Topic: Cylinder Base Gasket and O-ring Question

[BRAD CHIEDUCH]

Hello everyone, it has been a very long time since I was on the forum - moving, new job, etc. etc.  I am working on my project and have a question regarding the cylinder base gasket and cylinder o-ring.  My bike is bored to 915cc and with reference to the attached photos, with the base gasket laid on the cylinder the o-ring sits nicely inside the opening of the base gasket.  With the base gasket laid on the crankcase, the cylinder o-ring would seat against the crankcase at the front of the crankcase, but as you move towards the rear of the crankcase the o-ring can no longer seat against the crankcase.  Only about 3/4 of the o-ring diameter seats against the crankcase - to me this doesn't seem right, but I thought I would post this question and maybe someone could comment.

[newday777]

Hmmmm. It won't download.

[BRAD CHIEDUCH]

Sorry Newday, I have added photos (jpg versus pdf) and hopefully everyone can open.  The cylinder photo shows the base gasket around the cylinder o-ring.  The crankcase photo shows the base gasket on the crankcase and you can see how the o-ring would seat on the crankcase at the front and then not seat on the crankcase at the rear.  Am I missing something here?

[scottly]

Have you tried flipping the gasket over?

[newday777]

As Scott said it looks on backwards. Flip it over.

[HondaMan]

IF the flip-over isn't the answer: those O-rings on the bottom of the cylinders should be installed below flush, up into the cylinder block. If the machine shop did not install a recess around the edges of those holes for the O-rings (as is found in stock cylinders), then you'll have to omit those 4 O-rings. If that happens. then you might wish to spray some sealant on both sides of the base gasket, which will make it a b$#@h to remove later, but that's racing...

[PeWe]

Important that the o-rings around sleeves sit tight, stuck into the block.
I guess the sleeves are 74.2 mm outer, 67.5mm inner. (CycleX)

My cylinder with those sleeves, 67.0 mm pistons (890cc) have holes to vents 1-2, 3-4. Common when bore for bigger sleeves. Early cylinder type.

Sealed with Wurth High temp RTV to avoid oil leak coming out from the cylinder.  I hope that will keep it sealed forever.

I doubt o-rings only will keep it sealed when bored thru the vents 1-2, 3-4. Irritating oil weeping thru cylinder when warm.

#

My 970cc cylinder with 74.5mm OD sleeves (not stepped) got the bore in cylinder wider at bottom.
1.25mm x 6 mm groove created where tight fitted o-ring could be pressed down covered with ThreeBond 1184 ( as Honda Bond). 15000km no leaks.

[bryanj]

The O rings are supposed to fit in a groove arround the lliners in the block to stop oil travelling upwards, without the groove i would think they wont work

[PeWe]

The stepped sleeves get a groove since they are slimmer a little bit before exit at the bottom.
My cylinder with CycleX stepped sleeves have grooves. I doubt the o-ring will stop oil to weep where it has gone thru the ventilation between cylinders.

If the cylinder block somehow had thicker alu or welded + bored again where it went thru when boring the cylinder for sleeves.

It is not that easy to force sealer to fill the thin groove. My nearby shop did the sealing above.

The 970cc cylinder by me with a syringe I filled with sealer. Very sticky, stinking mess.

[BRAD CHIEDUCH]

Thanks to everyone for their replies!  I will attempt to clarify my question/concern.  I put some bluing on the cylinder o-rings and lowered the block on to the crankcase (without the base gasket) to determine the o-ring contact area.  As per the attached photos, the o-rings only contact the crankcase in the front for approximately 190 degrees around their circumference - the remaining portion of the o-rings falls inside the crankcase i.e. no contact with crankcase.  My conclusion is that this was an issue right from Honda.  The question I have is how to resolve the o-ring sealing.  Other members suggest abandoning the o-rings and use a sealant, but what sealant would work best?  Any suggestions here would be appreciated.

[bryanj]

Has the crancase been machined to take the liners?

[HondaMan]

My view: the O-rings should not be contacting the engine cases in the first place. They don't in a stock engine: they are recessed up inside the cylinder block by about 0.1mm/0.004" if installed correctly. They are there to prevent oil weeps around the head gasket area from crankcase pressures pushing oil up the sleeves. The base gasket is the one that seals between the cylinder block's bottom and the engine cases: as you can see, there are 'unsupported' areas under this gasket where cylinder studs may collect oil from up by the cam bearings, and it dribbles into the crankcase thru those open spots. The perimeter of this gasket does the sealing between the 2 engine parts. With the extra-big pistons and sleeves this sealing area is reduced, so spraying some sort of sealant onto this gasket would be a good idea. I personally have done it with Hondabond (very messy), spreading it with plastic gloves on both sides. This is time-sensitive, though, giving you approximately 15 minutes to put the rings and pistons into the cylinders before it starts to lose the ability to seal on the upper gasket surface as the Hondabond sets.

If you can find some FelCoBond, it comes in spray and tubes and gives you about 20-30 minutes instead. Hard to find today, but was a bike shop staple back in the day.

[Tracksnblades1]

Sorry Newday, I have added photos (jpg versus pdf) and hopefully everyone can open.  The cylinder photo shows the base gasket around the cylinder o-ring.  The crankcase photo shows the base gasket on the crankcase and you can see how the o-ring would seat on the crankcase at the front and then not seat on the crankcase at the rear.  Am I missing something here?

Oow,

Em cylinder ends are thin… What size are them bores?

[Tracksnblades1]

Has the crancase been machined to take the liners?

Looks 👀 to be oval machined if his oring witness marks are as round as the cylinder ends…?  🤔

[BRAD CHIEDUCH]

Cylinders are bored to 915cc and bore is approximately 2.658".  Yes, the sleeves get rather thin when bore diameters get this large.  The sleeves are not oval and everything is concentric and base gasket fits nicely around all the cylinders.  The issue is that the back side of the crankcase is not perfectly round and the o-rings do not contact the crankcase right from the factory.  I did speak with Mike Rieck and the solution is to remove the o-rings and fill with sealant such as Hondabond or similar.

[PeWe]

It is possible that base gasket is a little off if not better when flipped  it.

I got an RCS base gasket with too small holes for the inner 2 studs where oil is transporting up to head.
My shop's mechanic corrected them. Grind without melting the viton is the thing. The steel is hard.
Upper case for o-rings 16mm, exact as o-ring 11x2.5mm.
My base gaskets have 17mm holes here.

A small notice about 67.5mm bores on a CB750 SOHC.
For future purchase of rings etc.
- It is not 915cc.

Do the math:  radius x radius x PI x stroke 63mm x 4 cylinders.
67.5mm bores = 901.77 cc

(I calculated with PI truncated to 3.1415926)

68.0 mm bore = 915.18 cc
Dynoman sell that size. It must have bigger sleeves and case milling.

CycleX sell with stepped sleeves to fit stock upper case without milling the case wider.
They sell with 67.0 or 67.5 mm pistons. Those sleeves can be used with 836cc (65mm) too as they mention.


My K6 upper case needed massage with a Dremel grinder to fit a cylinder with those sleeves. An old K2 case not.
I think my 1975 case needed Dremel massage too. There were spots that were too tight to fully fit the cylinder with knock pins in place.
I think it fitted without knock pins that can be a bad surprise during assembly, find that upper case holes for sleeves need to be grinded here and there.

Clearance needed too. I guess 0.2mm can be a good number.

Clean the sleeve- cylinder groove with acetone before sealing them.
It is tricky to get it to fill the groove.
Acetone needed for cleaning fingers and cylinder. Honda Bond is sticky and ends up in long thin spaghetti strands....

See photo below before cleaning up the mess where it should not be "sealed".
That cylinder got plugged bottoms of triangular ventilation holes, cyl 2-3 each side of cam chain tunnel.
 Not much surface for base gasket too keep sand out from the engine next time lifting it so I made a bottom plug with JB Weld.
74.5 mm OD sleeves + 2.5mm wider at bottom creating an 1.25mm wide groove for sealing it.

[MRieck]

It is possible that base gasket is a little off if not better when flipped  it.

I got an RCS base gasket with too small holes for the inner 2 studs where oil is transporting up to head.
My shop's mechanic corrected them. Grind without melting the viton is the thing. The steel is hard.
Upper case for o-rings 16mm, exact as o-ring 11x2.5mm.
My base gaskets have 17mm holes here.

A small notice about 67.5mm bores on a CB750 SOHC.
For future purchase of rings etc.
- It is not 915cc.

Do the math:  radius x radius x PI x stroke 63mm x 4 cylinders.
67.5mm bores = 901.77 cc

(I calculated with PI truncated to 3.1415926)

68.0 mm bore = 915.18 cc
Dynoman sell that size. It must have bigger sleeves and case milling.

CycleX sell with stepped sleeves to fit stock upper case without milling the case wider.
They sell with 67.0 or 67.5 mm pistons. Those sleeves can be used with 836cc (65mm) too as they mention.


My K6 upper case needed massage with a Dremel grinder to fit a cylinder with those sleeves. An old K2 case not.
I think my 1975 case needed Dremel massage too. There were spots that were too tight to fully fit the cylinder with knock pins in place.
I think it fitted without knock pins that can be a bad surprise during assembly, find that upper case holes for sleeves need to be grinded here and there.

Clearance needed too. I guess 0.2mm can be a good number.

Clean the sleeve- cylinder groove with acetone before sealing them.
It is tricky to get it to fill the groove.
Acetone needed for cleaning fingers and cylinder. Honda Bond is sticky and ends up in long thin spaghetti strands....

See photo below before cleaning up the mess where it should not be "sealed".
That cylinder got plugged bottoms of triangular ventilation holes, cyl 2-3 each side of cam chain tunnel.
 Not much surface for base gasket too keep sand out from the engine next time lifting it so I made a bottom plug with JB Weld.
74.5 mm OD sleeves + 2.5mm wider at bottom creating an 1.25mm wide groove for sealing it.

Per.....were you hitting the Glogg before applying the 1184??

[BRAD CHIEDUCH]

Just sharing some more information, the cylinder work on my bike was done by CycleX.  The pistons are 67.5mm diameter and the sleeves are tapered at the bottom - sleeve OD at the bottom end is 70 mm.

PeWe, do you find the 1184 sealant easier to work with than the HondaBond?  The HondaBond as you noted is an awful stringy/gooy thing to deal with and I was hoping the 1184 is a little more fluid and would flow more readily into the o-ring groove.

[Tracksnblades1]

Cylinders are bored to 915cc and bore is approximately 2.658".  Yes, the sleeves get rather thin when bore diameters get this large.  The sleeves are not oval and everything is concentric and base gasket fits nicely around all the cylinders.  The issue is that the back side of the crankcase is not perfectly round and the o-rings do not contact the crankcase right from the factory.  I did speak with Mike Rieck and the solution is to remove the o-rings and fill with sealant such as Hondabond or similar.

That’s what the picture indicates, by the oring’s witness mark. The crankcase counterbore is oval or non concentric.

Are all four crankcase counterbores the same..? Non concentric..?   😳

[PeWe]

#

Per.....were you hitting the Glogg before applying the 1184??

It look like that!
Photo from end of November 2021 when the shops have started the sale of Glögg. My favourite with Cognac, 21%.
Tasty!!



The result should look better at first try if I should have got some spicy Glögg before.

That sealer must be exactly the  same as Honda Bond. It is more rinny than expected.
It will crimp when cured so more is needed to fill the gaps.

Acetone, tissues and razorblade a good help.

[BRAD CHIEDUCH]

TracksnBlades, the crankcase bore is not round towards the rear of the bike.  This is the area where the cylinder o-rings do not make contact with the crankcase.  However, as Hondaman pointed out, as well as other forum members, the o-rings do not contact the crankcase and this was how it was from the factory.  This whole discussion came about because I made the false assumption that the o-rings were to contact the crankcase.  I was wrong, but I'll get over it!   

[PeWe]

Just to be sure.
Do not forget the 2 o-rings.

11x2.5
ID (inner diameter)11 mm
CS (Cross Section)2.5 mm
A little bit thick for thin RCS. I have not seen any compression issues on o-rings when apart.

See red arrows on photo below.

- Base gasket (RCS) against docking cylinder held by rubber bands. Several to correct lenght and together with a strap. Open strap and take out the rubber band before it is squeezed by cylinder.

- Cam chain tensioner held in place by 2 straps, cut and pull out when all pistons are in.

I prefer to take photos for my 2nd thoughts.... did I forgot the o-rings or not???
The knock pins and cam chain slider other things to forget....

I have added the slider when pistons are in several times.
Just lift cylinder a little to slide in the bottom pin holding it without dropping it or any ring pop out....

[BRAD CHIEDUCH]

Thanks PeWe for the reminders!  I like the approach of holding the base gasket and cam tensioner in place with rubber bands - bloody genius !

[PeWe]

Thanks PeWe for the reminders!  I like the approach of holding the base gasket and cam tensioner in place with rubber bands - bloody genius !

That is something Mike Rieck has shown us before on this forum.

Too easy to get the 2-3 pistons in the bores to find that the tensioner is outside.

Some base gaskets have a really tight fit around sleeves, easy to damage the gasket when lower down the cylinder block with sleeves.

[HondaMan]

Too easy to get the 2-3 pistons in the bores to find that the tensioner is outside.

+4...