I keep blowing headgaskets

[Fiddy@norway]

I replaced pistonrings on my loncin 110cc recently because to old ones where shot, blue smoke spewing out and compression next to non-existing.

when I installed my new rings compression became unreal! I really got to put some muscle into kicking the bastard, but after a few hours of riding my head-gasket blew, kinda strange since I always use fresh gaskets, I pulled the head of and installed a new one and had the same thing happen again, the gasket blew after a few hours.

I ripped the head off again and installed yet another gasket, this time I used silicone between the gasket and the head and cylinder for maximum seal but it still blew!

why do this keep happening? the gasket-surfaces are perfect and the gaskets where all new.

the bike is sill rideable, but when it's cold it pops and bangs between the head and cylinder like crazy! and it also runs quite rough...

 

[Cactus Jack]

Your porting job has boosted cylinder filling and coupled with the high compression piston and better ring seal .. the 6 mm cylinder studs are too weak to handle it so they stretch like elastic ... allowing the gasket to get blown out ...

120 and up engines have 8 mm studs to overcome the problem ... and YX150/160's have 10 mm studs ...

I first came across the same problem over 30 years ago when I bored an XR75 out to 96 cc using a Powroll piston and sleeve kit plus ported and milled the head ... Every time I hit the throttle hard under load the engine would back fire ... but it ran perfectly if I throttled on easily ... I thought the valves were hitting the piston or intermittently sealing or something like that but everything was still spot on when I pulled it down to check ... the exact same engine with a high compression 80 cc bore kit didn't have the problem ...

I never did fully work out what it was at the time but years later I read an article on a 116 cc XR motor built by Powroll and they stated that with all the power the head will literally lift off with the stock 6 mm studs ... so they fitted hi quality 8 mm studs in place of the thinner stockers ...


XR 116 Powroll stroker motor !

http://www.vintagefactory.com/Powroll.jpg

http://www.vintagefactory.com/Powroll 1.jpg

http://www.vintagefactory.com/Powroll 2.jpg

http://www.vintagefactory.com/Powroll 3.jpg

http://www.vintagefactory.com/Powroll 4.jpg

http://www.vintagefactory.com/Powroll 5.jpg

Check out the modification options they offered back then ... shops got all their parts custom made to their own specs in their OWN country ... the USA ... Todays "modifiers and vendors" only offer what they can from China OR Japan ...

1975 Powroll Catalog/XR75 Pages

 

[Fiddy@norway]

I thought about the studs being to weak, I'm afraid to tighten in case they snap, but I also thought about another thing, I ran the bike for an extended period of time while the rings where bad, both me and my nephew ran the engine good and hard, oil-consumption was high and I had to top of the oil every few stops, since I use a cheap brand mineral oil, I suspect heavy carbon-buildup might have increased compression even more, could this be the culprit?
it hasn't really lost much power, but idle is rough and choppy, and it pops and flames on decceleration, neither of it was a problem before I replaced the rings.

 

[Cactus Jack]

Sounds like a bad exhaust valve or your ignition timing is out ... but you did say that the head gasket is leaking so an air leak can cause weird symptoms ... If the engine is carboned up bad , that can cause erratic ignition timing due to carbon glow and too high a compression ratio can cause spontaneous combustion aka "dieseling" ... The back firing with flaming can also be caused by carbon build up breaking away from around the exhaust valve seat ... some old engines rely on carbon to do the sealing ... You can spray water mist into the carb with a spray bottle as you rev your motor from cold to break up the carbon ... Since you've fixed the ring seal ... keep checking your valves for loss of clearance as the excessive carbon gets cleared out ...

As for weak studs ... they momentarily elongate under load like when you go to pop a wheelie in third gear or accelerate out of a tight corner in a taller gear ... You hear a loud sharp bang as the combustion blows past the gasket when combustion pressure rises sharply as you throttle on hard ...

The most accurate way to measure bolt or stud tension is not by torque but by stretch !!! Torque is inaccurate due to varying friction / drag of bolt heads and threads ...

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[Fiddy@norway]

I'll get some fresh studs, de-carbonize the head and see if that cures it, If I wack the throttle when the engine is cold it blows flames out the right side of the head, it gets better when the engine reaches working temperature though.

don't belive it has something to do with the ignition because I haven't touched that, my valves might be getting worn, last time I adjusted them, they where REALLY out of adjustment...

 

[Cactus Jack]

Check that your head and cylinder deck surfaces are flat ... they may be warped or something ... Fitting 8 mm studs would help ... A copper head gasket would'nt hurt either ...

 

[NRG]

Yeah, i might have an answer for this.

Sometimes when you carry out this type of work......you unknownly wind out the head studs when removing the head nuts.

So when it comes time to tighten the head back on.....the head nuts will bottom out on the stud, but not tighten the head down, giving you the impression that it is tight, but its not!

This will cause you to blow head gaskets all day long.

But wait..............................there is a tell tale sign.

If the spanner or the tool you are using springs back when you tighten the head nuts, that is a dead giveaway that the head nut has bottomed out on the stud, and hasnt exactly tighten the head down.

You can avoid this problem by always, tightening down the head studs before you put the cylinder or the head back on. You can use 2 nuts locked together on the head stud, or cheat like me by using vice grips on the unthreaded part of the stud.

With some engines you will get this spring back effect regardless, because the little chingas didnt engineer the thing properly funny buggers
it that case i use extra washers under the head nuts.....................

Hope this has helped.

 

[Tittan1500]

I would say coat the headgasket in some permatex copper spray, and re torque the head gasket after the engines first heat cycle.

Dont just go pound it off the bat, let it warm up and cool down once then re torque and you will be amazed how much the crush you lost on the gasket.


I go over the process here

Pit Bike Head Gasket Repair - YouTube