Why RTV and not the O-Ring?

Prepping to put engine back together with new rings etc and Wiseco MLS Head gasket.
(for ref my orig KZ1000st 1015cc engine is bored to 1075cc and has the Wiseco 2835XC kit.)
Anyways I saw a couple comments here and there lately where a couple people said they don't use the center O-Ring and instead only use RTV?
Sound legit? I have a new O-Ring and I have the official Kawasaki Service manual that says to use it and says nothing about using RTV instead.
So wondering if anyone can clarify why they would use RTV instead? Would the O-Ring not be better?? And if not, why?
I'll go with RTV if someone can validate or explain why ? I spent 2 hrs searching so far and still don't have the answer to that.
Plus I personally haven't come across anything in the Kawi shop manual they got wrong over the years but hey it was made 40 yrs ago, maybe the O-Ring was a bad idea?? 
Is it a case of people not wanting to bother with buying an overpriced O-Ring or is there a design flaw with using it vs filling the groove with RTV?
For the record the reason I took my engine apart in first place was because it started seeping a very small amount of oil on rides somewhere along front head gasket so to avoid making things worse I parked it 2 years ago until I could address it, and then life/kids etc got in the way of finding time to fix. But FINALLY made time and got it apart this week.
Did the steel straight edge tests and everything seems flat with no warping, but I also noticed there was no center O-Ring and just RTV was used.  I had a shop do engine work(bored cylinders and put in new Wiseco 1075 kit )  for me 12yrs ago or so when picked up bike as a project..  So they would have been the ones that used RTV,  and this removes any possibility of blaming the non-existent O-ring for the seepage.  They also used a fiber head gasket as well which seems in great shape with no blowouts, holes, cracks etc.. I picked up a Wiseco 1075c Head Gasket to replace and it is MLS instead of Fiber. Does fiber vs MLS make a diff as far as using the OEM O-ring vs RTV?    Anyways I was going to get this thing finally back together and on the road and now I am scratching my head if I should be using the new center O-Ring or RTV.  Feed me with your knowledge or experience on this curve ball. cheers, Steve 

I feel this is one of those questions that you'll get every answer for. I've heard people on this forum switching from the original o-ring to RTV due to leaks. The only experience I've had is putting a new o-ring in because the PO used RTV and it was leaking like a sieve.

I can recommend using the o-ring, I did after the RTV failed to do the job. Others will recommend the opposite from their experience. It's possibly the quality of replacement parts that may make all the difference.

Right on.  That is what I kinda hoped for, hearing which one people can vouch for and why.  I am neutral and can be swayed on this but if the consensus was half the world uses Rtv with no regrets and the other half is just as happy and leak free with the Kawi specified O ring then maybe that means it doesn't matter?  Though I wonder does that change depending on if you go with a MLS gasket, or fiber, or copper?  I def won't be drag racing this thing either, it's a frikken shafty afterall..  It's just my "dad needs to blow off some atress machine" .  Cheers

The stock 'O' ring is designed to be used with the 2 piece head gasket and is too thick for use with a one piece.
With a one piece it is effectively almost twice as thick as it needs to be and comes under a lot of pressure and distortion if used and can prevent the gasket from seating correctly.
The one piece gasket seals all around the camchain tunnel where the two piece does not which is why filling the groove with rtv to close a path for pressurised oil creep is all that should be needed.
A round viton rubber 'O' ring held in the groove with a few dabs of grease can be used but it needs to be a smaller section 2.5mm x 127 dia.
 

a subject where you will get a range of opinions that's for sure.

I have used a 1 piece head gasket with no O ring with 12:1 pistons and not had any issues.
I have always used the stock type O ring with a variety of 2 piece head gaskets and had no issues.

After 40 years my original O ring started a little  leaking. Last winter I changed it for a Viton O ring. They say Viton is much better than original. BTW is changed my carb plungers also for Viton ones, now the idle problems are also over

Mac0239:  Lots of differing opinions on this subject as you have already found out.  I went with the one-piece head gasket when I did the top end on my '77 KZ-1000A1 a couple of years ago.  To date I have had no problems with leaks of any kind (knock on wood) and I am happy with the results.  The process for installing a one-piece gasket is a bit different than the two-piece.  If you go to Redline Cycle they have an explanation of the install process and minor mods that must/should be done to avoid any leaks.  If memory serves me correctly Redline also includes some installation instructions with their one-piece gasket.  If they don't do this now you can call Jim there and he will clue you in on what you have to do.  I really can't recall why I went with the one-piece gasket now, but I would do it again if I had to.

Rick H.

2 piece  head gasket  = always use a o ring 

1 piece head gasket   = no o ring ,    That being said  , Joe at Pitstop  has a  file you can copy   stating   On all one piece head gaskets  dont use the o ring .
But instead of the o ring fill the o ring  groove  with Hi temp  rtv.  Make the rtv slightly higher that the groove so the head gasket compresses it.
 I didnt look up the exact verbage from Joe for this reply but  that's the jest of it. 
 I run one piece  copper head gasket  with the rtv in groove with no leaks , ( But I also have  wire o ringed  piston cylinders  to hold the boost in the hole
Dave

Thanks all for weighing in.  appreciate it.   My bad though , I took another look in the service manual and at the pic of the reassembly of head and with a closer look it appears to be a two piece gasket they use, so hence why they say use the O-ring.  Duh. Anyone need an O-ring? ha ha.  So off to auto parts store to add to my growing collection of hardly used gasket sealants.  Of course all the kinds I have either say don't use on head gasket or don't expose to fuel, or high temps etc.   Permatex seems to be the brand everyone supplies up here but must have 20 flavors so will find the one that seems best I guess.  Should be interesting trying to fill the cavity so it is just above flush , ha ha.  I heard someone say you can put a thick bead , let it dry then shave it to proper height.  Need one steady hand to cut it that precise!! So much for the quick n easy O-ring.  cheers. steve

thanks Rick I will have a look at the Redline site. cheers

wrote:

  So off to auto parts store to add to my growing collection of hardly used gasket sealants.  Of course all the kinds I have either say don't use on head gasket or don't expose to fuel, or high temps etc.   Should be interesting trying to fill the cavity so it is just above flush , ha ha.  I heard someone say you can put a thick bead , let it dry then shave it to proper height. 

I use this VTECH gasket maker.
Fill the groove proud, leave overnight to cure then slice flush wiht the block surface with a very sharp razor blade or similar.
Never had any leaks.
It's also good for the cam end plugs, crankcase halves, alternator grommet etc etc.
V-Tech VT-157 Vital Black RTV Silicone Instant Gasket Maker - 85g Tube | eBay
 

thanks man for tip.  so you just used razor blade and cut it flush , not raised as bit?   That sounds a lot easier to me than trying to make an even level cut above flush. And if no seepage then it works right.
cheers,
steve