The BSA A7-A10 Forum
Bikes, Pictures, Stories & more => Other BSAs, Other Bikes, Cars, Machinery & Tools => Topic started by: muskrat on 15.10. 2024 19:45
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G'day Fellas.
A mate bought a 1971 T120R from another friend (RIP) whom I rebuilt the motor some 30 odd years ago. On a 4 day ride a few weeks ago he was passed by a Honda four, he tried to keep up! Well that didn't last long and it nipped up.
I picked up the bike on the weekend and ripped the top end off. Mmmm that's a good one *eek* It'll have to go out to +60 now. The other (right) side is fine. I also noticed the left side little end to be just a tad tight.
The plot thickens. On removing the black base gasket I found a lot of the green base gasket that I used back then. So it had been messed with prior to my mate getting it and they didn't remove all the old gasket *pull hair out* I'm now looking deeper to see what else was bodged.
Cheers
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DOH *ex*
The bl@@dy barrels are cracked *ex* At first I thought that ring was a water mark but it is a crack *pull hair out*
Mate won't be happy to spend another $1000.
Cheers
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None of that was caused by revving too high.
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G'day TT.
Having a closer look I think when they bored it out to +40 it wasn't bored true to the base flange, so not true to the crank throw and thinner on one side. The base nuts, head bolts and rocker box's weren't as tight as I'd do. The pistons have no markings so are inferior after market, certainly nothing I'd use.
Other club members said he flew past them like a bat out of hell!
Cheers
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"Those who fail to learn from history are doomed to repeat it"
In the 1970's chasing Honda 4's was the primary means of destruction for Triumphs
Tell your mate he's living history *smile*
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Reckon that has to be a 'down to the last little washer' rebuild Musky. Nipped up?? That's a polite way of putting it!
Wonder how many flakes of 'inferior piston' are in the oilways?
If those aren't the pistons you put in it 30 years back, who knows if the skirt clearances were correct for whatever they are - looks rather as if they were inadequate or there was a lack of lube?
Presumably this'll be a new liners job now, or find a better set of barrels (and a person who can bore on centre) . . . expensive, as you say, whichever way one goes.
Glad it's not mine. Quite enough hassle with that sort of thing this year already round here!
Playing games with Honda 4s and similar only ever worked for me, a bit, sometimes, on bendy roads, and even then . . . brakes - argh!
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Do we know the comp ratio of those pistons? Look like 10 - 10 1/2? If the engine is tuned like that and being pushed it would need racing type clearances and be like a bag of nails cold. I had a 73 T140 and it wasn't a fast bike. The Bonnevilles of that period are around 50-55 hp and the Honda 4s are around 65+. Within months of the CB750 going on sale in the States the roads were littered with blown up Triumphs. *sad2*
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G'day Fellas.
I've got onto a set of barrels that are std but need to go out to +20 for $200. I'd already ordered +60 so will try to sell them.
I doubt very much that I would have used "no name" pistons (9:1), I haven't checked the clearance (will after dinner). I have flushed out the cases, oil tank and hoses. Tomorrow I'll pull the oil pump and have a good look.
I asked Rossco if he'd topped up the oil in the primary. He hadn't as he only did a few hundred miles since he got it. The primary was overfilled by about 1200ml *eek* I'd better check the g/box. There wasn't much in the oil tank (oil in frame) either *ex*
Now to find a reputable borer, my fella died a few years ago.
OK piston to bore 0.005", filter on the sump plug had a little swarf, oil pump all good. Looks like the timing side hadn't been touched as my green gasket was there.
Rear tyre is on backwards and 15 years old as is the front. Scotsman. See what I find tomorrow.
Cheers
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Those don't look like 9:1, i could be wrong, but are you sure?
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Those don't look like 9:1, i could be wrong, but are you sure?
(https://files.ekmcdn.com/montysclassics/images/triumph-pistons-e-9488-020-6t-tr6-tiger-t120-bonneville-1956-73-9-1-020-908-p.jpg?v=1c5b9576-f379-43ed-a74a-82f1bb91d794)
The seized piston looks same as these 9:1 examples.
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He tried to ride fast and the engine seized. I’ll assume the barrel wasn’t already cracked, because that should have been obviously noisy.
Fuel starvation is common but it’s unlucky if that causes a seizure without some missing/spluttering first.
Too-tight rebores are also common, but they usually seize at moderate speed on a long hill, not very long after the rebore.
If the oil supply fails, I don’t actually know what goes wrong first on a Triumph: pistons or bearings. Generally, if the lubrication system is working, it keeps working.
I’d venture to say those 9:1 pistons are marginally a bit high compression for a road Tri 650 nowadays. If the ignition timing is a bit off or the main jet a bit smaller than ideal, or the petrol octane rating a bit low, you could have an acute attack of detonation that you don’t hear at high speed.
The piston top is well coked up, which raises the compression ratio, causes hot spots and signifies oil in the combustion chamber. Oil reduces the knock resistance of the fuel.
Mine once seized because the top came loose on the Monobloc.
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Thanks for posting that pic of the 9:1. I'm genuinely surprised and would have put money on (than goodness I didn't *smile*) those being around 10:1.
Re oil, my bet would be a big end to go first. Engines that loose oil often throw rods.
It looks to me that the pistons got too big for the bore.
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G'day Fellas.
The bottom end feels fine, no "rock" of the rods at all, no (by feel) endfloat or up & down of the crank. Looking at the plugs it was fueling well, carbs will get a good service before going back on. Knowing Rossco he would have been using the cheapest fuel probably E10.
I'm wondering if the crack happened first *conf2* Bugga I clicked on the wrong barrels. The one coming is +40 to go out to +60 so am now concerned of wall thickness and cracking again. The Cafe is out to +80 with plenty of wall left. The Trihard has a massive flange but seems to be thin in the wall.
Time will tell.
Cheers
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Often Triumphs "go" first on the LH or drive side, mainly due to a (semi)blocked sludge trap not letting sufficient oil through the crank.
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Hi All,
20 or so years ago a neighbour melted the pistons in a Harris Bonny (the bikes built by LH Harris)
Running on the motorway these bikes need both fuel taps on or the cylinder away from the "on" tap goes lean
The flakes of molten piston got absolutely everywhere even into the cranks sludge trap *eek*
A couple of years after I rebuilt it he took it to France and filled from the "green" pump which is diesel and blew it up again *problem* *problem* *problem*
I hated that fkn bike *razz* *razz* *razz*
John
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The flakes of molten piston got absolutely everywhere even into the cranks sludge trap *eek*
John
That would be my worry. Maybe just my paranoia, but for peace of mind I'd feel obliged to hoick the crank, get the rods off (measuring sticks at the ready), flush everything out to within an inch of its life, etc, just to be sure. Time-consuming, carries the cost of extra consumables and maybe more if the journals have suffered or if shells are replaced while it's apart. But not I think a waste of effort even if there's nothing to see.
(I say this because a long long time ago I seized a piston on an A10 and particles had got pretty well everywhere just as John describes. 'Paranoia''s been my middle name ever since.)
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I too have seized an A10, the piston sized, pulled a skirt off the block then reduced it to flakes by hitting it repeatedly with the big ends. I felt the engine tighten and hit the clutch very fast so all the damage happened in a second or two. I was still able to rebuild it and its still going now but it took a case repair, replacement cylinder, rods pistons and a lot of cleaning.
Good call about the petrol taps, easy to overlook the flow at full throttle. I see there's a mod for Concentrics to up the flow through the float valve.
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I see there's a mod for Concentrics to up the flow through the float valve.
G'day Rob.
I used that mod on my A7SS racer. At 11 MPG on methanol I could have run the fuel hoses straight into the inlet tract *eek*
Cheers
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run the fuel hoses straight into the inlet tract. Isn't that called a Wal Phillips Injector? *smile*
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G'day Fellas.
Had a real good look at the barrels today. It looks to me like the barrels cracked first studying the wear marks. As I said the flange nuts weren't tight but the head steady was. Vibes/flex too much right wrist *bash* What I have noticed the outer wall thickness is thinner on that side. Sort of worries me of boring s/hand barrels to +60.
I know from machining castings/forgings their not exactly the same and when set up on the horizontal borer may be a mm or so off. Or like I said using the head surface to bore instead of the flange. *pull hair out*
Cheers
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G'day Fellas.
I forgot to mention, I pulled the crank flywheel bolt that holds the sludge trap, used my endoscope to see there is phu(k all in there. I then back flushed with oil then air. Nothing came out I wouldn't put on a sandwich *grins*
Cheers
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G'day Fellas.
It lives, but not without a few problems.
It started first kick (no smoke) so I quickly set the timing (Boyer). Ran it through a few heat cycles and set about balancing and tuning the carbs. Got them pretty close. Let it cool down and set the tappets, 3 & 5 thou. Started again first kick. Ran for about 3-4 minutes and I heard a tink and she stopped. Bugga the right side ex valve had stuck open kissing the piston *pull hair out* "Off with his head". Ordered two new guides & valves. Heated the head in the BBQ to 250C and the guides in the freezer for 1/2 hour. Old guides didn't take much persuasion to drop out, the new guides fell in most of the way and took a couple of whacks to seat home. Left side lapped in good but the right needed cutting. I don't have seat cutters so took it up to Bathurst. His cutters wouldn't reach the seats as the guides are too long. OK back to the BBQ, drop them back out and take 3mm off the port end. Drop them in again and back to Bathurst. This time the seats were cut and back home to lap them in.
All back together and started first kick. Again ran it through a few heat cycles. Took it off the bench and ran around the block a few times. Bloody oil leak, I couldn't tell if it was primary or gearbox so I raided the pantry. Food color to the rescue, red in primary, blue in gearbox *whistle* Give it a good wash and took it out for a good 60Km ride up too 120KPH in spots. No color to be seen. Must have been a dry seal that needed a good flogging. Changed all the oil (that food coloring is a bugga to get off the hands!). Took it out for another run of about 80Km, motor feels great. As it only has an oil pressure light, I bought a gauge to check. Hot at 2500rpm reading 60psi so I'm happy. Rang owner to take it away. Cost all up $1000.
Cheers