The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: t20racerman on 23.07. 2011 17:40

Title: Cylinder base gasket 'blown out'.
Post by: t20racerman on 23.07. 2011 17:40
Hi all

Been out for a thrash on my newly rebuilt A10 and the cylinder base gasket has been blown out the front of the barrels! All across the front it is sticking out and has clearly been under pressure as it is ripped in half where one of the studs was.
I was really surprised at this as the gasket was quite a thick one and was securely fixed to the crankcases with gasket glue. I used high quality 'Yamie bond' (Yamaha recommended crancase sealant for my TZ350) and tightened the barrels down (without pistons) after fitting the gasket to ensure it sealed correctly. I then rebuilt the top end and greased the barrel gasket joint. All the barrel bolts were tight and were retightened after 50 and 150 miles.

The engine breather is assembled correctly and there is no oil coming out of there. The rocker cover barely leaks at all and so I'm not convinced there is a pressure problem.

Any ideas as to why this happened?
Title: Re: Cylinder base gasket 'blown out'.
Post by: bsa-bill on 23.07. 2011 18:35
Hi t20racerman
Can't offer much in the way of an explanation but enquiring minds might want to know why fit barrels without pistons
Presumably you take barrels of again to fit pistons or you have a cousin called Merlin.

obvious question is going to be are the cranckase halves a true pair
Title: Re: Cylinder base gasket 'blown out'.
Post by: t20racerman on 23.07. 2011 19:26
Cases are indeed a matched pair and this never happened to me before in 30,000 + miles!
I fitted the barrels without pistons to ensure that the gasket glue sealed with the barrels in place and under pressure - the idea being that this would help make the 'perfect' joint!

I then rebuilt the next day WITH pistons in place.  *smile*
Title: Re: Cylinder base gasket 'blown out'.
Post by: t20racerman on 23.07. 2011 19:38
Just had a thought.....

Yammie bond is designed primarily for gasketless joints. The gasket that has blown out seems completely clear of bonding. Maybe it didn't bond? Still doesn't explain why it blew out though...
Title: Re: Cylinder base gasket 'blown out'.
Post by: bsa-bill on 23.07. 2011 19:50
Not sure I agree with your method, probably weakened the gasket when removing the barrels.
I'm using loctite 5910 these days basically a black silicon ( on both sides of a gasket) but it does seem to lift ok when removing and sets like rubber (not hard)
Title: Re: Cylinder base gasket 'blown out'.
Post by: fido on 23.07. 2011 21:27
Yes, taking the barrels off to fit the pistons means you were in effect using an old gasket which could have compressed slightly differently the second time is the barrels didn't sit in the exact same place as before. I wonder if the crankcase face is flat?
Title: Re: Cylinder base gasket 'blown out'.
Post by: muskrat on 23.07. 2011 21:35
I'm with Bill & Fido on that one t20.
I also think your right about the yammie bond being for metal to metal joints. I have used kawasakibond (same stuff) for the crankcase joint with no problems, but it was no good on timing side or rockerbox gaskets. I use permatex #3 for most gaskets.
Cheers
Title: Re: Cylinder base gasket 'blown out'.
Post by: orabanda on 24.07. 2011 01:54
Is the crankcase breather working correctly? (timed correctly, hole not blocked?)

richard
Title: Re: Cylinder base gasket 'blown out'.
Post by: t20racerman on 24.07. 2011 11:06
I'll re check the breather Richard but it does all look fine.

I'm feeling more and more strongly that the Yammie bond had no adhesion whatsoever on the gasket so I'll try something else next time. Still puzzled as to why a 3-4 inch strip of gasket should blow out though. Looks like the same has happening on the rear LH side too! I'll strip it down and report back but I have had one idea as to a possible problem - I fitted new base studs to the crankcases.... I wonder if the nuts were tightening down on to the base of the thread, rather than to the barrel itself? They were the correct studs for my thick flange barrels but it is certainly worth checking I feel. They look fine, but I can't really tell until the cylinders are lifted off. I had a similar problem on my T20 racer where the head gasket kept blowing, and I couldn't see why. It turned out that although the heads were tightening down, the last little nip up was pressing the head bolt studs down to the end of the thread, rather than tight against head!

Anyway, thanks for your thoughts - now time to get the spanners out :-)
Title: Re: Cylinder base gasket 'blown out'.
Post by: Triton Thrasher on 24.07. 2011 12:53
Cylinder base studs commonly loosen after a few miles.

Also- you can get a buildup of old Hermetite around where the studs go into the crankcase, holding the barrel up from mating properly with the crankcase face.
Title: Re: Cylinder base gasket 'blown out'
Post by: t20racerman on 24.07. 2011 20:46
All stripped and now I really am puzzled! It took me ages to lift the barrels as they were absolutely solidly stuck down to most of the gasket. The yammie bond on the bottom had bonded to the cases in most a, but the front is completely blown out, and the bottom LH side was on the way out. I've attached pics and you can see what I mean. Oddly, the LH side bit that was partly out was stuck down solid to the cases and I ripped a bit trying to lift it off as you can see in the pic.
Cases are a matched pair, surface was dressed and made flat before fitting new studs, studs were correct length, casings hadn't lifted around the studs (I'd slightly countersunk them before fitting), gasket glue had bonded, most of gasket absolutely solid, base of barrels seem flat...... ???

all I can do is rebuild I suppose :-)
Title: Re: Cylinder base gasket 'blown out'.
Post by: bsa-bill on 24.07. 2011 21:02
Hi t20racerman
Photos are all 0KB (empty file)
With out seeing the photos I'm assuming some Yamabond stuck ,some not then refitting is like using an old broken gasket.
if you clean it all off, put a smear of goo all round then the gasket, next barrels and fasten down I'll be surprised if it's any more bother.
If you wan to fit gaskets that can be removed and re-used the plan most used is goo on one side of the gasket, slight smear of grease or nothing on the other side, then the gasket will stick to one face and not the other.

Title: Re: Cylinder base gasket 'blown out'.
Post by: t20racerman on 25.07. 2011 12:52
Hi t20racerman
Photos are all 0KB (empty file)
With out seeing the photos I'm assuming some Yamabond stuck ,some not then refitting is like using an old broken gasket.
if you clean it all off, put a smear of goo all round then the gasket, next barrels and fasten down I'll be surprised if it's any more bother.
If you wan to fit gaskets that can be removed and re-used the plan most used is goo on one side of the gasket, slight smear of grease or nothing on the other side, then the gasket will stick to one face and not the other.

Photos there now (I hope)
I did use goo on one side and a light greasing on the top surface. I`ll try again and see what happens....  *smile*
Title: Re: Cylinder base gasket 'blown out'.
Post by: bsa-bill on 25.07. 2011 13:19
the front section hat has blown out has what appears to be bit's of something on the top of it, is this stuff that's got on it after you've taken it off,
Also there is what looks like oil on the cranckase joint is it oil or is that the Yamabond, if it's Yamabond it looks like some stuff I used once, didn't like it as it set hard
Title: Re: Cylinder base gasket 'blown out'.
Post by: muskrat on 25.07. 2011 14:18
Bill, Yambond is silver colour and similar to hylomar not hard setting. So with the soft goo not sticking to the gasket on one side and grease on the other and compression of the gasket the crankcase pressure has blown it out.
Cheers
Title: Re: Cylinder base gasket 'blown out'.
Post by: bikerbob on 25.07. 2011 15:51
Hi there
With regard to gasket sealing I have for the past 15 years used household bathroom and kitchen silicone sealer not the sealer for sealing window frames as is not silicone based. I use a minimum amount on each side of the gasket and then tap it all round with my finger this gives it a textured effect then tighten up and I have used the same gasket sometimes up to 3 times without any problems. With cylinder head gaskets I never use anything they go on dry, I always make sure all surfaces are clean and flat beforehand.
Title: Re: Cylinder base gasket 'blown out'.
Post by: olev on 26.07. 2011 12:57
Gday Adrian,
I think Yamabond is the same as Threebond 1104.
The stuff is brilliant. I'd be amazed if its the cause of your problems.
I use it metal to metal on crankcases and both sides on everything except head gaskets.
it sticks like sh*t to a blanket and its just as hard to get off.
Your idea about the studs sounds more likely.
cheers
Title: Re: Cylinder base gasket 'blown out'.
Post by: RichardL on 26.07. 2011 15:08
May be wrong,  but looks to me like very spotty contact with gasket,  and that the sealer has hardened.  I think that it does not make sense to use metal-to-metal,  machine-fit sealer where a gasket is required. I use permatex #2 on both sides of gaskets to enhance the surety of a seal and to keep the gasket in place. I know that better men than I grease one side,  but I have trouble seeing that as having the same sealing power as the Permatex. There is also no good reason to try to reuse paper gaskets.  They're available an cheap.

Awaiting thrashing,  or,  even,  agreement.

Richard L.
Title: Re: Cylinder base gasket 'blown out'.
Post by: chaterlea25 on 26.07. 2011 20:06
Hi All,
A peculiar problem allright *conf* *conf*
There must have been some pressure in the crankcase OR for some reason the crankcase mouth or cylinder base are not flat???
Clean everything and fit on the barrels without any gasket or sealer, then try with a thin feeler gauge to see if there is a gap between the 2  ????
Or it maybe as T20 says and the studs are a fraction long??

Bikerbob,
Petrol will dissolve ""bathroom sealer" almost instantly and in my opinion should be left for the bathroom jobs
Its also famous for blocking oilways and causing other problems *eek* *eek*

Regards
John O R