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Thread: AMAL Monoblock Boring
08-17-2011, 03:46 AM #1
AMAL Monoblock Boring
I would like to refurbish a monoblock carb by boring the slide area and fitting a sleeve to the slide. I found some old threads mentioning this process but how were the carb bodys held for boring?
The monoblock has a hole straight through from the slide area and I asusmed this was concentric. I made an arbor to hold the carb by that through hole but it runs out by 1/16" or so when spinning on a lathe. I am not sure what else to use as a datum/ holding point.
08-17-2011, 03:58 AM #2
Amal monoblocks?..... now they go back to my biking days
If the mounting flange / spigot is square to the through bore, mount by that on an angle plate / faceplate combo.
If it's a spigot you might need to make a dummy holding clamp to go on the angle plate.
If it's mounting is angled, it can still be done but will need a bit more fiddling.
Sometimes ??(IME later Amal carbs were not at all well made) the float bowl face is true to the slide bore in which case make up a larger cover plate & fix that to the carb to give you something to fix to the angle plate.
Oh yes, while you're at it, check the mounting flange for flatness etc and re-m/c if neccesary.
08-17-2011, 04:31 AM #3
I switched to Mikuni's to avoid just such activity.
I still have the Amal's, so I will have a look at them.
08-17-2011, 04:50 AM #4
Back 100 years ago when I was racing "Micro Midgets", there were 2 guys that custom built AMALs and BINGs. We used Bing Concentrics on a Konig 250V back then. There were probably just as many guys using the AMAL monoblocks.
Both guys are long gone. The one that was in Linoln, would set the carbs up on a "tiny" jig mill and then sleeve with stainless. He'd hone the stainless sleeve with a Sunnen hone to fit the new slides.
I blame that Konig for not being able to hear anymore. Boy was it fun at the time.
08-17-2011, 05:03 AM #5
A guy in the machine shop where I used to work did his on a vertical mill or jig borer with the body bolted to an angle plate via the flange and an extra support in the mouth. Apparently getting everything set up and aligned properly relative to the main jet required considerable creative comments! Apparently OEM wasn't impressively accurate. Material is a bit picky as to speed, feed and tool shape too when it comes to getting a good finish.
Seems easiest to bore and sleeve the body to take new slide from these folk AMAL - The AMAL Carburetter Company | AMAL - The AMAL Carburetter Company or their local agent. A while back there was talk of body repair sleeves and slide kits becoming available. Presumably from there. Dunno if anything ever transpired but I was asked if I could do the machining and fitting if a kit were purchased. Declined the job and heard no more.
All downhill since the GP and Standard range.
08-17-2011, 05:08 AM #6
08-17-2011, 06:17 AM #7
08-17-2011, 06:45 AM #8
08-17-2011, 06:48 AM #9
Lap the flange flat. Setting up to machine is the long way round and they always look a lot worse than they are. Sheet of emery on plate glass always worked fine for me, but then I was young enough in those days to get away with not knowing any better!
Don't fancy the idea of grabbing in a vice to machine tho'. Monoblock bodies aren't terribly strong.
08-17-2011, 07:05 AM #10
The best way to hold them by the slide bore is to make a shouldered mandrel to fit said bore, with a tapped end and put a (say)1/4 bolt up through the main jet hole.
X2 on not putting em in a vise.
08-17-2011, 01:56 PM #11
This is what I tried.
08-17-2011, 02:15 PM #12
I wish I could reclaim about 10% of the time I burned up on Amals back in the day! To bore Monoblocks I had a fixture that the flange bolted to. It was sort of L shaped and I would hold it in the lathe with a 4 jaw chuck. I am amazed that Amal is still in business! I clicked on the link provided by Clive603 and the first thing I see is the new "stay up" float, novel idea that, a float that does not sink!!! LOL
08-17-2011, 02:28 PM #13
mcman56 I think you will find that setup is a bit flexy. The one I made started with a bigger piece of stock. It had a plate about 1/2" thick up to the mounting flange. The flange bolted to the plate as well as a pilot and a bolt through the main jet bore. hold the lashup in a 4 jaw chuck so you can true up the bore.
08-17-2011, 07:18 PM #14
Monoblock? You late model guys!
There are datums other than the bore you want to restore on those critters. The needle jet threaded hole is establishment of the entire important axis of the device. Tool up to make it run true, have at it.
08-17-2011, 07:36 PM #15
''Monoblock? You late model guys! ''
Spose you're running a GP on your Zimmer
08-17-2011, 09:48 PM #16
If the bore's not horribly worn, but oval instead, I've found there's no need to bore & sleeve. Instead, turn a spud to .0005" larger than the bore, heat the carb body with a soft flame (propane works) & press in the spud. Then heat the spud & body together & let them cool. When cool, push the spud out with a stick through the jet hole, from the bottom. The bore will be round again. It's common that the body warps from overtightening the flange mounting nuts/bolts against the O ring, which stands about .025" proud of the surface. To prevent that, put a .012" shim (washer) between the carb body/mounting flange & the manifold. You can then tighten it down firmly without warping the body.
Mark in SC
Last edited by TAG; 08-17-2011 at 09:55 PM. Reason: Correct typo
08-18-2011, 07:33 AM #17
A new Amal monobloc isn't all that expensive comparied to a new Mikuni and you can get it prejetted to the make and model you own as I would assume it was OEM to begin with and you can even get it with a anodized slide too. Is the diffuser block still in the carb? This is the part that fits on the inside of the slide and makes the carb a smooth bore and without it the slide wears the body as it isn't long enough to support it in that bore. Most of them were thrown away because they could be a pita to set up and the carb would work without it...for a short time anyway. Dellorto SS carbs are similar but the parts are really expensive and hard to source. I've bored the slide bore on mine out just to 'true' and then made a new slide to fit the diffuser and new bore. In the end you probably have the time to play with this and if you screw up the body it was worn out to begin with and then you have to make up your mind if you want to buy a Mikuni and go through the time to rejet it or buy a OEM Amal and go riding. I'm just sayin.... Old bikes are a lot of fun and even more fun when they idle and run properly.