Exhaust to Head Spring

Hi, this may be a real amateur question, but I was wondering why on most classic racers I have seen, there is a spring from near the top of the exhaust to a fin on the head? Is it to change exhaust quickly instead of having to undo a bolt?
Thank You

On what bikes, for example ?

Some race engines have exhaust pipes that only push into the head, with nothing to actually hold them there - only the pipe clamped somewhere down near the footrests, etc.
Having the spring there is purely to hold it there - and counter the effects of vibration. Allowing a bit of flex makes them less prone to cracking.
Some japanese engines have the spring similarly there to take some of the load of holding it there, small studs not really being strong enough to retain it against the vibration of full bore running for long periods of time.

So, no, its not for quick change exhaust.
More to ensure its still there when you take it home later on... !

TBolt said:

Hi, this may be a real amateur question, but I was wondering why on most classic racers I have seen, there is a spring from near the top of the exhaust to a fin on the head? Is it to change exhaust quickly instead of having to undo a bolt?
Thank You

Click to expand...

On my Matchless G50, the header pipe is a slip fit over a steel stub in the cylinder head. Springs are fitted from the stub onto loops welded to the header pipe. The rear of the exhaust is mounted to the frame with two cotton reel rubbers. The springs retain the header in place and both the springs and the rubber mounts allow the entire exhaust to move a little to insulate it from vibration.

On Norton twins and Ducati bevel engines, springs are fitted to prevent the exhaust gland nuts from coming loose (damaging the threads in the cylinder head in the process). Springs are better than locking wire because they will tend to pull the nut tight if it loosens during a race. It is a safety measure as well as helping to seal the exhaust. The last thing any racer wants to see is a Black Flag with their number beside it because their exhaust is hanging off!

Some scrutineers like you to use locking wire through your springs, in case the springs break, which can happen as they become brittle with use.

Spigotting the exhaust header: The general idea is to remove the stress riser where the header "was" clamped into the head.

On Commando and big twin Featherbed racers, many choose to convert to a spigot (slip fit) arrangement at the head and hold the header in place with springs for the above mentioned reason. I have seen a few springs applied to the header nut to prevent loosening and major slack which will result in metal and thread fretting.

The spring from cylinder head to header also keeps header from falling off the bike if the thing gets loose elsewhere.

Thanks for the great replies!!! Now it seems so obvious, cheers.

I use a tag on each of the screws which hold the small plates to the rocker boxes, and run a spring to the exhaust pipe either side. As said above, it stops the threads in the head from loosening and wearing. I don't use the finned exhaust nut, with methanol it is not needed - only a stub with a step for the pipe to hit when it vibrates.

acotrel said:

I don't use the finned exhaust nut, with methanol it is not needed -

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I probably shouldn't ask, but do you think finned nuts are needed, with petrol?

Depends whether you believe the reverse wave in the exhaust pipe returns mixture into the combustion chamber. The heat from the port might have an effect on the charge density ? My feeling is that the fins are decoration, like those fins which some people put on timing and rocker box covers. If I was using petrol I would think about the ramifications, however motors run very cold on methanol, that is one of the main reasons I use it. Petrol motors can almost glow after long races on big circuits, and the oil pressure drops as the oil thins. For racing I would only use petrol in big bore two stroke motors, unless methanol was prohibited by the event promoter .

When you say it causes the threads in the head to loosen and wear, which threads are you talking about here?

The nut which normally holds the pipe into the head has a coarse thread which can become stripped and need a difficult repair. On one bike I threaded a piece of steel to make a tap, and inserted a helicoil with araldite. It is not what you want to be doing . The stubs simply screw in, in place of the nuts, and the spring holds the pipe back towards the head, so the stub tends to tighten rather than loosen and rattle around damaging the thread. The exhaust is also easier to remove and replace so work can be done.

Dances with Shrapnel said:

Spigotting the exhaust header: The general idea is to remove the stress riser where the header "was" clamped into the head.

Click to expand...

I remember Matt at CNW saying that they tried a spigoted header on a street Commando but there was a decline in performance due to the larger diameter pipe. I hope I've remembered that correctly. I wondered if you could weld a larger diameter where the pipe slips over the spigot and retain the stock diameter thereafter? Too awkward and costly to produce, perhaps? I have never had a gland nut on a spigoted header come loose on a race bike. I can see some negatives on a street bike, though.

The internal diameter of the stub and the exhaust pipe on my bike are exactly the diameter of the port outlet which is unmodified from standard. I have read claims the manx Nortons perform better with larger diameter pipes - I would never go that way. Actually it was pretty funny when one day, years ago I was in the workshop of a friend who had the most developed and successful manx in Victoria. His pipe was hanging off the rafter, and I looked at it and said to him 'gee that's a skinny pipe'. He said 'you bastard, you notice too much'.

If you have a look at the photo I posted, you will see the sleeve which fits over the stub welded to the pipe. The IDs all match up, the only problem I have is that the two into one pipe is made up of welded sections, and the flash is still inside the pipes where they join.

acotrel said:

I have read claims the manx Nortons perform better with larger diameter pipes - I would never go that way. Actually it was pretty funny when one day, years ago I was in the workshop of a friend who had the most developed and successful manx in Victoria. His pipe was hanging off the rafter, and I looked at it and said to him 'gee that's a skinny pipe'. He said 'you bastard, you notice too much'.

Click to expand...

Then again, have a look at the exhaust systems made by Andy Molnar for Manxes and G50s. The headers are as wide as the town drain, and yet they work well.

daveh said:

Dances with Shrapnel said:

Spigotting the exhaust header: The general idea is to remove the stress riser where the header "was" clamped into the head.

Click to expand...

I remember Matt at CNW saying that they tried a spigoted header on a street Commando but there was a decline in performance due to the larger diameter pipe.

Click to expand...

Don't use a larger diameter pipe.

'Don't use a larger diameter pipe.'

In the 30s there were single cylinder bikes with two exhaust pipes on twin port heads. Most other bikes used large diameter pipes. Post 50s bikes tended to use smaller diameter pipes. My feeling is that it is easier to vibrate a small column of gas than a large one, and that mass flow is less important. I've never known if the bridge pipe used on some twins is actually effective in doing anything except holding the pipes together better and stopping the threads in the heads from getting stuffed..