Robin Subaru milling question

125ccCrazy

125ccCrazy

Well-Known Member
#1
#1
anyone have any idea what the max the head on a Robin ex27 can be milled?? reason I'm asking is because of the timing chain, I'm not sure how much the tensioner will take up on the chain..... I was hoping to go about
.050 to .065 since the chamber is so huge and deep.. I would like to get the compression up to about 10:1 to 10.5:1.... I started porting the head and smoothing out the chamber so it can be polished as well, the exhaust port was really restrictive and had a ton of material to be removed to open it up.. The head has 28mm intake and 26mm exhaust valves....

I have alot more smoothing and then polishing to do on the chamber and exhaust port.. The chamber looked like a corn cob before I started...
 
minidragbike

minidragbike

Supporting Speed Nut!
#2
#2
A much better way would be to have the combustion chamber welded up, then that way you only have to mill it to square it up back flat. And that maybe about 0.005"-0.010"
 
125ccCrazy

125ccCrazy

Well-Known Member
#3
#3
minidragbike said:
A much better way would be to have the combustion chamber welded up, then that way you only have to mill it to square it up back flat. And that maybe about 0.005"-0.010"
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yeah that would be a sweet idea but having it welded is not really an option since I don't know anyone that could do that without charging an arm and a leg... I'm not looking to build an all out race engine, I just want to pump it up a little more for a buggy kart to play with in the snow and trails...
 
Phantom

Phantom

New Member
#4
#4
Decking the block surface will give you a boost in compression without touching the head. On the other hand that engine has a huge chamber.
 
125ccCrazy

125ccCrazy

Well-Known Member
#5
#5
Phantom said:
Decking the block surface will give you a boost in compression without touching the head. On the other hand that engine has a huge chamber.
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You are correct, the chamber in the head is bigggggg..... Not sure what the compression ratio is stock but it has to be pretty low, guessing around 6:1 maybe 7:1..... Welding and reshaping the chamber would be a great idea but thats not something I'm too familiar with as far as how much to fill, what the final shape should be ect not to mention the cost to have it welded....Milling was my only option to boost the compression but I don't know how much slop the chain tensioner will take up when dropping the head down...
 
Phantom

Phantom

New Member
#6
#6
125ccCrazy said:
You are correct, the chamber in the head is bigggggg..... Not sure what the compression ratio is stock but it has to be pretty low, guessing around 6:1 maybe 7:1..... Welding and reshaping the chamber would be a great idea but thats not something I'm too familiar with as far as how much to fill, what the final shape should be ect not to mention the cost to have it welded....Milling was my only option to boost the compression but I don't know how much slop the chain tensioner will take up when dropping the head down...
Click to expand...

You can have a chain custom made, milling,decking and filling the head is not a problem the only obstacle is the chain try automotive companies some of them take custom orders, you have to send your chain tho and specify how much material your removing etc. Just my 2 quarters
 
Phantom

Phantom

New Member
#7
#7
Another thing i believe the tensioners in these engines are actuated by a spring you could try to shim the spring for more tension.
 
125ccCrazy

125ccCrazy

Well-Known Member
#8
#8
Phantom said:
You can have a chain custom made, milling,decking and filling the head is not a problem the only obstacle is the chain try automotive companies some of them take custom orders, you have to send your chain tho and specify how much material your removing etc. Just my 2 quarters
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not sure about a custom chain, if anything a link may be able to be removed if needed... I'm gonna ask the guy at the machine shop I have all my machining done at and ask about welding the head up vs milling it..
Phantom said:
Another thing i believe the tensioners in these engines are actuated by a spring you could try to shim the spring for more tension.
Click to expand...
the type of spring that is used is not a coil spring like a valve spring so it can't be shimmed, it's more like the springs used on dirt bikes to keep tension on drive chain..... I think it will take up the slop with milling the head, I may get a new chain in the event this one is stretched...
 
Solo Cali

Solo Cali

New Member
#9
#9
125ccCrazy said:
not sure about a custom chain, if anything a link may be able to be removed if needed... I'm gonna ask the guy at the machine shop I have all my machining done at and ask about welding the head up vs milling it..

the type of spring that is used is not a coil spring like a valve spring so it can't be shimmed, it's more like the springs used on dirt bikes to keep tension on drive chain..... I think it will take up the slop with milling the head, I may get a new chain in the event this one is stretched...
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I knew a guy who milled .040 off his EX17 head and it did run, it was just loud from the chain slapping against the block and or head. The hot set up is a EX17 head ported along with a good valve job bolted to an EX21. The biggest hp gain issue is the single lope cam for these engines. I have not met anyone who has had one of these cams reground, but I'm sure it can be done. I wouldn't go to far with it, noone makes a billet rod, or flywheel so.
 
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N

NJOHC

New Member
#10
#10
Grant Goracy of Exotic toy builders has a cam and flywheel. I've never asked about the rod but QRC racing has a rod. The Subaru rod is one of, if not the strongest in the industry though and where they make all of their power with the stock cam, I dont think a new rod is really needed.

Edit: thats for the ex17 and ex21... I dont know what he has for the 9hp.
 
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Phantom

Phantom

New Member
#11
#11
the type of spring that is used is not a coil spring like a valve spring so it can't be shimmed, it's more like the springs used on dirt bikes to keep tension on drive chain..... I think it will take up the slop with milling the head, I may get a new chain in the event this one is stretched...[/QUOTE]


Oh ok i have a ex17 but i have never opened it to check the tensioner, so no luck there......hmmm
 
Phantom

Phantom

New Member
#12
#12
125ccCrazy said:
not sure about a custom chain, if anything a link may be able to be removed if needed... I'm gonna ask the guy at the machine shop I have all my machining done at and ask about welding the head up vs milling it..

Just my personal opinion and or advice, i think it will be a lot better if you weld the chamber i mean the chain wont be a prob anymore and look at the space you have in that head for welding. Welding a head it sounds difficult but its not you just fill up the spaces you want and then smooth it out and give it a shape.
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Solo Cali

Solo Cali

New Member
#13
#13
NJOHC said:
Grant Goracy of Exotic toy builders has a cam and flywheel. I've never asked about the rod but QRC racing has a rod. The Subaru rod is one of, if not the strongest in the industry though and where they make all of their power with the stock cam, I dont think a new rod is really needed.

Edit: thats for the ex17 and ex21... I dont know what he has for the 9hp.
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Grant would recommend the KX21 motor. He has alot of one off pieces and yes has the cam and flywheel for a hefty price. Another reason going the Subaru route is dollar wise not the best choice. He's the one who told me about the ex17/ex21 swap. All the parts are interchangeable, so if someone wanted to build a sleeper Subaru stuff all the parts from the 21 in a 17 along with the 17 head and yeah Grants cam and flywheel along with a small flat-slide maybe 24mm and you could beat most mild modded 6.5 Honda/Clones.
 
N

NJOHC

New Member
#14
#14
Grant would recommend the KX21 motor.
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That makes sense lol After all, he was one of the guys who helped create it. It does get expensive with these. But with the trophy kart class going on for the Ex27, more and more people are coming up with things for them. Though they may be expensive (I priced a cam for mine at 200$) they seem to respond quite well with HP and especially TQ.

Also, to the OP. Could you some how modify the chain tensioner to take up more slack? (non spring wise) I havent opened any of mine up yet either, so I dont quite know how they have it set up.
 
minidragbike

minidragbike

Supporting Speed Nut!
#15
#15
I would say to just don't mill it at all then. I would just fully port the head. That will increase the dynamic compression, and give you a little more TQ that way, plus a little more HP.
 
N

NJOHC

New Member
#16
#16
I agree with MDB. You seem good with engine work and it never hurts to try new things. And more TQ on these engines is some major thump. They already give out a lot of Tq. Have you contacted the PCP place I gave you a link to? They have something on their page about head mods for increased compression. Maybe you can email them and ask what they do?
 
125ccCrazy

125ccCrazy

Well-Known Member
#19
#19
minidragbike said:
Sorry not to much you can do.
I would sell it, and buy a clone. I have a buddy in the SFV selling a motor for pretty cheap. About 15hp on gas for $500, all the best parts.
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agreed, about all that can be done to the RS is porting, carb and exhaust, any other parts like cam, rod ect will have to be custom made and that expende just isn't worth it when you can build a clone, briggs with readily available parts.. For my RS I just ported the head, made an exhaust and adapted a 26mm carb... I fired the kart up but never rode it before selling it since the kids didn't seem interested in the kart..

$500 is a decent price for a nice engine...
 
65ShelbyClone

65ShelbyClone

Well-Known Member
#20
#20
I know it's after the fact, but something not mentioned in this OLD thread about milling the head and deck is that reducing the distance between crank and cam centerlines in an OHC engine will retard the cam timing. That's fine if you want to push the torque peak higher in the RPM range. It's not fine if piston-to-valve clearance is iffy and milling the head 0.060 or more is going to make a big difference.

Also, the EX27 has 9:1 compression with that "big" chamber.
 
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