Boring bar size ?

[Jeff Austin 1Participant @jeffaustin1]

HI,

Sorry for late reply, as is probably the case with most of the forum members I have been cursed by being a "practical man" so always other things to do, i.e. rcd unit tripping, bathroom to refurb, help friends (?) as in do you know anything about ? well the default answer should be "no idea about anything to do with that" but you tend to say yes don't you ?

Old mart, the 16mm boring bar is a decent option for me as I do odds and sods on the lathe for myself and could help others with the same type of work plus there are not any easy alternatives for bigger carbs, they are a flat slide carb with power jets (internal unusually) and are mounted at around 45 degress so the float chambers have to suit, the only bigger bore alternatives need more modification to fit them than is required to bore from 26 to 28mm, the reason for boring the carbs is engine upgrades, two stroke porting and capacity increase with better expansion chambers means that although the standard carbs will work the bigger bore size with correct jetting helps with a substantial power increase through the rev range,

Oily rag, Martin,

They are not pumper carbs but do have a "power jet" which unusually is internal to the carb, you are probably familiar with most power jet type carbs that have an external mounting on the intake side of the carb with a jet fed from the float chamber via an external tube,

re bell mouth changes, they are pretty good on the intake side with a very nice looking profile to the bell mouth shape, I still haven't had chance to get back on with the good jobs (bike engineering in my garage) due to other more mundane stuff to get out of the way, I could take a picture or two when I have a bit more time.

I agree that the youtube video of the diy boring bar is not a great example, I am of limited experience with turning, boring, milling etc but I do have a motto that there are only two ways to do a job, a right way and a wrong way, well that does not look the right way to me, the execution is poor, I cringed when the drilll bit was clearly off centre when drilling through the bar and as someone else said even the filing of the hole from round to square was a bit iffy, he would not have got his O level (GCE not gcse) metalwork at the school I went to, however the idea seems sound so I might have a go at making my own, it does look like the finished bar would work well for my purpose,

One of the replies mentioned being careful about getting the carbs mounted perfect for the obvious reasons of not getting the machining perfectly concentric, well the good news for me is that the carbs will mount directly into the three jaw chuck on the intake spigot so unless I have a meltdown (imminent) then it should not be a problem,

I will measure up my tool post asap and see if I can get a 20mm bar into it to clamp correctly, if not I will most likely go ahead with a 16mm bar and prcoeed with great care, the carbs that I am boring out are a spare set but as they vary between £200 and £600 a second hand set then I don't want to damage even the spare set,

Thanks for all the input, much appreciated,

Regards Jeff

Edited By Jeff Austin 1 on 17/01/2021 08:24:04

Edited By JasonB on 17/01/2021 12:15:47

[old martParticipant @oldmart]

Although you will be using a 16mm bar for a depth of 90mm, plus a bit more probably for clearance, the forces on the bar will be minimised with small depth of cuts and the soft material. The inserts for aluminium are very sharp, allowing depths of cut easily down to 0.012mm, 0.0005". Take your time and you may find it safer to do the boring away from the chuck. If you have a stop for the saddle, use it.

[Jeff Austin 1Participant @jeffaustin1]

Posted by old mart on 12/01/2021 16:28:45:

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.

HI

Been very busy so just got back to looking at this project, I have purchased the above recommended bar and insert,

I have since found out that there may be a problem with boring out this particular carb, it looks like even taking 1mm off (26 to 28mm bore) could be tricky as it may remove material that is shrouding an airway, also been given the information that when other engineers have bored these carbs from 26 to 28mm then they do it by boring them slightly off centre so as to avoid removing material from the bottom part of the carb so that the material shrouding the tiny airway is not removed, that means that I probably cannot do the job myself anyway now, I do have a small milling machine which is a Clarke CMD 300 but I think it is close on height to be able to do the job on this machine and I think that I would probably need some way of mounting the carb at a slight offset or tipped over at a slight angle, it does not need to be much to leave the material alone around the airway but it is getting too complex for me at this time,

Thanks to all

[JasonBModerator @jasonb]

Do you gave a 4-jaw chuck, that should be easy to offset the casting with.

Alternatively if you plan to do several then make a machining plate that can be held in the 3-jaw or on a faceplate with the carb mountings offset then it's a simple job to mount carb to plate and bore the offset hole in the same place every time without having to spend time clocking it up for the offset.

[Jeff Austin 1Participant @jeffaustin1]

Thanks JasonB

Actually I do have a four jaw chuck for my lathe so that is a possibility which I will be looking into, I also like the other suggestion of making a mounting plate for the job, as you point out this could be very useful as I have three to do initially and it may lead to doing more of them or similar jobs so the experience would be worthwhile,

[old martParticipant @oldmart]

You will have to be careful to get the offset the correct ammount and don't forget to mark the point of maximum deflection to align the carb bodies. If you offset 1mm, then the boring bar will just give you the 2mm increase in diameter without cutting into the sensitive part of the bore. Double check the settings before cutting metal. Keep the depth of cut small, 0.002", 0.050mm and remove the swarf regularly.

 The boring still has to go in line with the original carb bore, but slightly eccentrically, so a four jaw independent chuck will do exactly what you want. When mounting the finished carbs, you will have to hand blend the offset bore to the inlet manifold, best done before final cleaning and assembling of the bodies.

Edited By old mart on 31/01/2021 20:21:40

[Author]

Posts