will take the long way.
More question (and pics) will pop up, definitely!  *smile*
Cheers Tinu
Title: Re: Possible cause for seizing
Post by: muskrat on 30.08. 2019 10:24
G'day tinu.
Wise choice mate. We are all here to help, not trying to give you work or spend your money. In the long run if done right you will have a bike that will eat those hills without trouble.
The only reason Pommy bikes had a bit of a bad reputation was the back yard, ham fisted morons who worked on them.
Cheers
Title: Re: Possible cause for seizing
Post by: BSA_54A10 on 31.08. 2019 10:18
And you can never tell.
Shane bought Doug's old A10  as a spare  loaner bike for the 55th international.
Doug had run it for 15 years with few problems
On the test run up the Snowy Mountians a little end let go.
Had we have known what the bike was like inside it would have had a full strip down
'Both rods were unservicable having been bodged at both ends and they were not a pair
Every bush inside the engine was mashed from being fitted with a pin punch and horror, the crank was cracked full circumference.
Bolts had been drilled out and tapped randomly in random profiles from metric to AF dowel pins were missing oil pump was loose.

Preso Pete bought a rocket that another member had spent 5 years rebuilding .
HE sold the bike because he hurt his back & was no longer able to ride.
The DPO thrashed the bike when he rode it so again we thought it would be a good buy at the price.
$ 6000 latter is replacement parts the ideas are a bit different and how the DPO did not kill himself is beyond belief.

Winter is coming up so some time in the shed will not distract from possible road time.
Title: Re: Possible cause for seizing
Post by: berger on 31.08. 2019 13:21
" ham fisted morons who worked on them" YEP that's meeee musky when I was trying to get the front wheel spindle out as a young one does and my dad was working on his lathe and heard this click - bang sound and shouted ITS LEFT HAND THREAD. he had to make a new spindle a bit longer with a nut to fit as well for safeness as I had also slightly damaged the leg . still on the bike today--- cheers dad *thanks*
Title: Re: Possible cause for seizing
Post by: BigJim on 31.08. 2019 19:55
" ham fisted morons who worked on them" YEP that's meeee musky quote)
I'm still learning and pleased to have a vice!
Generally try to improve it and often make it worse.
Someone has to be last when it comes to toolwork.
Title: Re: Possible cause for seizing
Post by: tinu on 19.10. 2019 19:16
It took quite a long time to go a step further with my engine...
As you proposed, I had a shaft made to fit the small end properly. The result is quite ok as far as I can tell. The shaft fits smoothly into both small ends and all seems to be straight and parallel.
I will ask at the shop to change the small end bearings. They look not good. Maybe the guy at the shop can do some other checks to be sure...
still a lot of time left until next summer...  *smile*
Title: Re: Possible cause for seizing
Post by: RDfella on 20.10. 2019 12:34
It is possible to change SE bushes in situ, but much better to have the rods off the crank. That's a full engine strip, I'm afraid.