Topic: Honda CB750 HM341 exhaust

[kyle750]

I took HondaMan's advice and decided to look for stainless steel bolts to replace the old, rusted exhaust collar bolts. At my local hardware mom and pop store they had 30mm and 35mm stainless steel.  The originals at 32mm were extremely long and I decided to go with the 30mm and still left about 3mm of length outside the collar (before tightening down).  Best of all the price was only 40 baht for all 4 bolts and washers or $1.30 for the 4.  Not $20 plus shipping costs plus import duties plus VAT Tax plus waiting 1 month.   

Thank You HondaMan!

Late Edit:  My  collar assembly did not have any nuts on the other side of the collar (bolt and washer only) and the bolt attaches to the opposite side of the collar that is internally threaded.    I am assuming this is right and no additional outer nut was used?  Please confirm if this is correct??  Thanks!

[kyle750]

For those wanting concourse condition.  Seller's price is actually the Best price that I could find on Ebay and also original vintage Honda.    Other sellers have generic reproduction 8 x 32 bolts priced at $10 each 

https://www.ebay.com/itm/232727025585



 

[newday777]

Late Edit:  My  collar assembly did not have any nuts on the other side of the collar (bolt and washer only) and the bolt attaches to the opposite side of the collar that is internally threaded.    I am assuming this is right and no additional outer nut was used?  Please confirm if this is correct??  Thanks!

Just the bolt only. No washer  no nut.

[MauiK3]

Hi Temp anti seize on the threads if you can get some. Stainless likes anti seize.

Something like one of these:

https://www.amazon.com/s?k=high+temperature+anti+seize&crid=K2FOGO3TRTOR&sprefix=high+temperature+an%2Caps%2C280&ref=nb_sb_ss_p13n-expert-pd-ops-ranker_1_19

[newday777]

This is good antiseize

[kyle750]

Many thanks for all the replies.  Bolt only (no washer and no nut)  I totally forgot about the need for anti-seize when installing stainless steel bolts. Ready to install but I'm still waiting for Authentic Autosol ordered from the Official Distributor in Thailand to arrive so I can lightly polish the chrome on the pipes.

I bought this anti-seize a while ago but I'm not sure if it is any good? Also please notice the extra threaded length of the bolt that extends outside the collar.  These are shorter 30mm bolts and I don't know why Honda decided to use even longer 32mm length??

[kyle750]

Authentic Autosol delivered today 

Lightly polishing the pipes.  Outside of each of the 4 pipes is nice and shiny but the inside toward the frame there is considerable discoloration and oxidation to the chrome.  This photo was taken after only a few minutes of light polishing with a soft towel and small amount of Autosol. The transformation is immediately visible and remarkable.   

[kyle750]

Inspecting the drain holes carefully   They are open but a lot of thick black grime.    I can see how oil and dirt from the road can quickly and easily clog the drain holes and then moisture and condensation will become trapped inside the exhaust pipes and then RUST. I now fully understand why cleaning the drain holes should become a regular periodic check and part of a routine maintenance schedule and also should be included in the Honda owner's manual.

Late Edit:  I took bright white cotton swabs dipped in powerful degreaser into the drain holes and they came back out oily black with thick gunk. 

[MauiK3]

Also, when the bike is together, avoid starting a cold engine and only running it  a few minutes, be sure to get it hot to dry out the pipes.

[kyle750]

A quick google search explains the difference in quality of plating between inside and outside:

The chrome quality on the original Honda CB750 4-into-4 exhaust is generally not the same on the inside as it is on the outside.

Manufacturing processes for these vintage exhausts prioritized the highly visible outer surfaces. While the entire pipe was plated, the "show side" (facing outward) received a higher level of polishing and a more thorough plating process to ensure a deep, mirror-like finish.

Key differences typically include:

Plating Thickness and Finish: The inward-facing side often has a thinner layer of chrome and may lack the mirror-like luster found on the outside. This side was sometimes left with a "dull" or "satin" appearance because it would be hidden by the frame and engine.

Surface Preparation: The outer surfaces were meticulously polished before plating to remove imperfections. The inward surfaces often received less preparation, which can make them more susceptible to corrosion over time.

Because of this difference in protection, the inward-facing side of the mufflers is frequently where rust first begins to take hold on older systems.

[BenelliSEI]

You’ll likely find the drain holes only drain the sheet metal that is the muffler casing. They do not go through the main head pipe that’s inside. On older pipes, I use an 1/8” bit and go right through it all. Allows a tiny amount of exhaust blow through that helps keep the hole clear.

[kyle750]

You’ll likely find the drain holes only drain the sheet metal that is the muffler casing. They do not go through the main head pipe that’s inside. On older pipes, I use an 1/8” bit and go right through it all. Allows a tiny amount of exhaust blow through that helps keep the hole clear.

An intriguing solution.  Thank You Sir!  My understanding of how an exhaust/muffler system actually works is very limited.  I would think any change (even minor) would disrupt the internal pressure, air flow, and even the sound of the exhaust coming out the end of the pipes?

Upper exhaust has 2 drain holes
Lower exhaust has 1 drain hole

[kyle750]

Does anyone have a diagram of the internal baffling/cross section of a typical Honda CB750K 4-into-4 muffler.  According to a quick search:

Starting with later K0/K1 units Honda added welded internal baffle plates inside the muffler body. The common HM341 mufflers from this era typically contained two or three internal baffles through the chamber to meet stricter noise regulations. These models often featured a smaller diffuser rather than a full-length removable baffle.

As the early HM300 exhaust were less restrictive than the later more restrictive HM341 did they have less or different internal baffles?