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Discussion Starter #1
Hello everyone, I've just joined.
I have recently bought a CB 450 twin 1970 K2. The bike had been sitting inside for 25 years and not started in this time. I put penetrating oil down the spark plug holes, rocking the bike in gear for 10 days. No movement so I took the motor out and freed the pistons. I took the cylinder to a very experienced mechanic who said the bore was good. The pistons are standard, he told me to get a set of standard BLACK RINGS a good hone, lap the valves, new cam chain and we will put it back together. I the took it to a mechanical engineer they want to rebore it with oversized pistons and rings and soda-blast it. Not sure now what to do.

Seeking some advice and opinions, thank you.
 

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Would help to know where you are located since terminology varies with geography.

Generally, a mechanical engineer designs stuff on paper. Not where I'd go for help in this case.

A competent mechanic may be able to do the measurements necessary to make this decision but, at leasy around here, we take the pistons and cylinders to machine shop/machinist. They are the ones with the skills and equipment both to advise you and to do the necessary work.

Again, depending on your location, the words may have different meanings.

Welcome to the group.

Got pictures?:p
 

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Discussion Starter #3
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Hi JT,
I am from Sydney Australia, the guy I took the cylinder to has the title of a Motorcycle Engineer, he measured the top of the pistol only. To rebore cylinders costs $75AUD per each cylinder plus the new pistons, the mechanic did not take any measurements, looked at the bore, felt the bore for any lipping at the top of the piston and said it was fine. He is an excellent mechanic with a great reputation in our area, I am keen to order the rings, I don't think I can get them here in Australia.
 

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My CL350K3 sat unused from 1976 to 2010, so 34 years in a barn. The engine was stuck. I cleaned the pistons, cleaned the existing original rings, measured everything closely, ran a ball hone through the bores, reassembled it, and ran it. I have put approximately 7,500 miles on it since then, and it does not smoke or use oil. It runs very well. I did install a Tsubaki timing chain and KA tensioner though. I rode the bike this afternoon, with its original pistons, rings and standard bores that were machined in Japan in 1971.




My Triumph TR6R sat unused from 1971 to 2010, so 39 years in a barn. The engine was stuck. One ring was stuck in its groove and I broke it when removing it. I soda blasted the pistons, measured everything closely, ran a ball hone through the bores, installed new rings, all new bearings and seals, and my buddy and I did a valve job. I have put 7,500 miles on it since, and it does not smoke or use oil. It runs very well with its original pistons in the standard bores that were machined in Meriden, England in 1964.






My BMW R90/6 sat unused from 1998 to 2013, so 15 years in a barn. On this bike, the engine was not stuck. I disassembled, measured, honed the bores with a ball hone, and installed new rings on the original pistons. My buddy and I did a valve job, but used the existing valves and springs. I have put 6,000 miles on it so far this year, starting at the end of March when I got it running. It starts easily, runs great, does not smoke or use oil and the original pistons are running in their standard bores that were machined in West Germany in 1974. I rode it this morning.




My buddy and I did a top end job on my Harley FXRS this summer. It now has new rings on the existing original pistons, new valves and springs, and recut valve seats. I ran a ball hone (same one used previously on the BMW) through the bores, but we did not rebore. We measured the cylinders with a dial bore gage while the cylinders were at standard torque in a set of torque plates. The bike starts and runs well and does not smoke or use oil. The original pistons are in their original bores that were machined in Milwaukee Wisconsin in 1991. It rolled up 72,000 miles as I was riding it yesterday. I bought this one new.

A picture from 2 days ago:



In each case, I considered an overbore. After measuring and comparing to spec, I decided it was not necessary. See a pattern here? The way I look at it, using the original bore as long as practical just saves the meat for when it might be needed later. My bikes have not suffered from this at all.

Ray
 

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Discussion Starter #5
thanks so much Ray. love your bikes . this is my first resto project and my plan is just to get the bike up and running for transport, try and get another motor and rebuild it over time so as not to outlay all the money at once. The rings in my bike look fine also but never though of reusing them... until now that is... I can probably get a rings for $50 plus postage from the US.Did you take the valves out and lap them
 

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On the Honda, I took them out and put all the associated parts for each valve in a labeled plastic baggie. I measured valve stem diameter, valve guide diameter, inspected the face, cleaned off the carbon, but did not lap or cut anything. I reassembled them and put acetone in the ports and let it sit (overnight, as I recall??) and they all held. There are posts on here about why you should not lap a Honda valve, and I took that to heart. Click here, for example.
 

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Discussion Starter #7
thanks Ray ...got that .. could you tell me how I determine what size ball hone I need ?....the cylinder is 70mm sorry I have never done this before
 

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Just be very careful with the ball hone. They are the quickest way to make a cylinder out of round known to man! Well, maybe an exaggeration, but ask your motorcycle his opinion of ball hones. You don't say how many miles on the bike. But, I'd tend to go with your motorcycle engineer. New rings and a quick hone to put a cross hatch in the cylinder walls and put it back together with new rings.

Interesting comment on not to lap Honda valves. I'll have to read that.
 

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Well, after reading the link I came away with a different view. The link says not to cut Honda valves. It does not say do not lap the valves.
 

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There are different opinions about lapping valves. I did mine and have never had any issues.
 

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thanks Ray ...got that .. could you tell me how I determine what size ball hone I need ?....the cylinder is 70mm sorry I have never done this before
I did a little write-up back when I was working on my engine. It tells the hone that I used and where I got it. Click HERE.
 

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Hi JT,
I am from Sydney Australia, the guy I took the cylinder to has the title of a Motorcycle Engineer, he measured the top of the pistol only. To rebore cylinders costs $75AUD per each cylinder plus the new pistons, the mechanic did not take any measurements, looked at the bore, felt the bore for any lipping at the top of the piston and said it was fine. He is an excellent mechanic with a great reputation in our area, I am keen to order the rings, I don't think I can get them here in Australia.
The pistons are tapered, so you measure them at the bottom, not the top. I would not trust the engineers measurement.
TOOLS
 

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The pistons are tapered, so you measure them at the bottom, not the top. I would not trust the engineers measurement.
TOOLS
Was thinking the same thing. You measure the skirts...
 

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Discussion Starter #14
thank you everyone for all your help I have started to pull bit off the head I have another question so I am going to start another thread
 
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