Author Topic: KZ400 rebuilt engine head leak  (Read 702 times)

Offline sosedoff

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KZ400 rebuilt engine head leak
« on: May 15, 2018, 02:01:03 »
I've been working on this 75 KZ400 for quite some time and finally getting close to the finish line.
This is my first project and i've pretty much gone through the whole bike. Engine has been rebuilt:
0.5mm rebore, new gaskets, pistons, rings, cam chain, cam chain tensioner, seals, valves, valve springs,
head sandblasted, etc, the whole deal.

The first time i put the engine together and after the first 5-ish starts (~5-10min) there were no leaks,
at some point the cylinder head had developed a few small leaks, like air bubbling out the head base gasket,
head o-ring and 2 small leaks from the head cover. Head and head cover both have a rubber o-ring, so i put
some silicone over the the head, but didn't on the head cover. From what i read online you're supposed to
re-torque all bolts after a few good head cycles. So, i removed the head cover, got some silicone over the
head o-ring and then torqued everything to the manual's spec.

Started and ran the engine today for a good 10 minutes, head cover no longer leaks, but i noticed bubbling
on the right side of the cylinder head gasket. Before i re-torqued the head again, there was a bubbling on
the left side of the cylinder head gasket (near the front stud), which is gone now.

At this point i'm not sure what i'm doing wrong. I haven't checked the head surface for warpage, could this be
the case? The engine runs fine, revs up great, but i'm at the loss of what the leakage issue might be.

Any ideas or suggestions are highly appreciated. Thanks!

Offline doc_rot

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Re: KZ400 rebuilt engine head leak
« Reply #1 on: May 15, 2018, 14:26:17 »
The early kz400 motors are notorious for leaking head gaskets. You're probably going to have to replace the headgasket. You should check the head and cylinders for flatness when you do. Copper spray on the gasket helps with sealing. Ive gotten paranoid of leaking head gaskets so after a rebuild I run the bike just long enough to get it hot. then cool till stone cold, re-torque. then go on a short 10 min ride trying to get it nice and hot, then wait till its stone cold again and re-torque. i check it again after 50 miles.

Offline sosedoff

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Re: KZ400 rebuilt engine head leak
« Reply #2 on: May 15, 2018, 15:39:55 »
The cylinder base and head gaskets are brand new. Should i still get a new head gasket?

Offline jpmobius

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Re: KZ400 rebuilt engine head leak
« Reply #3 on: May 15, 2018, 17:12:02 »
Yes, replace the gaskets.  If they have been assembled into an engine, they are no longer new.  Most engines do not need any sort of sealant on these gaskets, but some are troublesome and need a bit of help to not leak.  As a rule, I never use silicone anything on anything!  Exception is applications where the original engineering included "make in place " sealing with RTV and no other gasket.  My suggestion is to take the top end back apart and remove absolutely every trace of silicone first and start again.  The base gasket surfaces are seldom out of flat, but they need to be checked.  They also have to be unmarked from any sort of gasket scraping or prying used to separate the cylinders from the case.  If there is injury here, it can be very hard indeed to not have leaks.  If there is any sort of significant damage, there are several remedies.  Machining the surfaces flat and using a thicker gasket is make up for the difference is a comprehensive solution, but a very great deal of work.  Minor defects can be restored with JB weld and an expert with a file.  In any event, once the surfaces are acceptable, if there is trouble getting things to seal I simply glue both sides of the base gasket to the case and cylinder with 3-bond (or Yamabond, Hondabond etc.) gasket sealant/adhesive.  You need something that actually dries fairly hard and keeps the gasket from moving around, so no weather strip adhesive or something fancy like Hylomar, and absolutely no RTV.    Replace any O-rings with new and make sure their mating surfaces are perfect.  The cylinder and head ALWAYS need to be checked for flat.  Most motorcycle components are small enough to be "surfaced" on a piece of glass and wet sandpaper  (#400 grit).  "Paint" the sealing surfaces with a sharpie or similar marker, some spray paint or machinists blue. Work the parts in a figure 8 pattern with a continuous supply of water being very careful to not exert any tipping pressure to the parts - hold them at the bottom very close to the sandpaper.  Once you have removed 100% of the "paint" the parts are flat.  I use Copper coat as well on problem head gaskets, and altogether back Doc_rot's advice.  Hang up the gasket and spray both sides.  Let dry completely.  Make sure the parts are completely clean and dry (yes, white glove test clean!) and do a final clean with alcohol or acetone.  When you have everything ready, spray another light coat of Copper coat on both sides of the gasket and assemble immediately.  DO NOT OVER TORQUE THE HEAD BOLTS! And tighten following the factory advised pattern and first through final torque values.  Let this set up for 24 hours.  When you have the bike ready to ride, heat cycle it a couple of times in the shop.  Don't ride it.  Get it up to normal temperature and then let it cool completely (overnight).  Re-torque the head bolts.  Repeat until the bolts are always tight.  Then take it for a short, very easy ride and check the bolts.  If the bolts are still tight, try a longer medium ride and re-check.  Repeat and increase as needed.  Usually it ends up being less trouble than the worst case I'm describing here, and commonly it is a good idea to check the valves etc. straight away after a few miles so checking the head bolts is not much of an extra chore.
« Last Edit: May 15, 2018, 17:19:15 by jpmobius »

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Offline sosedoff

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Re: KZ400 rebuilt engine head leak
« Reply #4 on: May 15, 2018, 18:31:36 »
Thanks for the info @jpmobius.
To clarify a bit on silicone - i didn't use it on any gaskets, just on the big rubber o-rings (head/head cover).
Before getting in there again, i might give it a few heat cycles again, then check the bolts.

Offline sosedoff

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Re: KZ400 rebuilt engine head leak
« Reply #5 on: May 15, 2018, 23:55:46 »
Oh well, ran the engine today for 10 minutes, the head gasket is leaking on the left side, under the exhaust, and in the middle.
It only reveals itself under 3+k rpm.

Offline sosedoff

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Re: KZ400 rebuilt engine head leak
« Reply #6 on: May 16, 2018, 01:13:23 »
The gasket kit that i got (i think, it came with the bike) was  this one:

Should i just get an OEM head gasket ? They're pricey (~100) but i'm not sure if they make any difference.

Offline datadavid

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Re: KZ400 rebuilt engine head leak
« Reply #7 on: May 16, 2018, 05:00:14 »
Pull apart and check for flatness first,  oem gasket wont help if head is bent.

Online advCo

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Re: KZ400 rebuilt engine head leak
« Reply #8 on: May 16, 2018, 11:19:35 »
Bring your cylinder and head to a competent automotive machine shop and have them check/skim the mating surfaces.

You’ll then need to check the ‘squish’ aka piston crown to valve clearance and may need to adjust with a thicker base gasket to compensate for material removed from the surfaces.

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Offline sosedoff

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Re: KZ400 rebuilt engine head leak
« Reply #9 on: May 18, 2018, 00:42:29 »
Uh, thats a total bummer, after all this time i spent rebuilding things.
Could one the head pin dowels cause a bad seal? I've got a set of new ones, but i cant remember if
i put both new or just one in the engine (one of the old ones was chewed up for some reason).

I just checked the manual, all the work could be done with the engine still in the frame, so this is good.
Alright then, here we go again :)

@advCo, how am i supposed to check for this "squish"? Is this something that the machine shop can do
as well? And the thicker base gasket? Are you supposed to get one or mod the new OEM one? I dont really know
anything about all this stuff.