YX150/160, 155z top end build up and magneto mod

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sean01

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Ok been thinking of doing a run through of the KLX style YX engine top end build process.
this will also include a few little hot up and reliability mods.

ok well firstly you'll want to drain the oil out of the engine so you don't make too much of a mess.

parts/materials needed..

top end gasket set (inlet manifold gasket, exhaust gasket, head gasket, cam gear gasket and maybe a base gasket)
piston kit
engine oil
really good sealant like Honda bond or high quality high temp silicon
degreaser of some sort and also strong truck wash
WD40
rags

tools needed..
8mm, 10mm, 12mm, 14mm, 17mm sockets and extension bars.
8mm, 9mm and 10mm spanner
possibly you'll need some allen keys(hex keys)
impact driver with star(Philips) screw driver bit
hammer
pointy nose pliers
variety of screw drivers
gasket scraper or Stanley knife blade
honing tool
variable speed drill
air compressor and air blower
bench vise

depending on the bike style you have, remove what ever appears to be in the way of the head coming off.

start by removing the inlet manifold, exhaust pipe and spark plug.
then remove the magneto cover and the cam gear cover and just crack the bolts that hold the valve covers on.

with a 14mm socket stop the magneto from turning while cracking the cam gear bolts.
once there loose roll the magneto till you have TDC on the cam(usually the mark on the cam gear with YX stamped). TDC on the cam is timing mark @ 9 o'clock (there is a arrow cast into the head).
now with a 10mm socket or spanner remove the bolt in the center of the timing chain adjuster. now remove the timing chain adjuster all together.
double check to see it is still on TDC and remove the cam gear bolts all together and pull the cam gear away from the camshaft.
now if your replacing the camshaft now is the time to undo the 2 screws in the camshaft retaining plate with an impact driver. remove the retaining plate completely.

on the opposite side of the engine remove the oil feed pipe from the clutch cover and the head.

undo the 4 head bolts(take note of where the brass washer is).
remove the 2 long bolts from inside the cam gear area(they screw into the cases from the head).


the head will now slide away from the barrel (may need a bit of prying)
at the moment your engine should look like this


to remove the barrel you will need to remove the 2 bolts that hold the bash plate to the barrel.
pull the bottom rubber chain guide out of the barrel.
now slide the barrel off (may need a little prying)

if your replacing the piston then start by removing the wrist pin clip from the clutch side (cam chain guide is in the way on the other side), then slide the wrist pin out (this may need a small screw driver)

and there you have it, complete top end removed and ready for cleaning, modding/prepping and assembly.

start with cleaning all gasket surfaces till there is no signs of the gasket at all.
give the barrel a quick wipe out with a rag.
fit the barrel in a vice, but don't clamp it on any gasket surfaces.
now with the drill fitted with the honing tool give the barrel a good spray with wd40.
start with a reasonable speed on the drill and start honing it, but always keep the hone going up and down the barrel.
also make sure the hone isnt chattering, if so then apply more pressure on the hone with the adjuster.
when your happy that there is no or minimal marks in the barrel lower the speed of the drill a pretty slow speed.
now this going to be the finishing hone and can be a little tricky. give the barrel another good spray with wd40.
start the hone and go straight to the opposite end of the bore..pull the trigger on the drill, get to the bottom and release the button then the same when coming back up the bore. do this several times then squeeze the hone legs in and remove the hone form the bore.
give it a spray and wipe it out and check out the finishing pattern, you want a nice even 45 degree crosshatch in the bore.
(hard to see in the pic but it is evident)


when your happy with the finish give it a good degrease then a good wash with HOT soapy water and another degrease. blow dry the barrel completely as it only takes minutes before rust will form.
use a tissue and wipe the bore out to make sure no dirt or grit is left in it..
ok barrel finished spray the bore with WD40.

if your porting the head now is the time to do it, for a simple clean up job you can leave the valves intact.
iv found the best and easiest porting for these is to open the exhaust port up to the exhaust gasket and same with the inlet.. just blend what you have removed back to the guide boss.

probably not a bad idea to do a leak test on the valves.
1 port at a time fill with water, kero or fuel and make sure the valves aren't weeping/leaking.
if they are then they will require work (re-lapping in or re-machining)

for cleaning the head i recommend you remove the camshaft.
to do this remove the valve covers completely and loosen the valve adjuster screws a little bit.
now the camshaft retaining plate should be removed already so from where the plate was you'll see 2 round shafts, these are the rocker arm shafts.
use a 8x1.25 threaded bolt (commonly 12mm/13mm head), screw it in to one shaft at a time and pull them out.
now remove the rockers from the head. now the cam can slide right out.


again clean all gasket surfaces and clean the head with degreaser then a quick wash with truck wash and blow dry and spray with WD40.

now for installing the piston and rings or just rings...
start with the center oil ring first then the bottom outer ring and then top outer, then install the second ring then the top.
space all the ring gaps 180 degrees apart except the center oil ring which will be 90 degrees from both the outers.
the black ring goes in the second ring groove and the chrome ring in the top.
if there are any marks or letters on the rings, they always face to the top of the piston.


install the cam chain side wrist pin clip, these can be tricky and can get lost easily when they slip.
now slide the piston over the con-rod, on the piston there should be some letters (EX on this one) that goes to the exhaust side of the engine. so facing down.
oil the wrist pin and slide it in from the clutch side then install the last wrist pin clip.
apply some oil to the piston and rings and dab it around making sure all the piston is coated (except the top).

for the base gasket, you can choose to either install the proper gasket or leave it out and apply some silicon gasket sealant.
using sealant increases your compression a little by raising the piston up the bore(by approx 1mm).
also when using sealant make sure the case and barrel gasket surface is totally free from any oil.(clean with a bit of fuel)
you can see the difference in the following picture. and don't go over board with the sealant.


ok now you ready to install the barrel. firstly make sure both the locating dowels are installed to either the cases or the barrel.. should be 2 on the cam chain side
slide the barrel down and slowly work the rings into the barrel. the bottom of the bore is tapper-ed and usually closes the ring gaps on its own. if it feels stuck, remove and try again. on occasions i have had to use something to help close the gaps but be careful not to damage the rings..

installing the head is pretty straight forward. but don't forget to install the bottom cam chain guide in the barrel and also the dowels, and the cam gear @ TDC with the crank @ TDC before fitting the head.
 
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which ever cam you install make sure the 2 lobes are facing the chamber as this will be pretty close to TDC on the cam. then install the rockers and shafts and retaining plate the same way you removed them. but make sure everything is super clean and lubed well with oil (especially the cam lobes)..
install the head gasket (this can only go on 1 way). again make sure both surfaces are clean from oil.
now slide head on and install the 2 8mm head bolts from the cam gear area, but only just nip then up.
then do the 4 head bolts and don't forget the washers with the brass one on the bottom exhaust side stud. nip them up evenly. then proceed to do them up tight but again evenly going from diagonal to diagonal nuts. then check the tightness of the 2 8mm bolts in the cam area.

line up the cam gear and slide it on the cam keeping everything @ TDC, the cam should be free to turn a bit.
once your happy with the cam timing install the 2 10mm bolts and hold the magneto with the 14mm socket and tighten the bolts firmly.(you can apply loctite to these bolts if needed).
the chain tensioner goes in now but remove the middle 10mm bolt and press the little button in and push the plunger up and if you've used the sealant method for the base gasket then do the same for the tensioner. remove the gasket and apply a light coating to it.. this takes up the little bit more cam chain free play you get from using no base gasket..
now install the 10mm bolt (don't forget the spring) nip it up good.

set the rockers, .003 thou on the inlet and .004 thou on the exhaust. once done fit the valve covers.

fit the inlet manifold and exhaust pipe (don't forget the exhaust and inlet manifold gaskets).

the rest is pretty basic, fit the oil tube from the clutch cover to the head making sure the alloy washers are installed, then fit the cam gear cover.
and now fit anything else you removed.
add oil and see if she'll start.

now to add to all this i would recommend that anyone upgrading the camshaft also upgrade the valve springs and install a magnetic oil drain plug.
AKUNAR does an awesome package which comes with new retainers as well because the springs are a little bigger on the OD.

also for the standard outer rotor kit that comes installed on some YX KLX engines its recommended that you do the magneto timing mod.
the idea is too retard the timing as these are set with way too much initial timing.


some extra info regarding ring gaps.
basic rule of ring gaps is .004 thou for every inch of bore size.
so a 60mm bore converts 2.362 inches, times that by .004 and you end up with .009 tho. the second ring gap is a little bigger but not by much.
so top ring gap is .009
second ring gap is .011
oil rings are fine as they are unless there touching.
now maximum allowable gap would be .012 for top and .014 for second and i class this as extreme gap..

now also you need to see if the rings have worn the ring grooves out.
i'v seen some rings where they have a step worn into them so obviously the piston would of suffered some wear too.
so fit the new rings to the piston. the ring to piston groove clearance is .001-.002, and more than that and it allows the ring to twist too much and not provide a good seal in the bore.

for piston and bore measuring it gets a little tricky as you need some special tools.
but piston to bore clearance should be no greater than .002 thou, a new bore and piston would have around .001 thou clearance..
forged pistons how ever run more clearance, .002 thou would be considered new clearance and .0035 would be maximum clearance..

a few things come into it when considering the clearance..
the piston skirts can collapse leaving a good cylinder but poor clearance.
pistons are actually tapered, measured just under the oil ring on the skirt and then slowly moving down to the bottom of the piston, you should see a noticeable taper.
if there is no taper then the piston is considered a bin job..

this is why i always prefer to use a new piston regardless..

bores can wear also from all sorts of reasons. poor service history, dirty or no air cleaner, rich tuned carby, overheating and so on.
bores also are meant to be tapered but no where near the amount that a piston is.
usually from top of the bore to bottom will have a bout .001 thou taper..
also if there is a lip in the top of the bore thats a good indication that it will probably be out of spec.

if there is anything anyone want to know or would like to add then post away..

ill probably add some extra info over time, so keep an eye on this thread..
 
Last edited:
which ever cam you install make sure the 2 lobes are facing the chamber as this will be pretty close to TDC on the cam. then install the rockers and shafts and retaining plate the same way you removed them. but make sure everything is super clean and lubed well with oil (especially the cam lobes)..
install the head gasket (this can only go on 1 way). again make sure both surfaces are clean from oil.
now slide head on and install the 2 8mm head bolts from the cam gear area, but only just nip then up.
then do the 4 head bolts and don't forget the washers with the brass one on the bottom exhaust side stud. nip them up evenly. then proceed to do them up tight but again evenly going from diagonal to diagonal nuts. then check the tightness of the 2 8mm bolts in the cam area.

line up the cam gear and slide it on the cam keeping everything @ TDC, the cam should be free to turn a bit.
once your happy with the cam timing install the 2 10mm bolts and hold the magneto with the 14mm socket and tighten the bolts firmly.(you can apply loctite to these bolts if needed).
the chain tensioner goes in now but remove the middle 10mm bolt and press the little button in and push the plunger up and if you've used the sealant method for the base gasket then do the same for the tensioner. remove the gasket and apply a light coating to it.. this takes up the little bit more cam chain free play you get from using no base gasket..
now install the 10mm bolt (don't forget the spring) nip it up good.

set the rockers, .003 thou on the inlet and .004 thou on the exhaust. once done fit the valve covers.

fit the inlet manifold and exhaust pipe (don't forget the exhaust and inlet manifold gaskets).

the rest is pretty basic, fit the oil tube from the clutch cover to the head making sure the alloy washers are installed, then fit the cam gear cover.
and now fit anything else you removed.
add oil and see if she'll start.

now to add to all this i would recommend that anyone upgrading the camshaft also upgrade the valve springs and install a magnetic oil drain plug.
AKUNAR does an awesome package which comes with new retainers as well because the springs are a little bigger on the OD.

also for the standard outer rotor kit that comes installed on some YX KLX engines its recommended that you do the magneto timing mod.
the idea is too retard the timing as these are set with way too much initial timing.


some extra info regarding ring gaps.
basic rule of ring gaps is .004 thou for every inch of bore size.
so a 60mm bore converts 2.362 inches, times that by .004 and you end up with .009 tho. the second ring gap is a little bigger but not by much.
so top ring gap is .009
second ring gap is .011
oil rings are fine as they are unless there touching.
now maximum allowable gap would be .012 for top and .014 for second and i class this as extreme gap..

now also you need to see if the rings have worn the ring grooves out.
i'v seen some rings where they have a step worn into them so obviously the piston would of suffered some wear too.
so fit the new rings to the piston. the ring to piston groove clearance is .001-.002, and more than that and it allows the ring to twist too much and not provide a good seal in the bore.

for piston and bore measuring it gets a little tricky as you need some special tools.
but piston to bore clearance should be no greater than .002 thou, a new bore and piston would have around .001 thou clearance..
forged pistons how ever run more clearance, .002 thou would be considered new clearance and .0035 would be maximum clearance..

a few things come into it when considering the clearance..
the piston skirts can collapse leaving a good cylinder but poor clearance.
pistons are actually tapered, measured just under the oil ring on the skirt and then slowly moving down to the bottom of the piston, you should see a noticeable taper.
if there is no taper then the piston is considered a bin job..

this is why i always prefer to use a new piston regardless..

bores can wear also from all sorts of reasons. poor service history, dirty or no air cleaner, rich tuned carby, overheating and so on.
bores also are meant to be tapered but no where near the amount that a piston is.
usually from top of the bore to bottom will have a bout .001 thou taper..
also if there is a lip in the top of the bore thats a good indication that it will probably be out of spec.

if there is anything anyone want to know or would like to add then post away..

ill probably add some extra info over time, so keep an eye on this thread..

sorry to lazy to read haha with the cutting of the sensor lobe what does it do ? retard the spark a smidge to make it easier to start ? iv seen it b4 but not to sure ? cheers
 
sorry to lazy to read haha with the cutting of the sensor lobe what does it do ? retard the spark a smidge to make it easier to start ? iv seen it b4 but not to sure ? cheers

i got a funny feeling people are gonna be "too lazy" to give you an answer
 
im not getting pissy mate your taking it the wrong with you are im just stating the facts
 
dont do this timing mod. my bike lost sooo much mid range power.
 
a little bit of power loss can be noticed on a stocker, but what would you prefer... broken kick start gears or cracked cases?
 
Wow dude, this is a flippin awesome tutorial, I shall print it off now and learn it word for word in bed, all ready for my installation. Im a bit scared about installing the camshaft and getting the cam chain at tdc, but I will give it my best shot.

Reps to you mate +++++++++++++
 
Wow dude, this is a flippin awesome tutorial, I shall print it off now and learn it word for word in bed, all ready for my installation. Im a bit scared about installing the camshaft and getting the cam chain at tdc, but I will give it my best shot.

Reps to you mate +++++++++++++

lol the bit yr scared of is the easiest of all
 
Bumping this thread.

Did anyone do that magneto mod?
 
i guess the only way to verify it is with a dail on the crank or somthing i dont think it is nessorcary when you can adjust your timing by moving the strator plate
 
^^ only if you have an adjustable one, like the Lifan 150 outer rotors
 
Hmmm, I just picked up a new head that I have to wack on tomorrow, side cover is off anyway, should I grind the mag?
 
how much kickback do you get trying to start it?
and how did you think your bike will go across the rev range running it with less timing ?
 

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