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NickL #880709 05/16/22 6:09 am
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Hi Nick,

Do you have someone locally doing the offset crank? Doing it yourself? For other reasons I rediscovered I have a spare crank and am now considering what an offset engine might be like.

Cheers
Ray


BSA 1969 A65F
BSA 1966 A65H
Triumph 1968 T120
Kawasaki A1R
& too many projects!
British motorcycles on eBay
NickL #880710 05/16/22 6:28 am
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Ray
I've basically followed Mark's drawing for the offset crank.
We had it cut in half and are machining up a flywheel for it.
It's just a turning exercise really, plus no timing side to modify for
oil pump drive, drive side different size etc.
As all my old offset Norton crank conversions were bolted together,
that's what will be done here. To make life easier we will run the
SRM 90 deg cam.so have the timing side as the leading one.
I am sorting out an ignition for it at the moment but the crank is
an ongoing project that will be done within the next month or two.

Last edited by NickL; 05/16/22 6:30 am.
NickL #880722 05/16/22 12:03 pm
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Thanks mate.

Ray


BSA 1969 A65F
BSA 1966 A65H
Triumph 1968 T120
Kawasaki A1R
& too many projects!
NickL #880907 05/17/22 11:18 pm
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So the late Thunderbolt head castings are a bit thicker than the early ones. (not thick enough) It seems more like the '71 in the bowl area on the twin carb but not quiet. The early T/bolt I have is small valve so not comparable. But I'm pretty sure the shape of the bowl is different.

Started hacking it on one side, hope I can get it good. I want it + 60% at least. frown

The seats will just go 42mm one isn't very central. The way they fit the guide is better on the earlier '70 without the recess because it has more grip on the guide.

The port roof needs to go straight and sweep down onto the valve or it can sweep down all the way (except for the valve spring area), but it cannot go down, then up, then down, because when it's going fast that wall is directing it, like a little car, if it aims down then up again it will leave the wall and air will break up. It isn't pushed against the bottom curve, but it needs to be smooth curve over, and the right volume with no ups, like a jump, or it will leave it. It can go wide around the valve and guide because it will be forced because it's blocked by the guide. And it needs turning onto the back of the valve, not across it. If it comes off the wall anywhere it will break up and eddie and block the port.

Last edited by Mark Parker; 05/18/22 2:38 am.

mark
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NickL #880917 05/18/22 12:37 am
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Yes, it's surprising about the many differences in the heads over the years.
As i've said before, the later motors were much improved overall. BSA
management may have been stupid but many of the guys there were not.

Last edited by NickL; 05/18/22 12:38 am.
NickL #881004 05/19/22 2:28 am
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Tentatively speaking so far so good. I have a Lightning 34mm port and 42mm valve, reading around 180 with a bell. This morning it read 178cfm, so comparatively on the same calibration from the plate, the Thunderbolt port is 173.3 if everything is right. But parts are rough and I need a patch not tape over a hole. And I need a bit of floor built up. and a proper seat cut with a permanent guide. It may go more with luck but the same will do, what ever it actually is. It needs the floor, maybe no a lot, to increase speed with fill where it's doing nothing. More metal and less air around that port would be good.

With full vacs 31" - the 162cfm hole in the plate is 36"w. Std head the vacs will eat the water. Speed's maybe over 40" with a hose shoved down there. The other side has to be hacked out to match so it doesn't pull to one side. So Matt can steer it.

[Linked Image from i.postimg.cc]

Look where it's aiming, those stud holes. Haven't hit one yet, and it seems good. There is always the other one. Ported Triumphs and Harley XR750s have tubes in them. Making an alloy plate to patch pushrod side. Weird shape, if it's thick cannot pound it to shape so once on no more hacking that side. It cannot be just JBWeld because it cracks.

If it sets over night and the head stays where it is it should be good.

Last edited by Mark Parker; 05/19/22 7:45 am.

mark
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NickL #881024 05/19/22 11:10 am
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LOL, those figures look bloody good, that's a massive increase and
being a small port it should be very good at partial valve lift too.
I used a couple of small pieces of 3mm ally plate in the floor of the
one i did up for Matt's 650 as it had been opened up way too much.
The side wall had broken through on that too so i put a couple of
1mm thick ally pieces in the tunnel stuck with jb weld. That' seems ok
and didn't crack again. Yes, the one you did before is still good, i have
it on the bench at the moment and there are no apparent cracks in that.
Be great if you can get both sides the same as otherwise we will be
limited to oval circuits...............
It seems that the straight tubes are better than angled down as previously
done, i'll have to get the carb end of the manifold modded a little to suit..
Great stuff Mark, much appreciated.

Nick

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NickL #881092 05/20/22 4:26 am
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How smooth bore are Webers? I know they have a thing in the centre and butterflies but they were the thing in the 1960s in F1 and some GT40s.

Side looks good and numbers are still good. Waiting for a little bit in the floor to set. .385" to .410" is about +1cfm which is better than less.

The Thunderbolt head has a little more on that bottom turn and the floor sort of curves down there for a while, so I just filled it a little to see if it adds flow or loses it. If it doesn't loose it should add hp. If I fill the turn at the port to head joint a bit more it may be the same thing. It's an odd way to do it but filling a little without loosing cfm will add power. One at a time or I won't know what's doing what.

That extra cam lift even though it isn't making heaps of extra flow will actually benefit as it's keeping the valve at a good height for longer, shutting later takes advantage of intake speed.


mark
NickL #881095 05/20/22 4:54 am
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The actual main venturis in the DCOE are very smooth and nicely flowed.
The butterfly follows after the main venturi so they are what spoils it, well,
not so's you'd notice anyway. The auxiliary venturis are where the fuel mixture
is dispensed they are before the main vents. These placements are what makes
the carb a large lump but it never stopped them being one of the best carbs made.

https://www.ebay.com/itm/291433107226

https://240260280.com/Tech/Carbs/Weber/DCOE%20Theory%20Operation%20and%20Tuning.html

Last edited by NickL; 05/20/22 5:10 am.
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NickL #881098 05/20/22 6:35 am
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Hi Nick

Thanks for the post on the Weber carb. I now just might manage to understand what goes on inside these.

regards

John

NickL #881166 05/20/22 11:57 pm
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John, if an idiot like me can get to grips with the webers (well, on a good day anyway)
I'm sure you'll see the see the pro's and con's of them.
I can honestly say that the trouble ive had recently with this bike's carb is the first running
and tuning problem i've ever had with them. AND, that was just my not recognising the
float level problem. Someone with half a brain would have seen it straight away. The first
time he used it it was fine then the next, it was rubbish,,,,,, obvious really that it must have
been something along those lines but it took me way too long to find it. My age is now greater
than my IQ i think............

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NickL #881169 05/21/22 12:21 am
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Here is a thing. If big pipes lower gas speed which in turn lowers inlet gas speed they can cause reversion or flatness. Combined with a hot late closing inlet valve it becomes worse because the piston is on the way up. Even if top end is eventually stronger. So inlet speed and inertia is what is adversely effecting it. A small high flowing port addresses that. And like at low rpm at high rpm it has more inertia.

Neck and neck with the best using that 42mm valve. I stuck the other head on to make the comparison and it was screaming so loud reading too high then he twigged, put a spark plug in that hole frown The left side is toward the wall. The Thunderbolt port may be a little bigger, but now the other side. It's still not exactly perfect in all the surfaces but basically a functioning shape. Usually it's a big fight, polishing stuff to try and coax a bit more.

Last edited by Mark Parker; 05/21/22 4:56 am.

mark
NickL #881170 05/21/22 12:40 am
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Yes i agree but this situation was made worse by me richening the mixture which is the opposite
of what you would expect if reversion was causing the problem. I had considered the fact that
he is running an exhaust that now has a short slightly reversed outlet 'mega' type end and thought
ahh yes reversion. but i was way off.

NickL #881184 05/21/22 8:27 am
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That was just a general comment. It should have included compression ratio because high comp means faster exhaust better pull on the inlet. Nothing to do with the malady with the outfit. That's funny change the exhaust a bit smile runs bad...

You should be able to run a 90 to 8,500 or more if you needed to. I don't know about leaning over it. I guess having a great heap of petrol it a tin between you and it may not be a whole lot better.

In theory if you get more flow through the same carb it needs leaning because there is more pull on the jets. That's just to have it on the best mixture. A short stroke may be a bit different. Just the bigger window under the valve. Should make an assembly video. There are some on youtube of pretty ordinary re-builds and then the kicking and smoke.

It would be great on that thing, like condense the build (some vids, they take 20minutes to fit a plug or discuss an AMAL)then show the starting and on board and testing or racing. If you have onboard from a go pro you can edit it and switch between trackside and onboard. Find kids doing video at school or Uni maybe. They have access to good gear. It could make a really interesting exciting film. Especially if it's running good.

I like the little hammer hit on the head getting that guide out. Saved me doing it.

This shows compression. What it does if it can survive. The red is 12-1, the others are 10.5 or 11-1, see what it does to it. Why they ran some triples at Daytona with 12-1 but others lasted the distance. That's me matching two odd cases with a big file. Not quite right, na, nearly, bit more, almost. Wanted the cylinder square to the crank. Valves were pretty close when I put it together. It sounded quite different in the intake, but but it was really nice. I never heard it rattle on 98 but after actually measuring it, I kept thinking what if it does it at rpm and I don't here it?

[Linked Image from i.postimg.cc]

It would be interesting dynoing it with that comp. This is from the data logger and shows acceleration. The better head is better but there is still that little dip down low, where you wait more. I have ridden Ben's once and not far, but I think it's pretty strong without waiting in that dip. It's impressive.

The torque at 6000rpm is around 68ftlb on the hp graph of that run. So around 77ftlb at the engine, 4ftlb better than testing on the actual dyno, with a better head and less compression, though that was very lean it pulled strongly into higher rpm.

Last edited by Mark Parker; 05/21/22 11:21 pm.

mark
NickL #881223 05/21/22 11:15 pm
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We used to run the old 750 to 8k as a parallel motor years ago.
I spoke to a couple of guys riding them and they said similar so
we'll go with that to start with. Comp ratio will be up at 10.5-1 i
would think, but when we are further forward i'll measure it and
see.
Matt's tried his gopro a couple of times but the stuff out of it
isn't very good.
Sorry about the small dent, it wasn't a little hammer hit i dropped
the head onto the vise while turning it around.

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NickL #881311 05/22/22 11:23 pm
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Allan Price said he used about 11.5-1. The EdV pistons can easy get that in a 744 I think. I bought a burette so I could see precisely and Ben's is just under 11-1 with Omegas. Allan said he used pump fuel, no additives.

On Ben's we have the pistons at deck height and the gasket giving the clearance. Paul's were a bit above it using the nice forged ones. I need to make it around 11-1.

Hacked #2 out this morning pretty quickly. Waiting for JBWeld to set. Not quite the same, but I'll test first. I just want similar flow. When it's almost set it can be pushed into shape. It's sort of good putting enough but not too much and making the shape without heaps to cut. Pushrod tunnel and floor still to be fixed.

Last edited by Mark Parker; 05/23/22 3:43 am.

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NickL #881391 05/24/22 3:21 am
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#2 didn't look quite the same and surprise surprise didn't flow the same by about 13cfm. It now looks the same and picked up 11. So doing the floor and then the pushrod tunnel. It's not much different but it directs the air. And it means I know what that depth and turn does. BSA did a B50 head and cut the bowl wrong, too deep, it was their best competition head at the time. And if you do not go there, you never know. You are just on a limit and think that is what it is.


mark
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NickL #881518 05/25/22 10:50 pm
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Mark, your PM folder is full.

NickL #881530 05/26/22 4:53 am
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Deleted one.

Fitted guides - the left hand exhaust guide hole was tighter using the undersize guide as a guide than the intake hole, if you can follow that. So I put the 2 in the inlet. At 200c and with freezered guides they all tapped in. The other side is the reverse. None just fell in. Now they are cold they all need honing, so even a std valve does not go in. The nitrided ones are fractionally bigger. I got a fancy guide hone a while back because every new guide does this. Crushes in a bit. If you wear them out get them K-lined rather than removing.


mark
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NickL #881546 05/26/22 9:50 am
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No one around here does k-liners Mark.

NickL #881565 05/26/22 2:08 pm
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Yep, that messes that idea up. They should last anyway the nitrided valves mean less wear. I'd put old guides back in my stuff and K-line by preference. But waiting is a problem when I can just slog MAP ones in. Cost is about the same, and may even last as well, plus I have my hone. It's just the idea of removing a guide and maybe moving the seat alignment.

The Trident used to stuff guides pretty quickly. Then smoke a bit. Usually pulled it down pretty often to fix leaks so could do guides. Solid copper tappet cover gaskets were a great improvement, but it still had push rod tubes. Oil tight one day, seeping out a rubber the next, and it never fixed itself. Hard to believe an engine needed to be so complex.


mark
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Hi Mark
Wondered what clearance you used for the guides, the valves seem quiet tight in them.
I know that decent guides and seat accuracy allows better fit but i still would have thought a couple
of thou on the exhausts is relevant, the inlets don't worry me so much but i would like your opinion.

Nick

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If they seem tight hone them a bit for sure. They are probably tightest in the middle where they crush.

I was just trying not to make them too big. Worn out before you run it, but too tight would not be good.

I just looked up Vizard's book .0018-.0021" is recommended on exhausts and .0014" on inlets, and I have nothing to measure with and the exhausts probably are small. I'm only guessing ie they fall shut dry but not with oil and maybe they should.

I usually used K/liners which are very anti seize and and straight, plus I'm not the one that sizes them.

A piece of rod with a slot with wet and dry in a drill or air tool can do it, if you haven't got anything.


mark
NickL #883112 06/16/22 10:36 am
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Few shots of the primary side showing outriggers etc.

https://imgur.com/a/BfK76ud

Last edited by NickL; 06/16/22 10:43 am.
NickL #883116 06/16/22 11:03 am
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I can scrounge a guide hone as my reamers won't touch that gear.
1 on the inlets and 2 on the exhausts are my fallback numbers when
using PM guides and having done the seats myself, so i'll use that.

Nick

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