Review: Akunar 67mm YX/Zonger Big Bore Kit, Valve and Clutch Springs

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sean01

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Ok just got a 67mm BBK and some extra's in the mail from Akunar in Thailand.

All Akunars bore kits are $125usd plus shipping which is usually around the $35usd

and since a fair few people buy from akunar but never review his products i thought id throw one up..

now this kit will make 186.85cc in a yx 150, 193.91cc in a 155zonger and 200.96cc in a yx160

Firstly the quality of the barrel looks good (brand new not reco'ed) and it even come with rubber dampeners in-between the barrel fins to try and lower some of the pinging noise these engines are known for..
and it actually look like one of the barrels used on the zonger 155

i have to add too that the quality of this bore kit compared to the ceramic kits available is far better

straight up i measured the sleeve OD and it is 71mm so cases will need to be bored at least 0.030 tho bigger or 71.07mm
the height of the bore from base to head gasket surfaces is 74.47mm
IMG00101-20100518-1723.jpg

IMG00105-20100518-1724.jpg


the piston is a cast flat top type with a 15mm gudgeon pin.
also there are 2mm thick pin boots so you can use the yx 13mm gudgeon pin in the 15mm piston..
the pin height with the pin boots included is 14.25mm

the 2 top rings are both chrome rings which is good as they last longer and can cop a pretty good flogging straight away.
both rings are marked so you know which is the top side of the ring and they also have the numbers 1 and 2 on them to show which ring is for which groove.

head gasket is made of copper and is 0.5mm thick and has a 67mm cylinder hole..

the base gasket is alloy and is 0.55mm thick and has a 70mm sleeve hole..(he has sent me the wrong one so i have to modify this)
IMG00109-20100518-1726.jpg

IMG00114-20100518-1814.jpg

IMG00112-20100518-1726.jpg


ok this is probly overkill for the review but now onto the technical side of it for the odd person that wants to know everything about these..
and for those wondering i use a micrometer and an inside dial bore gauge
IMG00115-20100518-1819.jpg


piston measures 66.916mm in the mic at the top of the skirt (just under the oil ring groove)
then measures 66.988mm at the bottom of the skirt

now the bore clearance im not 100% a fan of but should be it would be fine the way it is or you could give it a quick hone to suit your standards.. but im sure since akunar has been doing these for a long time now im going to trust what he has done..

ok at the top of the bore it has 0.0005 tho (half a tho)
in the middle of the bore it has 0.0015 tho (1.5 tho)
at the bottom of the bore it has 0.002 tho

im guess he does this for perfect ring seal as the skirt only comes up half way in the bore..

ok now i bought a cam off EPOD couple weeks ago but i needed springs, so here they are.
top set are heavy duty YX clutch springs
IMG00113-20100518-1727.jpg


i will edit this post when i get time to check the valves installed height on a YX head and length of these springs on coil bind.
that way everyone can see how much lift these springs will handle..
unfortunately im unable to check the seat and coil bind pressure, but i may take them to the engine shop around the corner from me to see when i have time..

ill probably be installing this kit either this weekend or next on another miniriders members bike (176thumpy) it will be in a yx150 bigfoot with either a 28mm or 30mm oko, but we are still waiting for the carb and exhaust system from akunar..

so keep an eye on this thread as im going to include some extra info like compression ratio, piston deck height etc
oh and for my end thoughts..
 
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Nice man. What did the bore kit set you back?

any idea what the biggest bore is that I could get as a bolt on? No machining to the cases/bigger crank etc?
 
cheers man.
ALL akunar bore kits are 125usd plus post. the kits come with head and base gaskets and obviously piston, rings, pin and clips..

id suggest you email akunar to find out, but im thinking around 63mm..
 
yeah ill get there lol, but if theres something in-particular you need/want to know, fire away mate..

only thing i havent included in the specs yet is the bore height and the piston pin height..

actually ill shoot them up now..
 
Man thats a pretty good price ay. Emailed them a while back about price/postage and they never replied :action-smiley-003:

How much was postage? Gunna shoot them an email and find out how big I can go, then hit bj up for the install.
 
1. what would be the best pin to put in the 13mm or the 15mm as it will no doubt alter the crank balance with a bigger piston and pin?

2. does a larger piston mass make the balance point higher or lower in the rev range?

3. are those gaskets enough to alter the compression to a sustainable level? or will you have to work something else out?

4. what kind of decomp system are you thinking? head button style?

5. Will kick-back be an issue?
 
ok just added some info. check for the bold text..

ok grnman

you can use the 15mm pin, the yx small end is 13mm hence the reason pin boots were supplied.

just weighed the pistons for comparison
standard yx150 60mm piston and 13mm pin is 129grams
67mm piston with pin boots and 13mm pin is 179grams
now also for comparison, a 65mm piston and pin is 158grams..

now i think everyone needs to know something about balancing a single cylinder engine..
they can only be balanced for a given rpm, meaning every other rpm is un-balanced..
so to get one balanced you need to choose what approx rpm youd spend more time at then get it balanced to that.
also note that i doubt these engines are even balanced to start with.
cause lets take the slinger for example, if it was balanced then it would have to be put on in a certain position for the balancing to be effective..

larger piston imo would yield better torque

no decomp.. never broke case yet..
kick back is an issue with all engines
comp i will be working out once i get some deck height measurements,

but going off .5mm deck height and 13cc chamber and gasket sizes it will be at 12.3:1, but this is just a guesstimate
 
ya waste with stock head but throw the s35 v2 and you will hit 20hp. then its time to replace clutch baskets every weekend.
 
ya waste with stock head but throw the s35 v2 and you will hit 20hp. then its time to replace clutch baskets every weekend.

it would have heaps more than 20hp man, if ya looked at no crf heres build and dyno run it was 22hp with standard size bore and stroke with v2 and s35(i think) plus some goodies.

Excellensio info in this thread ...

cheers cactus, i thought a review on the bore kit was well overdue considering how many people actually use them..
 
you can if you want, but there there to lower engine noise..

back in the air cooled days the heads and barrels were covered in these rubber dampeners or they joined the fins with alloy strips.
the idea is to eliminate the fins pinging thus making the engine noisy and giving false sounds if trying to hear for a problem.
and i have actually seen air-cooling fins snap off from vibration.
Siege Models
Siege Models
http://p1.bikepics.com/pics/2007\03\02\bikepics-817668-full.jpg

i have seen alot of people remove them and not knowing what there are doing..
 
you can if you want, but there there to lower engine noise..

back in the air cooled days the heads and barrels were covered in these rubber dampeners or they joined the fins with alloy strips.
the idea is to eliminate the fins pinging thus making the engine noisy and giving false sounds if trying to hear for a problem.
and i have actually seen air-cooling fins snap off from vibration.
Siege Models
Siege Models
http://p1.bikepics.com/pics/2007\03\02\bikepics-817668-full.jpg

i have seen alot of people remove them and not knowing what there are doing..


Well there ya go, i never new that *goes hunting through the bin for the little rubber things*

In that second pic that husky's got a michelin s12 on the front.....The best front tyre ever....... haha, that was totally irrelevant
 
i second the fin rubbers. i have also seen go-kart fins snap off due to this. its just unnecessary engine noise that you dont need. and not to mention breaking fins off, but i doubt you'd break pitty fins off. there not that big.
 
ok little update...

installed the yx H/D valve springs into my yx150 today.

there a sht load longer than standard springs but thinner coils so they can handle just about anything you can throw at it.. i used a 7.2mm cam it still had plenty of gap in the coils..
plenty of seat pressure too.
im yet to measure the coil bind and installed height..(i was in a rush today and totally forgot)

1 thing tho i had to use yx140 retainers as the springs are slightly smaller in diameter.(only fly sht)
if i had access to my mates lathe i would of turned the to suit but sadly he wasnt home.
im not leaving the retainers in there long as they are slightly different, but i gotta pull the head off again anyways cause the valve seals are shot and i didnt have any to replace them with..

but yeah, the performance these springs offer is awesome..
im running a standard valve but ported head, no base gasket(honda bond), standard bore and stroke yx150 with the 7.2mm cam and 30mm oko with 16/41 gearing and theres no way in hell i could get tht thing to valve bounce..in any gear..
it actually revved that much i started to get scared of valves pulling in half lol..

**EDIT**
also for anyone interested to know..
the 7.2mm cam was an akunar item as well and i had to use longer valve adjusters (TB V2 item i used). obviously they have to take a sht load of the base circle to achieve the lift..
 
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