Tech Tip: Triumph Exhaust Spigot Repair

Last week I had the opportunity to save an old Triumph cylinder head which was destined for the “junk” parts shelf

The cylinder in need of repair was a 1967-1974 Triumph 500 Daytona (T100R) head which has the “Hemi” style combustion chambers

Exhaust port #1 had damaged and worn threads beyond repair

Making an oversized spigot was quickly ruled out…

I had no choice but to bore the threads and tap to a larger diameter

If you would like to see how I tackled this job, please follow along 

Let’s dive in…

Preparation (3 steps)

#1 Cylinder head 

 

Preparation and checking over the cylinder head is vital to understand what type of work needs to be done and how it should be executed 

This 500 cylinder head was pretty “crusty” but with a quick dusting in my blast cabinet I had the head looking decent 

I run a file on the head gasket surface and exhaust spigot surface to remove any burs or “high spots” that would cause issues when the head is bolted to my angle plate

Measuring the major thread diameter, I find that this cylinder head has a bore of 1 1/2”, however the major diameter of the threads are 1.440” which is correct for  an 1 1/2” x 16 TPI hole 

Visually looking at the exhaust thread quality, I figured I would bore the hole to the next nominal oversize which would be 1 9/16”

The correct hole size for a 1 9/16” x 16 tap is 1.501” - this means I do not have much material to remove and very little room for error

Since I figured out my game plan, I decided to roll with what I have

 

#2 Mill set up

 

Before I set-up my angle plate I took the time to clean the milling table

I removed any burs by stoning the table and then finishing off with quick level check

My angle plate just got it back from Blanchard grinding so this will be a good time to check how straight and true the plate is

With the angle plate on the milling table I use my Mituoyo test indicator to ensure the plate is Pharrell with the table and also check that the plate is running “straight” - both areas of the plate is within .001” which is more than enough

Now that the plate is ready to go, it’s time to bolt the head onto the plate

 

#3 Head set up

 

With the head bolted to the angle plate (loosely) the goal is to make sure that the exhaust spigot surface is running level with the table or perpendicular with the spindle

I can do this in 2 different ways - first using a spirit level and matching the angle of the head to mimic the table or use a known ground plate or parallel with an indicator to find “0”

For this cylinder head I decided to do both

I first sat my spirit level on the exhaust spigot surface and used my mallet to get the head level

After I set my ground parallel down over the spigot surface I then measured using my indicator the work area using an indicator 

I was approx .005” off - tapping the head and focusing on a 1 1/2” diameter I got the head running true

 

Next up is to find the center of the existing hole

I decided to take a new spigot off the shelf and thread it as far as I could down the existing hole

The reason I indicated off the spigot is due to the wear of the threads 

If I decided to indicate off the major diameter of the threads, it would have given me a false "center" 

Once the spigot was installed I used my Blake co-ax indicator to find the center of the hole

Now I was ready to start boring..

 

Boring

 

Using my APT boring head and a new micro100 carbide boring bar I began to remove material .005” at a time to monitor the quality of the cut and to check if our prep work (leveling and centering) was paying off.. which it was!

Each pass I would bore to the bottom face inside the cylinder head

I get as close as I can to the bottom face each pass but not touching the shoulder, it is only on the final pass I manually feed the boring bar down to clean up the bottom inside face

The reason for manual feed is because I have more control and can visually see how much material I’m removing

I want to be able to clean the bottom face up without removing any unnecessary material and making the exhaust port “deeper”

Cleaning the shoulder the face up allows the shoulder to be concentric with the threads… this helps when I make the new spigot to ensure we have a good contact area

I use a telescopic snap gauge to measure the fresh bores hole and I get 1.500” which is .001” off to what I need to be - that is acceptable

I changed out my tooling and installed a chamfering tool I modified from an older carbide boring bar and added a nice deep chamfer at the top

A deep chamfer and the top will allow the tap to locate itself and makes it easier on the tool to start cutting 

 

Thread tapping

Now that the hole is ready I begin to tap the threads using my “new to me” tap

I add some cutting thread oil and tap the hole until the tap bottoms out

 

Once done I remove the tap to clean and inspect all threads for defects or burs 

Spigot prep

 

Now that the threads are complete in the head it’s time to finish up the job and focus on making a custom, oversized exhaust pipe spigot

I took a look at my stock pile and found a solid chunk of 4140 round stock (2” OD) that we can use

Usually I have hollow tube on hand but have since ran out (waiting for an order due the following week)

Measurements

Now that we have our stock selected we must take some measurements of our cylinder head which will help us when making the spigot

We need to obtain the bore depth using a depth micrometer

This spec will ensure that our spigot will fully seat inside the cylinder head - if done right, the threads should be flush with the face of the head

Depth is different from head to head

Turning, boring & threading

Making the spigot doesn’t take very long to produce

Each spigot requires a mix of boring, turning, tapering and threading

Once the ID and OD are complete we are ready for threading

I use a thread ring to check the spigot thread engagement quality - I try to get the ring to thread onto the adapter as smooth as I can with little “slop”

Now that the spigot is complete we head over to the JET mill and bore 2 (5/16”) holes in line of each other to accept the locking tool pin

Tightening the spigot

 

The last step is arguably the most important step; making sure that the spigot is tight

I take the cylinder head and bolt the unit onto my milling table

I use my custom made tool to together the spigot in place using all my body weight as leverage

With the spigot having fresh threads and good thread engagement now we can feel confident that this is a permanent fix that should last a lifetime

 

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 Ride safe!