A Good Ride Spoiled

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I’d advise caution asking a machine shop to inspect your bores. They may be sympathetic / sensible to the fact its an old bike, ridden like an old bike. But most are not, and will apply unsuitable modern, tight standards. So make sure you read the tolerance info in the workshop manual and ask them to check to that spec, NOT their modern opinion.

I once agreed with my machining guy that the valves and guides were clearly worn on my T160. So we fitted brand new, genuine valves and guides from a trusted source. When he’d finished, we were both surprised to find they were exactly the same as before !

Personally, if everything looks good, and if it’s all OEM stuff, I wouldn’t even change the rings or circlips. Cos unless you KNOW what you’re doing and what you’re buying, you could easily fit worse parts than you have.

All I’m saying is... if it were mine and I’d taken it down that far... I’d remove the barrels to allow inspection and cleaning for peace of mind.
Agree Eddie
An idiot can use a micrometer and get somewhere near if he doesnt think it's a G clamp, a bore mic is a different deal. Lots of measurements and judgement required.
 
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What's happened then is that the studs have let go which caused the loss of compression. Its not the fault of the copper gasket, although, like many here, I have had better luck with a composite gasket.

A good indication of this problem was in your early post-
"The lower front sleeve nuts are rear nut were no longer at proper torque when I removed them yesterday"

This is a very common problem as these threads are a weak spot in the Commando engine. Fortunately it's an easy repair.
Jim Comstock tested to destruction a variety of thread fixes awhile back. Prior to that some felt that the lowly helicoil was a poor fix compared to other methods.
The helicoil came out on top IIRC.

Found it


420 inch pounds= 35 ft lbs so one does need to limit torque to 30.
This might account for pulled studs on some heads. If at sometime in the past the head was torqued by feel, as some do, it would be easy to overdo it.





Glen
 
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Tornado

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Agree Eddie
An idiot can use a micrometer and get somewhere near if he doesnt think it's a G clamp, a bore mic is a different deal. Lots of measurements and judgement required.
How does one measure bore wear with a micrometer if that is what you are saying?
WS guide says taking a ring gap at several depths can tell wear amount. I guess it cannot determine out of round like a proper bore gauge.
 

Tornado

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What's happened then is that the studs have let go which caused the loss of compression. Its not the fault of the copper gasket, although, like many here, I have had better luck with a composite gasket.

A good indication of this problem was in your early post-
"The lower front sleeve nuts are rear nut were no longer at proper torque when I removed them yesterday"

This is a very common problem as these threads are a weak spot in the Commando engine. Fortunately it's an easy repair.
Jim Comstock tested to destruction a variety of thread fixes awhile back. Prior to that some felt that the lowly helicoil was a poor fix compared to other methods.
The helicoil came out on top IIRC.

Found it


420 inch pounds= 35 ft lbs so one does need to limit torque to 30.
This might account for pulled studs on some heads. If at sometime in the past the head was torqued by feel, as some do, it would be easy to overdo it.





Glen
Ive seen the video a while ago. Do we know wjy he heated the head and tested as 400 deg as most apply these fastners to cold engines?
Also would be nice to see what original stud in hole pulls at. Maybe the 30 ft-lb spec is wrong for these three studs. Im now nervous on achieving 30 even with a torque wrench since these can be out on accuracy 1-3 percent.
 

concours

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Measuring end gap of rings is absolutely helpful... if you know the rest of the story.
What was thegap when they were installed?
If they still measure good, in several places, and you know 3.14159264, then you can accept good bore. If not, the next level of inspection is needed, dial bore gage, etc.
 

Tornado

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Measuring end gap of rings is absolutely helpful... if you know the rest of the story.
What was thegap when they were installed?
If they still measure good, in several places, and you know 3.14159264, then you can accept good bore. If not, the next level of inspection is needed, dial bore gage, etc.
According to the workshop guide, it says you only need the difference in gap at different depths to estimate bore wear. This gives the change in circumference which you convert to change in diameter at each depth. Original ring gap is not needed to do this. But it cant give you a measure of out of round.
 
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Measuring the bore with a ring is fine for someone checking for wear but not for a machinist fitting new pistons to a bore.
The ring method will tell you how much taper is in the cylinder +- half a thou.
I suspect that there isn't much taper or out of round.
I remember this is the engine that you at first thought had something wrong with it as it couldn't be kicked through compression.
I came in to try it and found that it would go through, but it sure took a good stomp to get it through. More than my MK3, which tests excellent on leakdown. Part of that could be the extra length of the MK3 kicker.
For sure your bike had excellent compression so the bore seal was excellent.



Glen
 
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concours

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According to the workshop guide, it says you only need the difference in gap at different depths to estimate bore wear. This gives the change in circumference which you convert to change in diameter at each depth. Original ring gap is not needed to do this. But it cant give you a measure of out of round.
If some doofus set it up wrong, then your data is skewed.
The shop manual assumes your looking at a properly set up engine.
I deal with this phenom EVERY DAY
 
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Ive seen the video a while ago. Do we know wjy he heated the head and tested as 400 deg as most apply these fastners to cold engines?
Also would be nice to see what original stud in hole pulls at. Maybe the 30 ft-lb spec is wrong for these three studs. Im now nervous on achieving 30 even with a torque wrench since these can be out on accuracy 1-3 percent.
On head bolt torque, the big White Norton book calls for 30 ft lbs on the 3/8" fasteners and 20 ft lbs on the 5/16"

Glen
 
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Rent a torque wrench and ask to see the calibration papers which are required periodically to ensure accuracy, or if you purchase one it should come with certs. Inspect it, if not completely backed off don't accept it.
There's some very good torque techniques buried on here if you've not looked them up already. Wizards abound within these hallowed halls. I'm astounded daily.... Preaching to the choir again.
 

Tornado

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Rent a torque wrench and ask to see the calibration papers which are required periodically to ensure accuracy, or if you purchase one it should come with certs. Inspect it, if not completely backed off don't accept it.
There's some very good torque techniques buried on here if you've not looked them up already. Wizards abound within these hallowed halls. I'm astounded daily.... Preaching to the choir again.
Yes ive got three, each a different range. But they all have stated accuracy ranges of around 1-3 percent I believe. I have run a calibration check on then by using a luggage scale pulling at 1ft from pivot point. They can be adjusted by fiddling with a nut within the handle. This proceedure is shown on some youtube videos.
My concern is, as Jim C showed in the linked video, if even a helicoil pulls out at 35 ft-lbs, the 30 spec'd for those 3/8" studs will be either marginal or excessive. Even if the torque wrench is accurate.
 
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I heli coiled those three stud holes then torqued them up to 30 ft lbs about 15 years ago. The head has been retorqued a couple of times just because the tank was off for something or other.
No problems and no leakage. I was not at all nervous about torquing to 30.
This was before seeing Jim's video.
Ignorance is bliss!
I believe it would take more than 35ft lbs to pull those studs when cold. For one thing the hole gets larger with heat. Also the Al. softens up.
Perhaps Jim will spot this discussion and explain.

Glen
 

Fast Eddie

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Re the studs, inserts will be much stronger, that is a very fine thread to expect to hold torque in aluminium, hence the frequent failure rate.

You might wanna check out JS and cNw web sites, they both offer stud kits with studs that have a UNC thread on the head end. The also need inserts of course, but create a much stronger end result.
 

Tornado

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Re the studs, inserts will be much stronger, that is a very fine thread to expect to hold torque in aluminium, hence the frequent failure rate.

You might wanna check out JS and cNw web sites, they both offer stud kits with studs that have a UNC thread on the head end. The also need inserts of course, but create a much stronger end result.
Yes, if an insert is pulling out at no more than 35 as per Jims video, then bare fine stud must be at or above limit at 30. I would be very concerned putting 30 on those stock three studs. Perhaps the rating is inadvertently applied to all the 3/8 fasteners, instead of just the head through bolts that anchor in the iron barrel.
 
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I've never used a torque wrench on the rear nut or the two long nuts underneath in the front. Just tighten them securely. I do use a torque wrench on the four side bolts and the center front bolt set to 30 foot pounds. Also on the two 5/16 set on 20. When you retorque after the first heat cycle I find that if it takes a half turn or so on the ones I can get a wrench on it also takes about the same half turn to tighten the ones I can't. Mick Hemmings never uses a torque wrench at all in his rebuild dvd. Says just to tighten securely.
 

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How does one measure bore wear with a micrometer if that is what you are saying?
WS guide says taking a ring gap at several depths can tell wear amount. I guess it cannot determine out of round like a proper bore gauge.

The budget version is to use a telescoping gauge and measure that with a conventional outside micrometer (Unless you have a more expensive cylinder bore type gauge) but both of those rely on being able to measure accurately to tenths of thousands.

You would still need to get an accurate condition of the bore surface where the rings traverse the TDC position. (I sent a TR5T cylinder out for re boring and according to the shop doing it, there was on a stock bore 0.001" to spare going + 0.020" or 0.019" total wear)

Measuring things with a single ring (Besides checking ring end gap at installation) was little more than a means to get some idea of the bore condition for the (UK ?) home mechanic without excessive and unwanted expense so that procedure was added to many workshop manuals.
Is it the best way, probably not but only takes a set of feeler gauges.

The pictures only show a ball park condition, it is probably fine but to me cross hatch is a moot point unless combined with the path of the vertical scratches. (continuous or intermittent to the vertical and also at the TDC ring area)

You could over think it easily, if it was running fine before this hiccup, it would most likely need the minimum to reinstate after qualified examination. (Should you feel the need to take the cylinder to someone that can measure the bore with suitable measuring equipment for peace of mind.

Failing that fit the horn first.
 

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PS, You would also be looking at the piston ring contact faces with a jewellers eye glass, that will tell you plenty.
 

Tornado

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Today I got the rockers, springs and valves out. One exhaust valve was quite tight in the guide and needed a lot of force and wiggling turns to finally get it out. Not sure how The springs could move that one up to close. That one and other exhaust valve had no easily feel-able rocking in the guides. Intake did seem to have some rocking.
 
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