Quote:
Originally Posted by GrayBerry Dear All,
Just opened my engine head and cylinder. Looks like I may have to change my cylinder and piston.
My bike is a 2001 AVL 350 engine. I was told that re-boring cannot be done and I will have to purchase a new cylinder (not sure if these cylinders are manufactured these days).
However, one of the mechanic suggested to go the Cast Iron way. Any suggestions?
If I need to do that, then I will need a new crank, connecting rod, piston, cylinder and head unit. I was also told it may not be a direct drop in fit and will need some work on a lathe. Has anyone done this before and is it worth going this way?
Venky |
I am not sure why you were told the cylinder can't be rebored but first a few comments about cylinders and the need to rebore them.
IMO, there are only two reasons a cylinder needs to be rebored.
•If there are relatively deep longitudinal scratches that run down the cylinder wall such as something that might be caused by a loose snap ring or gudgeon (wrist) pin the cylinder may need reboring. These scratches will be very obvious to anyone looking at the bore.
•If there is taper in the bore where the upper area is 0.02 mm larger than the lower area of the cylinder a rebore is suggested. If the upper area is 0.25 mm larger, a rebore is required.
Just looking at the bore and saying something like "it's glazed" is not a reason to rebore.
If the cylinder doesn't have the scratches or taper I mentioned, usually the surface can be lightly honed to create tiny scratches in the surface, new piston rings installed and the engine put back together and it will be fine.
This honing process can even be done with some medium grit sandpaper.
To do it, the thing to remember is you are trying to make very small grooves which will hold oil to lubricate the new piston rings while they break in.
When doing it the sandpaper is rotated about 180° as it travels from the top of the bore to the bottom.
The rotation is then reversed on the next stroke down the bore so it creates a cross hatched pattern.
This will serve nicely for breaking in the new piston rings.
There is also no need to replace the piston unless it is showing signs of excessive wear. This is usually most pronounced on the lower piston skirt.
The piston ring grooves can usually be cleaned up nicely for the new piston rings. In fact, a tool is available for cleaning out the hardened carbon in the bottom of the grooves and with this tool it is a 3 minute job. (The width of the grooves should not be increased so some care is needed when doing this job.)
With a bore with no scratches or wear, new piston rings are all that are needed IMO.
If the cylinder does need reboring, unless it has been rebored at least twice in the past, it should be able to be rebored again.
Pistons and piston rings are usually supplied (generally speaking) in increments of 0.5 mm and a cylinder should easily have enough material for at least two reborings. (The first, 0,5 mm oversize, the second, 1.0 mm oversize.).
Actually there should be enough steel cylinder wall for a third, 1.5mm oversize boring but there may not be any pistons or rings available in that size.
Now, about the cast iron cylinders.
IMO, you should avoid them unless they are the last possible answer to the engines problems.
The steel lined aluminum cylinders remove heat from the bore and piston three times faster (better) than cast iron.
One of the prime problems with the old cast iron cylinders is they cause the engine to overheat and often seize the piston if they are run hard.
If you change the cylinder to a cast iron one you will not be able to utilize the power and speed your AVL has been able to give and engine failure can become a real possibility.
Of course if you always just putt, putt down the road this may not be a problem but if you are using the engines power often or riding at fairly fast speeds this problem is worth thinking about.
There are those who may disagree with me and that's fine but just remember, there are also mechanics who make their money by selling services that are not needed like, selling a total rebore+new piston/+new piston rings rather than just replacing the worn out piston rings.
Finally, reguardless of what you end up doing, don't forget to also have the intake valve and exhaust valve lightly lapped with their respective valve seats.
(Unless the valve head has cracks in the edges of its head, it should not need replacing.)