Rebores

Message

Henriettaah · Thu Dec 15, 2005 1:58 am

Does anybody know what is involved in a rebore?

i.e., what exactly happens? what is removed? what is adjusted/amended?

I'm still having problems with the bike. But, it does really well above 4k rpm. Its at the lower end that it is prone to going pfft and conking out. Keeping the revs right up can help when waiting to pull away...

I rang a garage for some advice, and they say it might be the compression - we did suspect this earlier. However, he said that when the revs are cranked up a bit, there could be enough pressure to override any leaks, which would explain its good response higher up.

I mentioned this to my neighbour, and asked him if the bike's allegeded rebore it has had previously might be a load of twaddle - he said not neccessarily - which would mean something hadn't been put back together properly/efficiently/tightly etc.

Hence my wondering hwat is involved with a rebore - what is done, and what could be neglected/not done fully on the rebuild to affect it in this way ??!

boom · #2 Unread post by **boom** » Thu Dec 15, 2005 3:31 am

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Henriettaah · Thu Dec 15, 2005 7:04 am

Good post.

Thu Dec 15, 2005 11:29 am

a rebore means removing the head then the jug (this is the cylinder) removing the piston.taking jug to machine shop and getting it bored out to oversize--buying a new oversize piston and a set of rings and gaskets--then putting it all back together and going for a ride and discovering that you still have the same problem.

#5 Unread post by **dr_bar** » Thu Dec 15, 2005 1:52 pm

9000white wrote:a rebore means removing the head then the jug (this is the cylinder) removing the piston.taking jug to machine shop and getting it bored out to oversize--buying a new oversize piston and a set of rings and gaskets--then putting it all back together and going for a ride and discovering that you still have the same problem.

BuzZz · #6 Unread post by **BuzZz** » Thu Dec 15, 2005 6:41 pm

Yeah, what he said....

And none of it happens for free.

Your bike would be more likely to just need rings and a piston rather than a full bore job, IF that. Your bike has Primary(sealed by the reed valves) and Secondary(sealed by the rings) Compression. The only sure way to know if something is amiss in the compression area is with a leakdown test. Well, you could throw parts at it until you went broke or fixed it if you really wanted to, but.....

A reputable shop should be able to do this for you and tell you where the leak is, rings, reeds, seals, case gasket, whatever it is. If it is indeed leaking. A few bucks might save you alot of time and even more money.....

Henriettaah · Thu Dec 15, 2005 10:25 pm

- Am borrowing a compression tester today, to we'll know if there is a pressure leak anywhere. That should save us a little time at the garage, if I can ask straightaway for a leakdown test.

Henriettaah · Fri Dec 16, 2005 7:14 am

Well.. I got hold of a compression tester - compression was 3.6:1, when it should be 6.4:1.

So when talking about taking the head off for a leakdown test, my friend commented on there being spring washers under the nuts for the head.. I got some plain washers to swap them, went to unscrew the nuts... and could do them by hand i.e with my fingers and no spanner

So, have swapped the washers and tightened them nuts good, but yet to get the tester back to retest.. I am optimistic (always a good idea with this bike!). Started well, anyway

BuzZz · #9 Unread post by **BuzZz** » Fri Dec 16, 2005 8:52 pm

That could be a good sign alright, Henry.

I'm a bit confused about the numbers you put up. 3.6:1 is a compression ratio, not a compression reading. The ratio is how much volume the piston sweeps compared to the amount of volume left when the piston is at TDC. it doesn't change without physically altering some internal engine dimension.

A compression reading will be expressed as an amount of pressure over atmospheric pressure. Something over 100 PSI or some such number.

Henriettaah · Fri Dec 16, 2005 11:21 pm

Thats just me working out the ratio from the PSI - for there to be a ratio of 6.4 : 1 the PSI should have been at least 100 PSI from what I can make out, which would make the reading I got of around 55 PSI at a ratio of about 3.6 : 1.

BUT after further reading on the net, it seems I did the compression test wrong.

So -

I'm going to report back later with a new result - the follow up compression test I did was disappointing, and didn't make sense - if the PSI was that low, from what I can figure the bike would barely start, no matter run pretty well (when it does!). So thats how I worked out I think I was doing it wrong.

I certainly hope it was user error anyway!