The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: chotus52 on 14.05. 2017 11:07

Title: Seizing engine
Post by: chotus52 on 14.05. 2017 11:07
Hi all, my 1955 shooting star was rebuilt about 4000km ago. It's fitted with 8.5:1 pistons & thorspark ignition. It runs lovely except when cruising at 100kph for maybe 10km the engine locks up. By the time I roll to a stop, the engine kicks over & starts normally. It seized twice & I have since done a compression test & have 160psi in both cylinders. Any thoughts?
Title: Re: Seizing engine
Post by: orabanda on 14.05. 2017 11:13
What brand pistons are fitted?
Title: Re: Seizing engine
Post by: JulianS on 14.05. 2017 12:16
Check the oil feed.

When it was rebuilt was the oil pump to crankcase gasket one with holes which were the same size as the holes in the pump body? Some gaskets being offered have small holes which obstruct the oilways and significantly reduce oil flow. See first photo.

Second photo show a new old stock pump gasket you can see the difference in size of the holes.
Title: Re: Seizing engine
Post by: muskrat on 14.05. 2017 12:20
G'dah chotus52  *welcome*
 The reason orabanda asks is some brands need more clearance than others. Forged need more than cast.
1: not enough piston bore clearance.
2: getting hot (see 1:), retarded ignition or lean mixture.
After two partial seizures I'd be lifting the top end to see the damage.
A few years ago my 51 (57SS top end) A7 nipped up. a little scoring on one piston & bore. A light hone and new rings. Bad piston in good bore, good piston in bad bore. Still fine after10 000 miles.
Cheers.

JulianS got in 1st with a good point

Now get over to Pictures, Stories and Intros to give uss your story.
Title: Re: Seizing engine
Post by: orabanda on 14.05. 2017 13:53
Musky,
You are onto it!
I have had to increase bore clearance to 0.006"to stop seizures if JP pistons are fitted.
Then; no problems; no more seizures!
Title: Re: Seizing engine
Post by: Triton Thrasher on 14.05. 2017 15:52
Your symptoms very much resemble insufficient piston to bore clearance.

Was the rebore done by someone who usually works on modern, or water cooled engines?
Title: Re: Seizing engine
Post by: chotus52 on 14.05. 2017 18:49
The rebuild was done by the previous owner so not sure of any clearances. SRM 8.5:1 pistons fitted 4000km ago.
Title: Re: Seizing engine
Post by: Triton Thrasher on 14.05. 2017 20:08
100 km/h is 64 mph, which is usually not engine labouring rpm in top gear, on a 500.

Unless someone has messed about with the sprocket sizes.
Title: Re: Seizing engine
Post by: muskrat on 14.05. 2017 21:01
G'day chotus52.
Where abouts are you in this great land? We have members in most states.
Do you have access to an endoscope camera to have a peek down the bore through the plug hole? If not rip the head off and show us some pictures.
My little A7 will sit on 70mph all day.
Cheers
Title: Re: Seizing engine
Post by: chotus52 on 15.05. 2017 00:23
Hello Muskrat, here in Cranbourne, 35km southeast of Melbourne. Previous owner rebuilt the engine from the crank up. Sent the crank away to get the required end float exact, SRM 8.5:1 pistons (clearance unknown), new valve guides & valves (told me the guides are a tight fit to Ensure no blow by, as engine used to be a smoker) & geared it up one tooth on the front as it pulled so well after the overhaul. Dynamo rewound, 12 volt system, thorspark ignition etc etc etc. After the 2nd seize, I cleaned out the fuel tank (some small flakes of debris inside), replaced a clogged inline fuel filter, which I suspected was causing it to lean out & ping plus found the needle jet loose in the carby. All fixed & back together but seized again yesterday Disappointing.
Title: Re: Seizing engine
Post by: muskrat on 15.05. 2017 09:19
G'day again chotus52.
I think the nearest member to you is 20km NW of Melbourne.
The damage will only get worse the more it happens. Time to get the topend off.
"(told me the guides are a tight fit to Ensure no blow by, as engine used to be a smoker)" The guides are a shrink fit in the head, are you sure he didn't mean the pistons or ring gaps?
Cheers
Title: Re: Seizing engine
Post by: JulianS on 15.05. 2017 09:39
Guides a tight fit sounds worrying.

Did he mean less than normal clearance between valve stem and guide bore or oversized guide in head?

If the former that could well be the cause of your seizure, valve and guide nipping up when hot due to lack of clearance.
Title: Re: Seizing engine
Post by: muskrat on 15.05. 2017 13:01
That is feasible JulianS. But wouldn't they be "taped" shut by the piston  *eek*.
Cheers
Title: Re: Seizing engine
Post by: RichardL on 15.05. 2017 13:15
If Julian's observation is the cause, I'm thinking removing the rocker box and looking for bent pushrods might tell you right away. The question is, would this be a seizure or just ceasing to run? Then, would it start again with bent pushrods? Honestly, I don't know the answer. It's possible that the "...guides are a tight fit..." comment is a sideline to the main issue, and may just be an indicator of the PO's thinking as to clearances.

Richad L.
Title: Re: Seizing engine
Post by: JulianS on 15.05. 2017 13:22
That is feasible JulianS. But wouldn't they be "taped" shut by the piston  *eek*.
Cheers

They could well be! And there would be witness marks on the piston crowns.


I think the only way forward is, as you suggest, to have the top off and check .
Title: Re: Seizing engine
Post by: worntorn on 15.05. 2017 13:37
Sure does sound like valves tightening up. I had a bike that did this and it certainly made me think it was pistons nipping up the first time it happened.
This would always happen when climbing a mountain grade on a hot summer day.
It would lose power, slow down rapidly, then eventually shut down. Kicking it over at roadside immediately after the event found the engine free but with virtually no compression.
After sitting and cooling for a few minutes compression returned and the bike would start and run normally again.
I took it apart but found no damage. I surmised that one or both valves in each cylinder ( twin) was not quite closing  on its seat when the engine got really hot, ie on a long steep grade in hot weather. Luckily the valves weren't sticking in the fully open position.
A light ream of the guides solved the problem.

Glen
Title: Re: Seizing engine
Post by: RichardL on 15.05. 2017 13:51
Glen,

Does this mean valve(s) was (or, were) only stuck just before seating but otherwise opened fine? I suppose that would explain no damaged pushrods but still not running.

Richad L.
Title: Re: Seizing engine
Post by: worntorn on 15.05. 2017 14:09
Yes, the valves must have been sticking just off the seat but still opening and closing normally for the rest of the cycle.
 If they had been sticking in the fully open  or even half open position there would have been contact marks on the piston crowns as minimum , but more likely a bent valve or two.

I was very lucky.

Glen
Title: Re: Seizing engine
Post by: Triton Thrasher on 15.05. 2017 17:49
That is feasible JulianS. But wouldn't they be "taped" shut by the piston  *eek*.
Cheers

Pushed nearly shut and probably bent.

But a valve can't lock up an engine at road speed.
Title: Re: Seizing engine
Post by: worntorn on 15.05. 2017 18:14
A stuck valve can't lock an engine, but to the rider it feels much the same as an engine that is nipping up.

Glen
Title: Re: Seizing engine
Post by: Triton Thrasher on 15.05. 2017 21:37
A stuck valve can't lock an engine, but to the rider it feels much the same as an engine that is nipping up.

Glen

Any loss of power feels a bit similar.  Sometimes a seizure can make you hit the tank with your nose.
Title: Re: Seizing engine
Post by: worntorn on 16.05. 2017 03:01
My mistake , I missed the " locking up at 100 kms" part in the first post, . I wrongly thought this was more of a soft seize.
Quite right , there is more happening than a stuck valve.

Glen
Title: Re: Seizing engine
Post by: chotus52 on 19.05. 2017 02:12
Thanks for the imput guys.
Some more info:
It was apparently blowing oil out past the valve guides & the guy made the new guides a snug fit during the overhaul. Compression is still good (160 psi in both cylinders). The bike came with a box of spares, including a 15/16" monobloc Amal carby fitted with 270 main jet. According to my instruction manual, it should be 1" & 270 jet for a Shooting Star. The carby fitted is a 26mm concentric Mark 1. The main jet size on the concentric is unreadable but it "looks" smaller than the 270 in the monobloc, so I'm putting the 270 into the concentric & have lifted the needle one more notch which is now fully lifted. Hoping this is the problem solver.
Title: Re: Seizing engine
Post by: muskrat on 19.05. 2017 13:23
G'day chotus52
No amount of carb changes will solve the problem of multiple seizures. The next one may send the piston through the barrels if your lucky or the rod through the cases if your not.
 Any seizure causes damage be it valve/guide or piston/bore. So it's had two or three, don't push your luck!
Cheers
Title: Re: Seizing engine
Post by: chotus52 on 21.05. 2017 08:03
Muskrat, yes, I think that is the case. Top end coming off. I just find it strange th previous owner never had it nip in 4000km since the rebuild. He did tell me he never did 100kph on it. Cheers
Title: Re: Seizing engine
Post by: Triton Thrasher on 21.05. 2017 12:59
Muskrat, yes, I think that is the case. Top end coming off. I just find it strange th previous owner never had it nip in 4000km since the rebuild. He did tell me he never did 100kph on it. Cheers

Maybe he did have it nip up.  Maybe it hasn't done 4,000 km since the rebuild.
Title: Re: Seizing engine
Post by: chotus52 on 21.05. 2017 14:25
It was definitely a nip up as I was a little slow to grab the clutch on the second seize & it locked the back wheel. Strange that by the time I rolled to a stop, turned off the ignition & kicked the engine over, it felt like normal compression & started first kick & ran perfectly normal. Same on all three occasions. Oh, I heard it start to detonate each time just before it seized. After the first time, I put 98 octane fuel in her & was doing up to 80mph on winding roads. I Guess every time I backed off for bends, it got a drink of fuel. The three nipups have been at steady 60 mph for 5 or more miles
Title: Re: Seizing engine
Post by: Triton Thrasher on 21.05. 2017 15:00
If the main jet is correct, but the needle is in too weak a position, the engine will be safer (from seizing) at 80 mph than at 60 mph.
Title: Re: Seizing engine
Post by: chotus52 on 22.05. 2017 02:08
The main jet size marking was unreadable but I compared it with the correct 270 jet & it looked smaller. I've since fitted the 270 since the last seize. Running fine but not tested on highway yet. I wonder if that could have been the problem.
Title: Re: Seizing engine
Post by: Triton Thrasher on 22.05. 2017 06:41
Could be.
Title: Re: Seizing engine
Post by: muskrat on 22.05. 2017 09:12
But cruising at 60mph your not on the main jet till you hit a hill. Mark your throttle body and twist grip in the closed position, then the twist grip in the wide open position. Now divide into 4 segments. You'd be surprised how little throttle is used when just cruising at 60mph on flat ground.
In saying all that a too small main jet will affect the needle taper.
Cheers
Title: Re: Seizing engine
Post by: JulianS on 22.05. 2017 09:39
The main jet size marking was unreadable but I compared it with the correct 270 jet & it looked smaller. I've since fitted the 270 since the last seize. Running fine but not tested on highway yet. I wonder if that could have been the problem.

If the cause of a seizure was too weak a mixture then I would expect to see serious signs of a weak mixture on the plugs - colour towards white .

The A7SS would also have been fitted with a 1 inch bore carb, so the 15/16 one fitted would tend to give a richer mixture  due to smaller bore.

I still think you need to look inside or that threat of seizure will always be there.
Title: Re: Seizing engine
Post by: orabanda on 22.05. 2017 09:42
Musky offers good advice for when tuning.
A mark on the side of the twist grip, and a piece of masking tape (temporary) stuck to the throttle body on the left. Mark the tape in quarters (or eights) and setting up the carby becomes easier / more accurate.

Richard
Title: Re: Seizing engine
Post by: chotus52 on 22.05. 2017 14:02
Thanks for all the tips gents. I'll advise the outcome. Regards
Title: Re: Seizing engine
Post by: chotus52 on 06.06. 2017 11:56
KCC pistons
Title: Re: Seizing engine
Post by: chotus52 on 06.06. 2017 11:59
I mean JCC pistons
Title: Re: Seizing engine
Post by: Topdad on 06.06. 2017 13:35
What are the bores like ??
Title: Re: Seizing engine
Post by: bikerboy on 13.06. 2017 01:34
Sorry to come in late but IMO it was always going to be pistons. Have you measure the bores yet? I must admit I dont like the sound of "tight" valve guides either I would check that while I had the head off for sure
Title: Re: Seizing engine
Post by: chotus52 on 27.06. 2017 02:34
Thanks for all the pointers. We pulled the top end, cleaned up the pistons & honed to 6 thou. It seems like problem solved. Cheers