revtothelimit - motorbike forum

You’re welcome to borrow my tappet block removal tool rather than buy one if you like

Nidge wrote: Fri Oct 02, 2020 8:49 pm You’re welcome to borrow my tappet block removal tool rather than buy one if you like

Ooh there’s a job for a man with a knack!

Nidge wrote: Fri Oct 02, 2020 8:49 pm You’re welcome to borrow my tappet block removal tool rather than buy one if you like

That's very kind. It's gonna be a few weeks/months yet but cheers.

Mr. Dazzle wrote: Sat Oct 03, 2020 9:29 am

Nidge wrote: Fri Oct 02, 2020 8:49 pm You’re welcome to borrow my tappet block removal tool rather than buy one if you like

That's very kind. It's gonna be a few weeks/months yet but cheers.

No problem - let me know if/when you need it

Will do.

While the cylinders and head are off being fettled I'll put new washers and seals etc. in the rockers so they're all tip-top.

I also need to get one carb float bowl, at the moment one side has the kind of bowl where you can take the main jets out through the bottom and the other has a mismatched solid bowl.

If there is no oil already leaking from the drive side ends of the rocker shafts in the rocker boxes, there’s a lot to be said for leaving them alone.

Mr. Dazzle wrote: Sat Oct 03, 2020 3:32 pmI also need to get one carb float bowl, at the moment one side has the kind of bowl where you can take the main jets out through the bottom and the other has a mismatched solid bowl.

The first Concentrics were supplied with no bowl drain plugs, shorter main jet holder, “two-stroke type” needle jet and needle and a pilot jet screwed up into the body of the carb.

If the bike ran well, it’s unlikely you have two carbs full of different parts though.

The carbs were previously a right mish mash of bits. My FiL paid a guy to do 'something' to the bike not long before he stopped riding due to ill health. In the end he had to fight to even get the bike back off him....total scam.

Its got all new genuine Amal bits inside 'em, but I think the setup is way off. It previously had standard jetting (with gunged up bits) and that's what I put in too. However its straight through both inhaling and exhaling plus its supposedly got high lift cams, so I'd be surprised if it needs standard jets.

Rocker box wise, its got oil leaking from both ends of the shafts. At least I know the rockers are getting oil!

Mr. Dazzle wrote: Tue Aug 18, 2020 6:01 pm So maybe I should get a new head after all.

People say that to me on a regular basis.

Mr Dazzle wrote:the gears are on the left

The pedal that changes them is on the right.

The cogs themselves are somewhere amidships.

Yeah the pedal is on the right Took a while to notice that mistake eh?

When I first rode it it took ages to realise the gears shift 1 up 3 down. I had be forewarned that first was really tall and that the ratios were quite close. Having never ridden the bike before, and with the fueling being well off, I actually struggled to tell which gear is which!

Also not helped by the fact both Speedo and Rev counter are more general themes than indication. Unless you believe a 69 Bonneville can do 150mph at 10,000rpm that is.

It will pull off in fourth and wasn't running smoothly, so it wasn't abundantly clear which way the gears go. Made a lot more sense when I figured they were 'backwards' though.

Nowadays they seem to call it a racing shift pattern.

I don’t know if a close ratio set is saleable, to someone obsessive about originality on their Thruxton.

I’m told it costs about £1,000 in new parts to put five gears in it.

AFAIK it had close ratio from new, the original owner (who didn't road register the bike) did it for racing.

Update - without pics sadly.

The head is all fixed up. They had to weld up and then re-machine the offending guide hole. It's now got brand new guides everywhere, it also needs new valves (old ones distorted) and new springs (old ones were too short after overheating).

Cylinders have been re-honed. A re-bore wasn't needed and it's already at +40. It's keeping the old pistons but with new rings, he's gonna gap them correctly for me.

I shall order a complete gasket set tonight and we'll be ready to put it all back together in a few weeks.

Good time to fettle/rebuild for spring

'Course.....what I need to do as a priority is figure out why the valves and springs got so hot they distorted in the first place! I've personally only done ~100 miles on this bike so far and I'm assuming this problem has been building for a while.

I am guessing poor fueling. The cylinders were black inside, but is also had piss poor compression and basically no oil control. Hopefully now its got nice healthy bores etc. I can take a proper look at the fuel.

If you look at my pic on page 1 you can see the exhaust headers have got pretty toasty based on the colour. They're supposed to get hot, but I'm not sure how hot is OK.

The fact that it was burning oil could make it run hot.

Otherwise, weak mixture, particularly needle position, or ignition timing wrong (in either direction).

I picked up the bits from the mechanic today.

I'm keeping the old pistons - which aren't actually that old I don't think - but it's got new rings throughout. The rings have been correctly 'gapped'; you need to make sure that when the ring is located in the bore it's got a tiny gap left in its circumference, if you don't the rings will seize in the engine when everything expands. However if you have too much gap it won't seal properly. I left that job to the mechanic.

The cylinder head has new guides, 2 new valves and new springs. It's also been vapourmatt cleaned (basically very fine grit+water blast) and the valves have been lapped in. It's been pressure tested and it's airtight. I imagine this is the best condition the head has been in for several decades.





Bores have just been honed, didn't need anything else. I'm gonna replace the tappet block O-rings, I'm not even sure it's necessary but I might as well while I'm in here. That also means I can give the outside a coat of black paint. In the meantime the bores are coated with a bit of engine oil to stop 'em rusting and it's tied up in two layers of plastic bag awaiting reassembly.

That several decades probably extends to 'ever'.

Too small ring gaps cause engine disaster. Gaps bigger than spec have no real effect.

For optimum pressure sealing, the gap in the second ring should be 30% bigger than the gap in the top ring.