G5 blown head gasket

[Al_Bolton]

My G5 blew its head gasket recently - for the second time in about 500 miles. Last time I just replaced the gasket (with a composite type from the club) and cleaned up the mating surfaces. I carefully torqued the head down to 35lbft and all was well.

On investigation this time, I found the head bolts were loose! Almost finger tight. I pulled the head and checked it over. I enjoyed rubbing it on a piece of plate glass with an abrasive or three for an hour or so - this revealed it was slightly warped on one side. But it cleaned up nicely. The barrel surface seems straight.

I also checked the mounting flange on the Amal carb too - this was distorted too but was easily sorted.

In one of the technical articles there's a mention of head gasket problems with the Lightweights - the article describes junking the non-copper piece of these composite head gaskets and using all copper. The composite gasket available from the club has a mean little bit of copper in a ring on it as standard, so this didn't help much. But I decided to see if I could find an all-copper head gasket - and immediately found one on eBay in a Paylen decoke set. I'm not sure if anyone else has found a source of all-copper head gaskets for these bikes? Let us know if you have because I think they MUST be better - and they are reusable too.

I'm about to put the top end back together on the G5 so I'll let you know how it goes...

Al

[1608]

If after the first fitting you found the head nuts loose, it suggests that you MAY not have re-tightened them after a warm-up. A short while after the first torquing the gasket etc. will compress and the head nuts may become loose.

[Al_Bolton]

No, I ran it for a few miles and retorqued (what a pain that was!).

Will do the same again and we will see!

Cheers,

Al

If after the first fitting you found the head nuts loose, it suggests that you MAY not have re-tightened them after a warm-up. A short while after the first torquing the gasket etc. will compress and the head nuts may become loose.

[Stuoyb]

The advice that I had was to torque the head to 25ft/lb, leave for 24 hours, tighten to 35 ft/lb and then re-check every 24 hours for 3 to 4 days - only possible if you don't want to ride the bike straight away.
I did this whilst I was rebuilding the bike and after 200 miles re-checked again and there was no movement. It has been fine for 3 years.
The head gasket was the standard composite one in a club supplied kit.
It seems that this method allows the gasket to become fully "settled" before being exposed to the pressures of combustion and heat.
It has also been successful on four other bikes that I have rebuilt.

[Al_Bolton]

Seems a lot of work but worth a try! Will report back. Another thing - the sleeve bolts should have a thick washer under them - I noticed that one of the washers I have is thinner than the others - examining photos of the head from old pictures, the thick washers I have look correct - so I've found a replacement for the thin interloper.
I think the tighten it up again after a run is probably most important.
All good fun!
Al

The advice that I had was to torque the head to 25ft/lb, leave for 24 hours, tighten to 35 ft/lb and then re-check every 24 hours for 3 to 4 days - only possible if you don't want to ride the bike straight away.
I did this whilst I was rebuilding the bike and after 200 miles re-checked again and there was no movement. It has been fine for 3 years.
The head gasket was the standard composite one in a club supplied kit.
It seems that this method allows the gasket to become fully "settled" before being exposed to the pressures of combustion and heat.
It has also been successful on four other bikes that I have rebuilt.

[Ozmadman]

I'm not sure if anyone else has found a source of all-copper head gaskets for these bikes? Let us know if you have because I think they MUST be better - and they are reusable too.

I'm about to put the top end back together on the G5 so I'll let you know how it goes...

Al

There was also an aluminium version, not sure how successful they were but I have one as a spare should I need it. My bike had a slight leak as well (by the oil ways) when I got it. I checked the bolts and they were loose as well. I had a look between the fins once cleaned up and saw it had a copper gasket so I had nothing to loose by just tightening it up so that's what i did and it has been fine since!!!

Paul

[Al_Bolton]

Well, the G5 is back together. I fitted the copper gasket and the damned thing leaked oil from around the pushrod/oil feed area - plenty of compression though! I tried a retorque and it made little difference, so off came the head again...

The lovely copper gasket had a high spot on it! Looked just like there had been a (sizeable) piece of grit caught under the gasket...that's what you get for working on your driveway. Weird though as I made sure all was clean - a gremlin must have put it there when I looked away for a second!

I rechecked the head mating surface - gave it a very rapid scrape on glass again and then put it all back together with a new composite gasket.

Perfect - no oil leaks, no nothing. Compression is around 145 psi now compared to the measly 90-ish before and the bike feels really good on the open road ( I saw an indicated 75 MPH! ). I'll retorque the head again next weekend. So far, so good. Now to sort the next problem, the idle is too rapid - due to a worn slide in the carb it seems. Always something...

Al

[bigwol]

Well, the G5 is back together. I fitted the copper gasket and the damned thing leaked oil from around the pushrod/oil feed area - plenty of compression though! I tried a retorque and it made little difference, so off came the head again...

The lovely copper gasket had a high spot on it! Looked just like there had been a (sizeable) piece of grit caught under the gasket...that's what you get for working on your driveway. Weird though as I made sure all was clean - a gremlin must have put it there when I looked away for a second!

I rechecked the head mating surface - gave it a very rapid scrape on glass again and then put it all back together with a new composite gasket.

Perfect - no oil leaks, no nothing. Compression is around 145 psi now compared to the measly 90-ish before and the bike feels really good on the open road ( I saw an indicated 75 MPH! ). I'll retorque the head again next weekend. So far, so good. Now to sort the next problem, the idle is too rapid - due to a worn slide in the carb it seems. Always something...

Al

Did you heat the copper gasket up a nice cherry red and aneal before use Al?

[Al_Bolton]

Yes, certainly did. Always do that with any copper gasket, new or old. But that wasn't the problem - the problem was something sneaked in there under the gasket...I noticed it made a small mark on the barrel mating surface too.

Just got to work on the G5 and the petrol tap lever broke off in me hand...It's one of these rather excellent modern gas taps which doesn't leak and cost £3...luckily I have a spare (at home).

Al

[Did you heat the copper gasket up a nice cherry red and aneal before use Al?[/quote]

[Ozmadman]

Yes, certainly did. Always do that with any copper gasket,
Did you heat the copper gasket up a nice cherry red and aneal before use Al?

Just for future reference, does anyone know whether an aluminium head gasket needs the same treatment as a copper one??

Paul