PROJECT 900

First test results:
Piston skirt 68mm
Top & second ring end gap .020 (0.50mm) limit 1.0mm
Bores are unlined with no ridges.
Book says stock bore is 67mm, so this has been rebored to oversize 4, unless Len can tell me different.
Given the info' to date, what is the general prognosis guys ?

 

Well, 67mm is stock bore for the 853cc "900" engines, while 68.5mm is the standard bore for the 891cc "900" engines, which is what your bike should have, correct? (the 891cc version).


So 68.0mm would be a 4th oversize (wow!) for an 853cc engine, and not be possible for an 891cc engine, and gosh knows what else it might be if it's not one of those...........what size cc's are cast into the front of the jugs?


I think it's quite unusual to see an engine with a 4th oversize set of pistons in it.........I've never seen one.

 

I will go & see what is on the block, Len, but I'm begining to feel better. 8)

 

U have to remember Chacal that he measured the piston skirt not the bore. Although I haven't been into the parts nearly as much as you I have never seen somedody do a 4th bore even in the days of two stroke dirtbikes you when 1 and maybe 2 and got a sleeve put it.


I'd say time to buy a new book hehe..

 

It says 891 cm3, so we have a whole different scenario.
There must be a part of this manual that I havn't found.
What do you think, a hone & rings, perhaps?

 

As long as the piston to bore clearance is within spec, a hone and new rings will do it.

There should be a piston to bore clearance spec in your manual and it wouldn't be any different between the two engine sizes.

 

Found the right section of the book, now, Hillsy.
Piston clearance = 0.03-0.05mm. Limit 0.1mm
Would I need just standard rings?

 

wiz to answer that I would hone or have the cylinders honed then you will have to measure them and the pistons you are going to use. It just has to be we can't tell you the rings you need until everything is cleaned up and measured properly. Then it will be apparent by the measurements as to what you need.

 

Yep - what he said ^^^

 

I will re-use these pistons, they are well in spec'
How's this for a clunk test.

 

Urk. Also, way off topic but an interesting indicator of our "global economy" I have the exact same electronic dial caliper. The display and placement/color of the buttons are EXACTLY like mine except mine came in a black plastic case. Sold by a different company I'm sure, but sourced from the same Chinese manufacturer no doubt...

 

I'm an analog type of guy myself............

 

Wiz,

I have sent you a message re piston/piston ring sizes based on my experience.

We have 891cc bikes...when I re ringed mine I just got standard rings...even after a light hone they work perfectly. 4th Oversize could cause problems.

 

Wiz,

I have sent you a message re piston/piston ring sizes based on my experience.

We have 891cc bikes...when I re ringed mine I just got standard rings...even after a light hone they work perfectly. 4th Oversize could cause problems.

Any oversize rings will require tailoring to the piston. I think Rick might have a detailed posting here on fitting an oversized ring to a piston so that it sits neatly on the piston with appropriate gaps as per specs.

Let me know if you need further information.

Cheers.

 

So am I, I just bought an analogue multimeter & I had to ask Polock how to use it.

 

Make sure you zero it before each and every ohm test.........and also, if the internal battery is weak, they tend to read incorrectly.........

 

Some rust off the holding down bolts dropped into the crankcase when I was lifting off the block, what's the best way to make sure that it doen't cause a problem?

 

Tips for cleaning pistons, any?

 

Common issue rebuilding an old motor. Fill the crankcase up with cheap 10W or 20W motor oil by pouring it down on the crank/conn rods and flush it all clean; then drain the oil and replace with "real" oil before you start the motor. I don't like to use solvents in a bottom end because you can never be sure it's all out; and a thinner liquid can carry crud INTO places you don't want it.

Carb cleaner and a toothbrush for cleaning pistons. If the tops are all coked up, use a wooden or plastic scraper; you can use a tiny brass wire brush in your Dremel too but be careful around the ring grooves. Even a brass wire brush can "soften" (round off) the edges.

 

. . . and then polish the crowns and combustion chambers !!

Spring for the 3 angle valve job too. Course then you'll need to re-shim.