When to rebuilt engine

B Burdine97

B Burdine97

XS650 Addict
Messages
131
Reaction score
25
Points
28
Location
Minnesota
Hey guys! I have a couple of symptoms telling me my engine is getting old and probably needs a rebuilt. First off when I first got the bike i did an oil change, and changed the sump filter and the filter in the side cover. I noticed there was a decent amount of metal shavings on the magnets on the sump filter. Also there were a couple chunks of the cam chain tensioner in there, not a whole lot though, and Im not hearing any excess engine noise via cam chain rubbing on bare metal. The bike has 24,000 miles on it and I couldn't tell you if the engine has been rebuilt or not, Im guessing not. I havent done a compression test but the bike starts pretty easily with the choke on and a little bit of throttle. My question is, how much longer can I run this engine? Is the site of the cam chain tensioner in the oil pan an indication that the engine needs to be rebuilt as soon as possible? Is 24,000 miles on these motors without a rebuilt ALOT, or is that pretty average?
Thanks!
 
Hi BBurdine, I'd do another oil change now, see what's in the sump that's metallic and plastic. If the tensioner is not making money noises, adjust (to specification) clean / replace filters as needed....new oil.
You will get some plastic coming from the tensioner over time, what you need to find out is how much. The comment about metal shavings "decent amount" is non descriptive, please clarify with a picture if you can.
Complete a compression test, that will also help in your overall evaluation.
My 75B, it was @ 22,000 miles and needed a re build when I purchased it, had a small amount of metal, lots of plastic in the sump, cam chain was rattling around, low compression on [L] side, burning oil (100ml / 64 miles), blowing oil through the blow by.....so yours sounds ok, do another oil change and see what comes out, check compression, might want to replace air filters too, it will love you more......
 
The ferrous chunks on the plug magnets are most likely bits from the #4 starter gear, which tends to self destruct over time, especially when tension on its spring is insufficient; search and read, there's plenty here. Cam chains and dampers are often due before 24K miles, but this may have been done by a PO; keep an eye on them. As far as how far these bikes go without major engine work, that depends on how they've been maintained. I've seen XS650's running strong with over 50K miles on the original top end, and I've seen motors that were clapped out at 15K or less.
 
Good to know grizld. Unfortunately I know nothing about what the previous owners have done to this bike. I believe it went through 3 or 4 previous owner before me. I'm thinking ill rebuild it this winter most likely as I wont be able to ride, I live in the great snowy state of MN.
 
If I plan to rebuild the motor this winter mostly to achieve better compression and renew the cam chain and cam chain tensioner, would it be worth while to have the cylinder bored out a bit more, and install new pistons and rings? Or would I be able to use the stock bore size and just renew the pistons and rings, assuming there isn't any scoring of the cylinder walls. And leave the bottom end untouched.
 
If it was me, i'd go O/S pistons, gudgeon pins and rings, re-seat / re-face valves and plane the head, it's all pretty standard. Mine at 20,000 miles was worn, went straight to 0.50mm (2nd oversize), rebored the jugs to suit, new cam chain and tensioner, gaskets etc....The bottom didn't need anything. First you would need to find out the wear on the cylinders, also check the little ends for wear......

This was original pistons.....rings were stuffed, cam chain and tensioner stuffed......
ENG4.jpg
 
Okay thats good information to know. Yeah Id figure I would make more of a decision when I have the engine actually apart, just thought Id ask before I did that and gather some info. Is planing the head same thing as decking it? Just removing a little bit of material to make the surface flush again. If so how much material did you remove, cause Ive read something on hear about not wanting to remove too much material otherwise timing will be off.
 
My engine has 53,000 miles on it. It has never been opened. After a bit of neglect, I've found metal bits and cam chain tensioner bits in the sump. I give it some love, such as a cam chain adjustment, etc and she's purring along nicely again. My last oil change was clean. I'll keep going.
 
We always skim the heads, I would think its the same as 'decking it", the link below has tolerances on engine components, couldn't find the max / min on the head though. Like Marty said, his hadn't been opened before, mine was a virgin too, so it would be OK to shave / skim / deck, there would be enough meat there. I would really look at the valves and seats......

http://www.xs650.de/literatur/wsh/pdf/03-motor.pdf
 
Just for the sake of reference, a total skim (deck) value of 0.016" (0.4mm) would retard the cam timing by 1° (at the crank), and increase compression ratio of a stock XS650 8.4:1 engine to 8.7:1. If that's disturbing, you can get copper headgaskets of appropriate thickness to make up the difference, or double the base gasket...
 
TwoManyXS1Bs said:
Just for the sake of reference, a total skim (deck) value of 0.016" (0.4mm) would retard the cam timing by 1° (at the crank), and increase compression ratio of a stock XS650 8.4:1 engine to 8.7:1. If that's disturbing, you can get copper headgaskets of appropriate thickness to make up the difference, or double the base gasket...
Click to expand...

We're you up all night working that out 2M? I don't find it disturbing, it's bloody amazing mate....have to break out the slide rule now..........where did you pull that from, especially the (1° at the crank)?
1° Is a lot of degrees for 0.4mm, it's also a fair chunk to shave off.....
 
had mine decked and used no extra or copper head gasket. runs strong, but then its bored 1.5 over. no replacement for displacement
 
The effects of head milling (decking, skimming) on XS650 camshaft timing and compression ratio.

Using these manual excerpts for the 1978-1979 XS650 engine:

EngineSpecs-2F0.jpg 77-06-22_M033-01_CamshaftSprocketSystems.jpg DID-219-ChainDims.jpg

CAMSHAFT TIMING:

The 74-83 camchain has a link pitch of 7.774mm
The cam sprocket has 36 teeth
This gives the sprocket an effective/mechanical chain link circumference of 36 * 7.774mm = 279.864mm
The cam sprocket rotates once for every 2 rotations of the crank, for a total of 720° of crank rotation.
That means that (279.864mm / 720°) = 0.3887mm of camchain travel per degree of crankshaft rotation.

Moving the head up or down, the camchain will remain taught at the front, rear slack absorbed by the tensioner.
So, moving the head 1mm is the same as moving the front camchain 1mm.
Move the head up, the camchain appears to move down relative to the cam, advancing the camshaft.
Move the head down, the camchain appears to move up relative to the cam, retarding the camshaft.

Move the head down (from milling/decking/skimming) by 0.3887mm (0.0153") will result in 1° of camshaft retardation.


COMPRESSION RATIO:

The static compression ratio is the mechanical ratio of (DisplacementVolume + CombustionChamberVolume) / CombustionChamberVolume.
I.E., a displacement of 9 and a head chamber of 1 gives a 10:1 C/R.
A simplified way is CompressionRatio = (DisplacementVolume / CombustionChamberVolume) + 1

We'll do the calculations for one cylinder in millimeters:

Bore * Bore * Pi/4 = CircularArea of one cylinder in square millimeters
CircularArea * Stroke = DisplacementVolume in cubic millimeters for that cylinder
CircularArea * SkimValue = Volume lost to the CombustionChamber from milling/decking in cubic millimeters

To get it in Cubic Centimeters, divide that mess by 1000

Example, for the Standard Bore:
Bore = 75mm
Stroke = 74mm
Pi/4 = (3.14159 / 4) = 0.7854

75 x 75 x 74 x 0.7854 = 326922, then (divided by 1000) = 326.9cc
That's DisplacementVolume for one cylinder. Multiply by 2 to get total displacement = 653.8cc

If the Mill/Deck/Skim value is 0.4mm (0.016"), the CombustionChamberVolume will be reduced by:

75 x 75 x 0.4 x 0.7854 = 1767, then (divided by 1000) = 1.767cc

The documented CombustionChamberVolume is 43.6cc.
CompressionRatio = (326.9 / 43.6) + 1 == 8.5:1

Reduce the CombustionChamberVolume by 1.767cc, now is 41.8cc.
CompressionRatio = (326.9 / 41.8) + 1 == 8.8:1

For convenience, for the stock/standard bore and unmolested head, here's a table of Mill/Deck/Skim values and expected results:

Skim.....Reduce Cmbr CompressionRatio
0.0mm 0.00cc 43.6cc 8.5:1
0.1mm 0.44cc 43.2cc 8.6:1
0.2mm 0.88cc 42.7cc 8.7:1
0.3mm 1.36cc 42.2cc 8.7:1
0.4mm 1.77cc 41.8cc 8.8:1
0.5mm 2.21cc 41.4cc 8.9:1
 
Last edited:

Similar threads

IronFist_650
This doesn't look healthy, bits from cam chain tensioner?
Replies
9
Views
413
5twins
5twins
notmoving
Oil Filters
2
Replies
39
Views
1K
5twins
5twins
denisducati750
Cam Chain Tensioner
Replies
5
Views
637
denisducati750
denisducati750
denisducati750
Cam Chain Tensioner
Replies
0
Views
323
denisducati750
denisducati750
B
Post engine rebuild running issues
Replies
5
Views
433
Jim
Jim
Back
Top