Advice needed - engine tightening up - help - tips

[trevinoz]

Josh, that is exactly what happens with too tight a bore.
Trev.

[Josh Cox]

Hi Trev,

Thanks for that.

I do not understand how the engine could stop due to rings being too tight, then the engine can be started straight away ?.

As stated I am no expert, but how does the engine instantly start working after stopping due to effectively ceasing, the ceasing force would have to be greater than say, 35 BHP, that is a considerable amount of force ?.

I am really interested in knowing the outcome of this issue.

[RichardL]

Without having experienced this lock-up my own self (thankfully), it would seem that the leap between the thinest possible film of oil, and no oil, is very small and the time for the pistons to contract the equivalent radius would be rather short. I think the engine stops  quickly at the no-oil point, else, it seems we would not see so many scored pistons as, would they not be, melted, instead?

Richard L.

[Brian]

When this happens the motor will usually restart almost immediately. I had it happen many times in my racing days and if you are quick enough you can pull in the clutch and coast for 50 yards or so and then let the clutch out and the motor will restart.

Unfortunately it almost always scores the piston and usually traps a ring or two in the piston. If it happens you have to pull the top off the motor and check it out. Most times the damage is very small and can be easily cleaned up. At the worst a set of rings and a hone and away you go.

Letting the motor work or labour is the worst thing you can do, revs dont harm anything. This is another reason to be wary of higher than standard gearing. Better to rev a bit higher than have the motor constantly under load.

I have told the story before but I bought my 51' A7 cheap because the previous owner didnt understand about gearing and kept seizing the motor and couldnt work out why.

[Josh Cox]

Well consider me educated, thanks gents.

[groily]

Just on Brian's revs and gearing point - couldn't agree more. Revs hurt nothing (though they can make unattended-to oil leaks worse). I run my A (standard iron motor) on one extra tooth on the g'box, and it pulls it just fine. But I also run a much revvier and low-torque 500 with an extra tooth . . . The next time the clutch is off, it's going back to standard. Just the few % difference is too much - too much throttle needed, not on the cams, poor fuel consumption and just generally not as nice as it was. Reason for doing it was to try to get a slightly more relaxed cruising pace on country roads, but it was a bad plan cos it didn't achieve it. No seizures though, luckily!

[Triton Thrasher]

Boring old bikes to too-close tolerance seems to be common.  Apart from the strange info in the service sheets above, engineers are maybe used to modern water cooled cylinders.

I use "another well-known 650 twin" with 71mm bore and get good results from 5 thou skirt clearance on Hepolite and Taiwanese "Harris" pistons.  Some original old pistons may have had different requirements.

[A10Boy]

I think what causes this is a build up of excessive heat local to the rings, which is is due to high levels of friction.

As we know, when the rings heat up they try to expand but if there is no ring gap the closed ends push against each other and have nowhere to go, causing them to press excessive force onto the bore in turn causing excessive heat. This excessive heat is very local to the rings and piston surfaces, and when the engine stops, it only takes a few seconds for that heat to disperse releasing the force on the bores. The heat also atomises the oil locally compounding the problem.

Of course, this problem might not be that, but it looks very likely.

[Richard]

i am a bit late in replying and have only read a few of the replies but I wanted to tell you that this is exactly what I experienced when I first purchased my super rocket, I duly treated it as new when I first got it keeping the speed on the low side for 500 miles and building up the speed gradually, during which time I increased the speed (revs) for short periods then reducing down to what I call running in speeds until I was happy that there was no noticeable problems.
When I went over 60 mph for any length of time the engine would start to tighten up and I pulled the clutch in before the engine actually seized, I then ran it in so to speak for a little longer but the same thing kept happening for any lengthy duration over 60mph, so after about the 5th time I took the bull by the horns and took the top end off, the pistons had tightened up in the bores and subsequently the rings grooves had pinched the rings.
I found out when apart that the pistons were 80 though oversize and had been supplied by SRM who also did the rebore now they reckoned that due to the make up of the pistons the clearance needed in the bore was less than BSA would recommend, I took the barrels along to our local BSA dealer (well he was in the 60,s and 70,s and is still trading) who measured them and declared them to be to tight a tolerance regardless of what SRM said. So after finding out that SRM had sold the remaining stock of 80 thou oversize pistons to MCA Hammond?s obtained a set from them and rehoned the bore to what they said was the correct tolerance, I duly fitted the parts back on rode it steady for a couple of hundred miles and have had no problem since, and ask any of the lads how I mistreat my super Rocket (70mph from Dover to Swindon continuously with only one stop and the last 6 miles between J15 to J16 M4 at 85 to 90mph)
So it would be worthwhile to take the top end of and check the pistons and bore even if for peace of mind.
Sorry for the book
Richard

[nigeldtr]

Trev,

Just finishing of a couple of "house" jobs and then I will bring her into the workshop - hopefully the next couple of days. Will keep you posted. I can't see it being anything else but we will see!

Regards

Nigel

[Triton Thrasher]

SRM who also did the rebore now they reckoned that due to the make up of the pistons the clearance needed in the bore was less than BSA would recommend,

So they bored it tighter than spec and it seized whenever it reached full temperature but it still wasn't their fault.

Glad you got it sorted!

[Richard]

So they bored it tighter than spec and it seized whenever it reached full temperature but it still wasn't their fault.

TT
I do not see the reason behind your statement they did the rebore tighter than spec so if it was not down to them who's fault was it?
Richard
please do not read this wrong it has been written with a smile on my face and its not wind

[bsa-bill]

Don't want to interfere here Richard but I think there was maybe an ironic smile on TT's face when he typed also i am I right TT

All the best Bill

[Richard]

Bill
Ahh I see what he means now its been a difficult day and the brain is a bit slow
Richard

[Triton Thrasher]

Sorry for being obscure.  It was probably more sarcastic than ironic.