Machining head O-rings

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DougB
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Machining head O-rings

Post by DougB »

Hi, I know some people use o-rings to seal the head without a head gasket (Janbros I think you mentioned it before?) I've had several bikes in the past that have O-ring heads but I've never done a conversion job.

Does anyone have any detailed technical info on carrying the job out? It's something I'd be interested in doing while I've got a head on the bench before rebuilding my spare motor.

Cheers, Doug.
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Re: Machining head O-rings

Post by JanBros »

you can do it on a lath (easiest, but the lath needs to be big enough) or on a mill.
it's not a dificult job, and the main advantage is you can open the head without needing a new gasket.
that's as far s the combustion chamber goes. the water can easily be sealed with silicon sealant but than you need to be sure both barrels are equal in height.

best is to have a big O-ring arround the water jacket to, but that's not easy (etch 'n' sketch method on a mill), or you pay someone to CNC it.
If you go for the big O-ring arround the water, make sure you use a head that hasn't bean skimmed a lot already, so you still have the little edge sticking out of the barrels, AND have the outside skimmed 1/10th mm or so, so that there isn't too much stress on the little remaining edge. I didn't do this with the original head and they started to crack :(
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Re: Machining head O-rings

Post by DougB »

Thanks for the advice. I'd looked at using an Oring around the outside and decided it would be too much trouble for the reasons you mentioned. I'd thought about machining a head similar to the oring groove that's been put into a head MJ posted a pic of (a long time ago)

viewtopic.php?f=19&t=769

Just 1 oring channel close to the combustion chamber then liquid sealant on the rest of the head.

I'm just not sure what diameter oring, I can get orings made in Viton in various diameter cord and to whatever size I need, I'm just not sure what size works best, what do you use in yours?
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Re: Machining head O-rings

Post by JanBros »

I think they are 60mm inner diameter, I use 1.5mm thick rings.
If you do it, you need to cut the groove with the width of the O-ring or just a touch bigger so it stays in by itself, and make the depth 80% of the thickness of the ring. As rubber rings can not be squished together, they only deform, the volume of the groove must be just slightly bigger than the volume of the ring and with 80% you achieve that.

I wouldn't use glued cord arround the combustion chamber (I do use it arround the water jacket), just use full viton rings.

offcourse you could always add your name to the list in this post, only one more needed (if the others are still in) :wink:

viewtopic.php?f=2&t=11307&hilit=cnc+head
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Re: Machining head O-rings

Post by Luders »

Yeah I agree Jan, seems like a lot of work only to benefit from not using a head gasket.

Stick your name down and get yourself a billet head and you can change the combustion chamber inserts to suit.
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Re: Machining head O-rings

Post by maccas »

I'm still in for a head Jan!

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Re: Machining head O-rings

Post by 500bernie »

Hi Doug,

BDK will mod your head to fit o rings.

Cheers,
Bernie :)
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Re: Machining head O-rings

Post by DougB »

I spoke to Brock at BDK and they said they have done it before but he didn't entirely recommend it due to material thickness (I think he mean't around the outer side of the water jacket), I've emailed him a few links from this site to read through and he said he'll look into it.

In the meantime I've dropped the head off at Blue Haze in York, he's going to get back to me if he's happy to give it a go, I'll pass on the oring sizes you mentioned Jan.

Simply I can't afford a CNC head, much as I'd like one, then as I don't have the machining tools myself to finish it I'd be begging or buying workshop time and I'm pushing favours as it is recently. At the moment I'm seriously considering getting my ACU and heading out racing next year, very likely on a KR, and I'm just trying things that I'd likely do to any engine I use on track. I've got 4 cyl heads now so don't mind having a go at converting one then see if it runs ok as a sort of test bed for a future engine if that makes sense? It does to me anyway :-)
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Re: Machining head O-rings

Post by Gerrit »

What about machining the cylinder instead of the head? I have a Yamaha TZ250 4DP cylinder and the "O" ring grooves are cut into the top of the cylinder. The same applies to the KR750 cylinder block.
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Re: Machining head O-rings

Post by JanBros »

Gerrit wrote:What about machining the cylinder instead of the head? I have a Yamaha TZ250 4DP cylinder and the "O" ring grooves are cut into the top of the cylinder. The same applies to the KR750 cylinder block.
2 reasons :

there is even less meat in the KR cylinders than in the head, because of the much bigger holes for the KIPS valves.
secondly : whenever you need to change barrels (different barrels for different tracks, trying out new tuning on different barrels, top-end failure destroying a barrel, ...) you need redo the machining in the barrel.

Do it once in the head and you are done. swapping inserts or when a top-end goes bust, just put in a new insert. no need machining new O-ring grooves
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Re: Machining head O-rings

Post by Gerrit »

Jan, the distance between edge of cylinder bore and KIPS valve bore is 6mm (at least on the cylinder I just measured). On the 4DP, the smallest distance between edge of cylinder bore and water passage is 7.5-8 mm. The "O" ring groove is 3 mm wide however (and a little over 1 mm deep). (I'll measure the KR750 cylinder later.) If you use a 1.5 mm "O" ring you still have 4.5 mm minimum to play with. Use a 1 mm "O" ring and you have 5 mm to play with. Given that the grooves are shallow I don't see any problem strength-wise.
By the way, do you use "O" rings with a different grade of rubber for the ring around the combustion chamber? I don't know whether Yamaha does, but Kawasaki used blue "O" rings for the KR750 combustion chamber sealing, black for the edge of the water jacket.
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Re: Machining head O-rings

Post by Gerrit »

I just measured my KR750 cylinder; the distance between edge of cylinder bore and water space is 10-11 mm, the distace between edge of cylinder bore and inner edge of "O" ring groove is 3 mm, groove width 2.5 mm, depth about 1.2 mm. The Big KR has a larger cylinder bore, of course, 68 mm vs 56 for both 4DP TZ and KR1-S. I'll have to check whether I have any of the blue "O" rings at home, as I recall they are quite thin.
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Re: Machining head O-rings

Post by JanBros »

the "meat" - part was in my head but concerning more when you also want to O-ring the KIPS valves, for which I found a solution with my CNC-head.

you need to use viton-rings, they can withstand higher temperatures.

and the size of your measured grooves are odd.
rubber can not be squeezed together, it can only de deformed. so the volume of the groove needs to be just a tiny bit bigger than the volume of the ring. this is achieved by making a groove as wide as the ring (advatage of this to is that the ring will not fall out when assembling), and the depth of the groove must de 20% less then the diameter of the ring. That way the groove is for about 98,xx% filled.

for example, if you use a 2.5mm ring, it's cross section is pi*1.25² = 4.9xxmm²
the cross section of the groove is 2*2.5=5mm²
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Re: Machining head O-rings

Post by Gerrit »

I don't know what size the 4DP "O" ring is, but I should have some rings for the KR750. I'll see whether I can find them tomorrow and check their dimensions.
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Re: Machining head O-rings

Post by DougB »

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Ok quick update, it worked out ok so far. The orings have sealed and I used 3bond to seal the rest. It's done about 20 miles and so far all is ok BUT!! There's always something.. During the first heat cycle I got a couple of drips of coolant from the front of the head/barrel in just one spot. When it cooled I re-torqued the head bolts. Next heat cycle it was ok. Test ride all was fine, then as the motor cooled I got another single drip in the same spot. Since then it's done a longer 20 mile ride and all seems fine. I'm hoping the 3bond has just sealed up whatever was the problem? Anyway it's a test really so I probably should just be happy it got me home!
Last edited by DougB on Mon Sep 19, 2016 10:03 pm, edited 2 times in total.
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