View Full Version : sad story...1970 Bonneville

02-14-2011, 6:04 PM
I rolled the dice and bought a nice cosmetically restored Bonneville..3 weeks ago. Started and ran, but didn't idle at all..so bought it assuming I might be lucky. Was told it had fresh rings and valve job. Got it home and started working on the Amals..couldn't get idle any lower than 1500..was about to quit and go order new Amals..but went for one last ride around the block...and it seized.

So I have top end off..one very scored piston and barrel...but other side looks fine. Valves and piston crowns look fine..both sets of rings look ok. Others have told me I did this to myself by letting it idle too long while playing with carbs.

So I just found this forum so I thought I would ask you guys the same questions....

1. Could this be a lubrication problem..dreaded sludge tube issue? Is there any part in the lub
system that might be left/right sensitive..ie..only seize one side?

2. Make sense that it could seize with very little load on it...I was riding around at 20-25 mph in lower gears....actually seized while coasting into drive way

3. Small rod end bushings are beat...symptons that it has lot more miles on it than I was told(11k).

4. Do I dare fix top end and try again or do I have to spring for a total teardown at this point?

02-14-2011, 6:21 PM
Sorry to hear that man!!! If it was me and I was going to ride it I would like to know it was in good shape. Therefore I say complete re build by a reputable shop.

02-14-2011, 6:37 PM
First . Is it a lube problem ? Was the engine making alot of noise ? Did you see oil pumping in the tank before you rode it ?
Was it a air leak . They tend not to let idle drop down but they will cause a high idle then they fall flat and die . A air leak can cuase the to hot sieze ( scored on all four corners ) Is there no sign of ptting on the piston ?
Who did the engine work ? If it was some home guy he might not have known were to set all the clearances .
Pics would help alot .

02-14-2011, 7:11 PM
I used the good piston to check clearance...bores were around.0045 both sides. bad piston was scored across skirt on both intake/exhaust side..but all damage below ring lands. I saw oil returning to tank..Am suspecting fuel
starvation..probably should have not tried with old carbs so long?

No one has answered my question about lube system....is there any thing that could affect oil pistons on one side only? I assume pump either works or doesn't. How exactly do pistons get lubed/? splash from bottom or presure feed from somewhere?

02-14-2011, 7:15 PM
oh yeah...noise...did hear what I thought might have been a hi pitched noise that i thought was an exhaust leak...i probably should have shut it off then. It was about 60 degrees out so I wasn't too concerned about idling..this may be a self inflicted wound?

02-14-2011, 7:24 PM
Here is my thoughts on what might have happened and what you should do and maybe Trumpnut and Tourch will agree. Your going to have to tare the engine down anyway. First and foremost check that pesky sludge trap if it's full of crap that may have been the problem, however I've seen this too check the crank to see if it has been turned or not. When the crank is turned the sludge tube hole should be cleaned out again very good a metal sliver may have clogged the oiling holes on the right side. Which will cause that side not to get any Oil and score only that side and could seize the motor. Anyhow tats my thoughts.

02-14-2011, 8:50 PM
Sounds like a blockage some where (be it sludge tube or the oiling lines) like said above. I still stand by my first comment. Tear it down. Then you know what you have to work with and it gives you a little more piece of mind.

02-14-2011, 9:24 PM
the sound of squealing is usually a top end seizure, that along with the story of "fresh top end work" would lead me to think it was bored and honed to automotive specs of 2.5k clearance, this is very common when people take air cooled cylinders to a water cooled automotive machine shop.
chances are you will need to buy a new set of pistons now because once they heat up they loose their shape. you may get away with the same size as the ones inside because the bore is actually 2k too small right now.
Take the cylinder head off and measure the bore and check on the top of the pistons for the overbore size. order 2 new pistons and take the jugs to a real motorcycle machine shop that uses a bore plate and have them correctly honed to 4.5k clearance to the new piston measurements. then put it back together and check your pilot circuit before you atempt to start it.
good luck, Call me if you need help
and yes the sludge trap could still bite you in the ass.

02-14-2011, 9:44 PM
The pistons are oiled by splash from the crank . The oil pump send preasured oil to the crank . from there it fills the sludge trap which seperates the particails from the oil by centerficail force . then the oil goes out of the crank through the big end bearings . The flywheel passing through the oil in the bottom of the case flinges it to both the piston skirts and cam shafts . The other side of the pump scavanges oil from the bottom end and sends back to the tank where it is restricted and a oil line comes off going to the rocker shafts .
I would agree ( With Torch ) that most likeky they made the clearances to close . What kind of pistons are they ? If the oil was returning to the tank and there was no knocking noise then I woulden be to concerned with the sudge trap . I would just redo the top end then check the oil preasure to make sure the trap is not clogged or anything . A novice engine rebuild to clean a oil sludge trap can cause more problems then the chance of the trap not being clean . I have seen more bottom ends come apart because guys "clean" there sludge trap with out the knowledge of reistalling rods and cleaning the oil passages to it . Most oil failures I have seen is from a bad seal on the timing cover . The one that nobody ever replaces or the timing side of the crank damaged because sombody drove it out of the cases with a hammer so it doesn't seal .

02-15-2011, 8:31 AM
In response to the replies above...Based on the condition of the pistons, I believe the engine was on standard bore..just new rings and a valve job...i also checked ring end gap and all looked good. So I conclude clearances were not the issues. Also..if pistons are splash lubricated...why would one side seize and not the other?

As for carbs...pretty sure pilot system was in bad shape...and slides pretty worn. I cleaned the carbs and it got a little better..but not much. I did let it idle for a while at around 1500 rpm while attempting to tune carbs. Could I have over heated it?

How about timing...I had not gotten that far so I have no idea if timing was way out...although it was not kicking back when I started it?

Any idea how many miles it takes for an engine to clog the sludge tube. I am trying to talk myself into not splitting cases..mostly because I have never done a motorcycle motor and don't have to tools. I did re build a Austin Healey six once...but it was simple..I replaced everything but the block, head and crank..no detective work required.

Also..one other clue..the worn out small end bushes....is that a lube problem or just proof that this motor has a lot of miles on it?

02-15-2011, 11:02 AM
ok then check your oil lines and see if they are hooked up backwards, the feed line should be going to the front pipe. the return line the the rear pipe. the feed line has the large silver hex fitting with a screen filter in it on the oil tank.
and when you fire up a Triumph the first thing you do is check for oil return pressure going back into the oil tank.

02-15-2011, 11:18 AM
If you had oil back to your tank before you rode . Have you considered a rich mixture on the one cylinder . To much fuel will wash the oil off the cylinder walls . If it was ign. there would be signs of it on the piston crown . See if your oil smells of fuel .

02-15-2011, 1:05 PM
I think I saw oil returning to tank when I first checked. Assume if hooked up backwards would not have run even as long as it did. I am pretty sure carbs are way too lean...not rich....

I just had some surgery so it will be a while before my wife allows me into the garage...so I will have to wait a bit. One more question...do the oil pumps wear out and partially fail on these things?

02-15-2011, 5:44 PM
I am screwed....even tho I just had a toe cut off yesterday..I hobbled out to the garage with a mirror
and checked....PO put hoses on backwards. Total run time, including idling was about 20-30 minutes. Any chance at all that the bottom end survived?

02-15-2011, 6:12 PM
Look man I'm really not trying to bust your bubble and as much fear you have right now about splitting the cases and a full rebuild. I've got one question for you Do you have a deep fryer and you can cook French Fries right? Sounds like you got motor rebuilding exp. You can do this man. Just look at it as a motor and you can do it, in all actuality it is easer then rebuilding a small block Chevy 350 or any other auto motor. Once you get into it you will realize the Brits are really a simple minded type of ppl. Read my build I am a car guy and this is my first and it's no sweat.

02-15-2011, 6:33 PM
I am screwed....even tho I just had a toe cut off yesterday..I hobbled out to the garage with a mirror
and checked....PO put hoses on backwards. Total run time, including idling was about 20-30 minutes. Any chance at all that the bottom end survived?NO

02-15-2011, 9:58 PM
I am screwed....even tho I just had a toe cut off yesterday..I hobbled out to the garage with a mirror
and checked....PO put hoses on backwards. Total run time, including idling was about 20-30 minutes. Any chance at all that the bottom end survived?

here is the deal..technically you should split the cases and replace all the various bearings and do it right. but I have had a couple of customers bikes where the the owners have done the same thing and the engines were still running the last thing I heard.
and on my son's pre unit I was riding his brand new engine breaking it in hard ripping along at full throttle in 3rd gear when the motor let out a scream and locked up the rear tire, when I stopped the bike the whole oil line assembly had blown off the engine , (the stud stripped out) we go it back to the shop fixed the oil lines and he has been riding it for 3 years since doing 4 hour freeway rides to central cal,
so could you dodge a bullet? maybe.. you could just give it a try, if you hear any rattling down in the cases shut it off and split the cases.
It could explode or it could run for a long time, it's a gamble.