Honda Twins banner

1 - 5 of 5 Posts

·
Registered
Joined
·
46 Posts
Discussion Starter #1
Hi all,
My CB450 just had new pistons (0.5mm oversize) and rings installed... it was a barn find with seized pistons. I've ridden it only 100 miles since the rebuild, and I checked compression today while the engine was warm. I got 145~150 on both cylinders. Throttle wide open, both spark plugs out, no choke.

Is that normal? Good? Bad? Is it even worthwhile to check compression right after new pistons/rings are put in, or do they need to break in / bed in first before a compression reading is valid?

The top end on this bike seemed to be in OK shape, so it was left alone in the rebuild. My mechanic said he tested it by pouring some gas on top of the valves while the head was off the bike and all the valves were closed... and he said no gas leaked down through the valves, therefore they should be OK. I'm new to this stuff, so, is that a valid method of checking the condition of valve sealing? Seemed clever to me but I'm not sure if it's meaningful.

Thanks all!
Max
 

·
Registered
Joined
·
703 Posts
No harm doing an early compression check, but redo it later once the rings have fully bedded in and you'll get the steady state/run-in reading.

On the petrol leak test, I asked one of the older-hand pro engine builders round here the same thing. His view is that the valves rotate constantly in normal usage, and the fact that they seal properly at one point - in a cold engine, as opposed to at full combustion temperatures isn't really that meaningful. If you are getting a leakage statically on the bench, then you have a problem for sure - but if you aren't, it's no guarantee that the valve will seal properly when running.
 

·
Registered
Joined
·
46 Posts
Discussion Starter #4
Thanks for the links 76Twin - those are great. I received the bike from my shadetree mechanic shortly after the rebuild, and I'm not exactly sure what he did from mile zero, but I've been riding it with a pretty liberal right wrist since I got it... Glad to know that this has actually been good for it!

It's my first bike and I'm learning a ton from it... if I end up needing to re-hone the cylinders and install new rings myself so that I can tend to its break-in from mile zero in the future - well, that's just an opportunity for more fun and learning.

Until then, I guess I'll follow the "if it runs, ride it" mentality!
 

·
Registered
Joined
·
77 Posts
Very cool choice for a first bike. I'd check it again after a few miles more for curiosity sake than anything.
It will take a little time for all those new parts to get happy in their new home and seat properly etc.
 
1 - 5 of 5 Posts
Top