270 Offy

[mayhugh1]

The Offy's exhaust ports are connected to the collector through 5/8" diameter stainless steel tubes. A drawing in the manual shows these tubes being derived from 90 degree bends formed on a 2" radius. This is a pretty tight radius for 5/8" hard tubing, and even though my tube/pipe bender has a 5/8" die, its bend radius is 3". The difference didn't seem significant until I tried laying out a paper exhaust using the larger bends. The tube intersections became even more difficult; and the overall look was, well, ugly. So, I spent several days making and testing a 2" die for my bender. One of the photos shows it next to the bender's original cast iron die.

I tested the die using some stainless drops in my scrap collection. I had a few pieces of 5/8" tubing in three different wall thicknesses: .035", .049", and .065". The new die created near perfect bends in the .065" and .049" tubing, but the bend cross-sections in the .035" tubes were noticeably oval-shaped. They were improved when filled with Cerrobend, though.

I continued using the original roller that came with the bender since, after a bit of re-work, it fit the tubing perfectly. I accidentally discovered it was important to not snug the roller up against the tubing before sweeping it around the die to make the bend. Without a quarter inch or so gap, it invariably marked the tubing. Using the bender in this way seemed counterintuitive, but its instruction manual was clear about closing the roller against the tubing.

I ultimately settled on .049" for the tubing wall thickness since there's little space on the exhaust flange for a .065" wall thickness after the tube is swaged. And, since I'll probably have to make a lot of bends before I get four tubes properly fitted, I didn't want to deal with the Cerrobend. The swaging tool I'd already machined was designed for .035" tubing, and so a new one had to be made to avoid scrapping the already finished exhaust flange with its tube recesses.

The ends of the tubes must be swaged after they're bent, and this creates a work-holding problem. Since the swaging operation requires everything my half-ton arbor press can deliver, a stout fixture that won't mar the tubing is required. My solution, shown in the photos, was a clamshell machined from a couple pieces of red oak.

For practice, I soldered some of my swaged test pieces to a dummy flange to compare the three varieties of silver solder that I have on hand: Silvalloy 355 (from Brownells), Prince & Izant 50 ni3, and some cadmium bearing Silvalloy 35 (purchased from McMaster- Carr). The cadmium bearing solder flowed noticeably easier and would create the need for less metal finishing, but the Silvalloy 355 gave the best color match to stainless. Since I'm currently planning to polish the finished exhaust, I'll likely use the Silvalloy 355. If I knew I was going to paint the finished exhaust, I'd for sure use the Silvalloy 35. The solder manufacturers recommend the black (rather than white) flux for metals with tough oxide coatings such as stainless steel. I tried both, and the joints were definitely wetted better by the black flux.

I used up on testing what little .049" wall thickness tubing I had on hand and am currently waiting on some to arrive. Unfortunately, it seems to be lost in transit, and the supplier wants to wait a few more days before shipping a replacement. - Terry

 

[Peter Twissell]

Nice work!
I've recently been assembling the exhaust system for my radial, all in stainless at 1.5" diameter. I cheated by designing the system to use available mandrel bends. Still, there were plenty of parts required machining and tig welding. My tig skills are limited, so some filing is involved to achieve an acceptable result.
I can only manage a couple of hours at a time - filing stainless steel soon drains the enthusiasm!

 

[gbritnell]

Hi Terry,
When I first built my small tubing bender it would bend SS up to 5/16 but with the roller type shoe in some tubing it would leave an impression at the end of the bend. I changed the roller shoe to a rectangular shape ( rectangular block with the necessary radius cut into it) and that eliminated the depression at the end of the bend.
gbritnell

 

[tornitore45]

I've recently been assembling the exhaust system for my radial, all in stainless at 1.5" diameter.

1.5" diameter? what kind of a model is that? Sounds like a full scale engine.

 

[Peter Twissell]

See topic 'Gert Big Radial'

 

[Peter Twissell]

https://www.homemodelenginemachinist.com/threads/a-gert-big-radial.31859/

 

[tornitore45]

https://www.homemodelenginemachinist.com/threads/a-gert-big-radial.31859/

Interesting, to put it mildly.

 

[Peter Twissell]

Thanks (I think), but let's not pollute this thread any further.

 

[kinggt4]

Beautiful work! You are a very talented guy. A few Q's: I noted you have a Tormach - what model? Does the glass beading remove the tooling marks or do you need to finish the parts before blasting? Thanks for the great presentation.
George

 

[mayhugh1]

George,
I have an original 1100 which I purchased about a year after Tormach started production, and I've been very happy with it. It's probably been used a dozen hours per week since received and has been very reliable. Yes, the machining marks quickly disappear in the bead blasting cabinet with no other finishing required. - Terry

 

[kvom]

Are you running PathPilot now, and with the TTS holders?

 

[mayhugh1]

Are you running PathPilot now, and with the TTS holders?

Still running Mach 3 because I use the same computer for my mill and lathe and don't have the room for two set-ups. I always use the TTS toolholders except if I'm going to rough out a lot of material with a 3/8" or 1/2" cutter. - Terry

 

[mayhugh1]

After my piece of 304 stainless tubing arrived, blanks for the four exhaust tubes were formed using the special tools made earlier. Silver solder doesn't play nicely with poor fit-ups, and the Offy's exhaust has eight interconnected joints to deal with. The collector's taper makes things more interesting and will affect the order of assembly.

Construction began with the machining of a ferrule to provide a temporary slip joint between the collector and the #1 (front) exhaust tube. This ferrule was silver-soldered inside the exhaust tube and made to have a snug fit inside the collector for consistency while fitting up the other three tubes.

An orientation was selected for the collector that placed its welded seam on the opposite side of the exhaust tubes. As mentioned earlier, shrinkage occurred along the seam as the weld puddle cooled and created a slight bow in the collector. When tig welding a seam in a piece of stainless tubing, a nice bead can be laid down on the outside surface under the protection of the shielding gas. On the inside however, unless shielding gas is also flowed through the tube, nasty carbides will form on the backside of the seam. (Sone welders call these boogers.) These carbides are difficult to drill through and damaging to tooling, and so an orientation that avoided them was chosen. As a result, the exhaust will end up directed slightly away from the the engine. A fortunate result was that with respect to the exhaust flange, the bow compensated for some of the collector's taper and made machining the fish-mouthed tube ends a bit easier.

The #1 tube was the first to be soldered to the exhaust flange. This step established the alignment of the entire exhaust with respect to the engine and in turn affected the machining of the remaining three tubes. It was carefully fixture'd and solidly supported during soldering. I forgot to take a photo of this important setup before soldering, and so I went back later for a staged photo.

The rest of the puzzle involved machining the ends of the other three tubes so they'd fit up closely against the collector while sitting inside the recesses machined for them in the exhaust flange. I started with the #4 tube and worked my way forward. Another oak clamshell was made to accurately grip the bent/swaged tube blanks while their ends were prepared. It was used to support the blanks in the bandsaw for rough trimming as well as in the mill during their final machining. Thanks to the bowed collector, 90% of the final end preparations could be accomplished using ball end mills. Only the #2 tube required fine tuning with a file.

The three completed tubes were then soldered to the exhaust flange using the collector as an alignment tool. The flange solder joints that I practiced earlier required a lot of metal finishing due to my left-hand trying to feed the solder while I was blinded by the flame of an over-sized acetylene torch. So, I came up with a technique to take me out of the operation except for holding the torch.

Pads were machined from .003" thick silver-solder ribbon that I fluxed and inserted in the machined recesses in the exhaust flange under the tubes. Rings of 1/32" solder were formed and inserted down over the tube just above the flange. (Forming silver solder is a lot like forming music wire.) With some torch manipulation, I was able to draw the pad solder up and the ring solder down to fill the gap around the tube in the flange recess. This resulted in joints that required little clean up.

After finishing up the soldering on the exhaust flange, I noticed it had acquired a slight warp. After straightening it, the fit of the #2 tube to the collector was now more than I wanted to see. Although the soldering fixture had originally been ground flat, a heat-induced warp occurred in its weak area between the #2 and #3 exhaust openings. Rather than bend the warp back into the flange, I'll deal with the gap while soldering the flange.

With all four tubes attached to the flange, the last and now even more difficult soldering operation will be to attach them to the collector without creating a mess needing days of filing. I have some more solder practice ahead with the gap filling silver solder I've ordered. - Terry

 

[Charles Lamont]

Don't forget the 'finger holes' in the trumpet!

 

[mayhugh1]

Before the collector was finally soldered, the ports for the exhaust tubes needed to be drilled. There were just three 7/16" holes, but they had to be drilled through a tapered thin-wall 304 stainless steel tube which was problematic.

The mandrel that was used to form the collector's taper was repurposed as a sacrificial backup to reduce the trauma of the drill breaking through the collector's wall. With the mandrel held in place with a vise stop, soft jaw material was used to safely support the tapered tube. I came across this material several years ago in a Youtube video that showed it off supporting a raw egg while its top was machined.

'Soft jaws' is sold by Tormach as a jar of beads that when heated in hot water becomes a pliable mass of plastic. A pair of wood end blocks aligned the collector in the mill vise while the pliable material was packed around it. A heat gun was used to extended its working time. The vise jaws were left slightly open (.050" or so) until the material cooled and hardened to a Nylon-like consistency.

The holes were spot-drilled with a 3/8" 60 degree v-drill, and then drilled using a 7/16" carbide drill. The sound coming from the mill told me it wasn't happy with what was going on, but I ended up with three round holes in an undamaged collector,

This was my first experience with Tormach's soft jaw product, and I found it a little messy to work with. Unless it's at the right temperature, it tends to stick to fingers and anything else it contacts. I was concerned about it sticking to my vise, which I attempted to protect with plastic sandwich wrap. My concerns were unfounded since once the material cooled, it completely unstuck itself and was ready for reuse. Soft Jaws isn't something I'd routinely use, but in this case it solved a difficult work-holding problem.

Despite being advertised otherwise, the Silvaloy 355 (56% Ag) that I used on the exhaust flange wasn't a color match to stainless steel. I decided to use it in ribbon form only inside the close-fitting joints on tubes #3 and #4. I couldn't locate a suitably colored wide gap solder for tube #2. Instead, I used Silverbraze 50ni2 (50%Ag, 20%Cu, 28%Zn 2%Ni) which is advertised specifically for 300 series stainless steels but with tight fit-ups.

The exhaust assembly was heated while supported in a fixture that used a weight to pull the collector tightly against the tubes as the solder ribbons melted. The two close-fitting joints came out as expected, but hand feeding the Silverbraze into the #2 tube's wide gap was much less satisfying. After globbing on enough solder for a generous fillet, I realized I'd have to do the same for the other two joints so they would match. This hadn't been part of the plan, but once I got started, it was difficult to stop. I used the tapered mandrel one last time in a jury rigged horizontal rotisserie that helped keep additional solder in place. The result was pretty much the mess I'd hoped to avoid.

After spending a full day filing and sanding, the exhaust was finally polished using a new technique I'd been recently considering to avoid the risk of a buffing wheel. Starting with an 800g final sanded surface, I used a cloth rag in shoeshine motions with 1200g Clover lapping grease to buff the complex shaped exhaust to a chrome-like finish. - Terry

 

[gbritnell]

Terry,
Truly remarkable outcome on the header. When I built the header piped for both my inline 4 cylinder and flathead V-8 engine I had the same questions as you for making the openings in the collector pipe.
My collector pipe was formed with the first cylinder (bend) I made a mild steel fixture plate with the same bolt mounting holes. I used the fixture for fitting the subsequent pipes and for silver soldering. To make the openings in the collector pipe I drew a magic marker line around each pipe joint then used a Dremel grinder to make the openings, just staying away about the thickness of the pipe wall. I did it this way because the attachment points were more elliptical than round.
gbritnell

 

[kvom]

Can you explain the "collector pipe" for someone ignorant of IC engines? Where is it in the finished exhaust?

 

[gbritnell]

The collector pipe is the main tube which 'collects' the exhaust gases from all the cylinders pipes.

 

[kvom]

Now I get it. The holes are where the three tubes are soldered.

 

[ddmckee54]

I was wondering if you were going to open up the collector for the #2, #3 and #4 pipes before you soldered them - or if you were going to try and open them up through the openings in the exhaust flange. Did the edge of the openings try to distort any while you were soldering them?

Don