If you are not going to machine a standard boring bar, you will be limited by what you can grip in the tool post.

Why not make one to suit:-

Boring bar – home made

There are lots of things to consider. If you are going to pursue this as a business, and the job will repeat itself in the future, it is worth investing in the tooling to do it as fast and as easily as possible. That also extends to the method of workholding.

If it is a one-off, and you want to use the tooling more generally, than there are different considerations.

Inserted tool boring bars come in 16, 20, 25 and 32mm shanks as standard. The largest three of these four may not fit your toolpost, but you can make a dedicated holder (see https://www.youtube.com/watch?v=-55cbSUWSho).

If the things being bored are always 26mm minimum diameter, the most rigid off-the-shelf solution is a 25mm bar. But this is so tight to the bore that swarf management can become a challenge.

A 16mm bar 90mm deep is greater than 5:1 length to diameter. For a carb., you surely need a good surface finish for good gas flow and you have to take into consideration that it is an interrupted cut.

My suggestion would be a 20mm bar as it is just under 2 1/2 times more rigid than a 16mm one and it will still allow the cuttings a way out.

If the carbs. are die-cast aluminium, as Jason says, the sharp, polished inserts specifically for aluminium will help a lot in whatever bar you choose.

For boring a carb out a couple of mm,I woud make a boring bar from 19 (3/4 inch) mm dia silver steel and use a high speed steel tool bit with a lot of rake ,the cost is minimal and will last years,most of my home made bars have lasted 50 years. Bars to skim out aluminium die castings do not need to be that rigid,better to have a smaller bar so that you can see how the tool is cutting ,I do have a number of modern insert type bars I would not use one on a carb job.If you get serious about going into classic motor cycle resoration machining services,you will need a lot larger lathe with a large bore through the spindle, and get some product liability insurance or avoid making things such a wheel spindles or fork repairs,

Jason B recommends a 16mm boring bar, and I think that is the best size. The actual tip height of boring bars is somewhere near half the nominal tool height, so if the toolpost has enough depth to hold the bar, it can fit a lathe that could not manage a 16mm standard tool.

ARC sell a nice bar 16mm which takes CCMT09 type inserts. 060-325-39516

An insert suitable for zinc or aluminium CCGT09T308. 060-326-01938

The ARC site shows the dimensions of the tool. It will be long enough for your carb bore. I would recommend very small cuts and lubrication such as wd40 or ac90.

Back in the sixties, I had a pair of Amal monoblocks bored out to 1 3/16" for a Royal Enfield Constellation. The centre part of the carbs was in place and bored simultaneously. Some time later, one of the central parts failed, the cause was blowholes in the zinc casting.

What carbs are they? Flange or spigot fitting? The old flange type had a habit of being overtightened and the face not being flat, which has to be addressed when mounting them. Keep the speed fairly low and check for any runout before cutting metal. The problem of tiny drillings into the bore getting clogged up and the problem of removing tiny burrs from the holes without changing their fuel or air flow is a worry.

 As already mentioned, the long stickout of a 16mm bar can cause problems, another reason for taking tiny cuts. I, too would prefer 20mm bar size, but it could be difficult to hold in your lathe.

Edited By old mart on 12/01/2021 21:28:51

Jeff, whilst working on carbs, be vey careful when drilling as the Anzac has a tendency to grab hold of the drill and go much deeper than you intended. Example, drilling an oversized hole for the fuel banjo bolt !

Bigger is usually better with boring bars but you can get away with smaller with soft materials and a sharp cutting edge.

Martin C

A solid carbide boring bar would certainly do the job, but that depth is at the limit of one of my 12mm ones and a 16mm one would probably cost more than a set of larger carburettors. If a 16mm steel bar is used, the compound and cross slide gibs should be tightened up a little and the minimum of tool overhang to reach the required depth is important.

Jeff,

Just noticed your reply that these are Keihin TA09's, from memory these are 'flat sliders' and you stated …

"the carbs work very well as a standard setup "…..

Well, yes they do, but they can work even better by some other modifications. Changes to the spray bar and separating out the secondary air feeds from out of the bell mouths will give far better performance. The spray bar mods require the reshaping of the main jet well, and the secondary air feed mods improve the mass air flow capability through the carb, probably by as much as the boring out, if not more (just under 15% for boring, empirically around 18% to 20% for bell mouth changes). These are easy mods which can be performed on the lathe whilst set up to bore out the main airway.

Just as a reminder are these 'pumper' carbs?

Martin

GHT was asked this question about his similar design; his reply, IIRC, was that silver steel was harder and more likely to be true to nominal size.

If you can hold a 20mm bar securely, it would be better than a 16mm one. Maybe a special toolpost could be made for large bars. I have made three special toolposts and also a mounting to directly use a larger toolpost on the cross slide expressly for using 25mm boring bars, but which will hold all the standard tooling.