Topic: HM300 vs HM341 Exhausts

[GLRoberts99]

It's clear that everyone wants HM300 exhaust systems for their K0 / K1 rebuild, but how do you recognise a genuine HM300 system?

On my K1 project, number 1 exhaust is clearly marked HRM CB750 HM341 with light stamping. Exhausts 2 and 3 have HM300 stamped quite deeply and nothing else, and number 4 exhaust is badly pitted in the stamping area. All are badly corroded and should really be replaced, however, if the 2 & 3 exhausts are genuine HM300s then I'd like to invest in repairing them, but if they are replicas then I'd be better off spending money getting new replicas.

The questions I have are;

1. How can you tell if the exhaust is genuine?
2. Where should the numbers be stamped on the exhaust body?

TIA


Gareth

[34barab]

I think both stamps indicate OEM Honda pipes.  HRM CB750 HM341 indicates it went to a European market.  HM300 is from an earlier model, possibly K0.

[BenelliSEI]

HM300’s were original to K0 and  K1 (1970/71).

[Tom]

If you have a good number 2 pipe that’s the hard to find one, when the bikes are left on the side stand the small drain holes where the mufflers join the pipe do not face the ground and hold water and eventually rust out No2 is the hardest to find followed by the No1 pipe and then the NO 4 usually scraped then 3 is the most common good luck they are out there and you are half way there.

[Don R]

 I used to watch ebay several times a day and list by newest ads. I was able to find 3 decent pipes.

[GLRoberts99]

I've taken some photos of the numbering to show the difference between the HM300 stamping and the HM341. I'll have a look to see what it'll cost to restore the HM300s because it'd be nice to keep the original exhausts on the motor if I can.

[ekpent]

 The style of the baffle will tell you of you have a 300 or 341 if the pipes still have them. The 300 is a tube that can be wrapped and the 341 is just basically has a cap or "diffuser" at the end of the pipe.

[Stev-o]

I'll have a look to see what it'll cost to restore the HM300s

I recently had some Kawasaki pipes rechromed, I considered it a deal at $425 for three pipes.

Most shops wont chrome exhaust as they dont want to contaminate their tanks.

If your pipes have rust holes it may not be worth restoring them, most repairs that I've seen did not look good.

[caluser2000]

The '70s exhausts didn't last that long at all before showing signs of rust out from the inside.-3 to 5years.

If I owned a 4 piper with rusted OEM mufflers  I'd get an adapter made up to fit a single muffler each side or buy a good used or new after market 4 into two system or decent 4 into 1 system/headers that the muffle can be replaced  on, not sound like the hounds of hell increasing my deafness on a long rides, doesn't look like a drainage pipe and I can keep the center stand. And preferably stainless steel.

The reduced weight ioss an obvious benefit.

[Kevnz]

FYI, a respected Honda dealer and racer from the '60's and 70's told me his shop often replaced mufflers under warranty. Said the problem was leaded gas, when we moved to unleaded, problem went away.

[caluser2000]

FYI, a respected Honda dealer and racer from the '60's and 70's told me his shop often replaced mufflers under warranty. Said the problem was leaded gas, when we moved to unleaded, problem went away.

No it didn't. My brand new OEM 4 into 1 system I bought for my 1976 CB550F in 1990 were showing signs of rust rot at 5 years . Using unleaded fuel just increased the time a bit before you saw rusting the outside. Same happened on my brand mew 1981 CB650 Custom 4 into 4 mufflers. As well as the Japanese market CB750F2D muflers which I bought new in 1986 and made sure the exhaust drain hole were clear.

The bike would most likely be on it's second owner at 5 years.

These bikes were designed to run unleaded fuel from when they were first manufactured.

[bryanj]

Same as Honda knew that 750F2 valve guides wore out quick but only replaced them under warantee if they failed in first year. Low mileage riders had to pay for the repair when the smoke zgot bad

[caluser2000]

There's also the early V-4 valve clearance issue. That tool they came out with was just a big hoax. I read tech docs from the head mechanic at Honda in the US at the time and he was quite clear the "special" tool hid the fact that the tool just camouflaged the wear issue.

[Kevnz]

Mufflers rusting out in 5 years is a bit different from rusting out in 6 months, though.
I owned Jags in the 80's, and you would be replacing 1 muffler every WoF ( 6 months, there are 4 of them). Once unleaded came into use, mufflers seem to last forever. Have not replaced 1 in the last 20 years.

[Stev-o]

The '70s exhausts didn't last that long at all before showing signs of rust out from the inside.-3 to 5years.

Simply not true. I have multiple '70s original exhaust on my bikes including CB750, CB550F, CB450 & SS50.
 
And I recently sold two sets of CB550K exhausts, one was in mint cond and the other in VG cond with only 1 rust area, not all the way through.

And if you want to talk two-stroke bikes, they never rusted out! 
[have original exhaust on my H1, RD350, TS250 & Puch 50]

[caluser2000]

The '70s exhausts didn't last that long at all before showing signs of rust out from the inside.-3 to 5years.

Simply not true. I have multiple '70s original exhaust on my bikes including CB750, CB550F, CB450 & SS50.
 ! 
[have original exhaust on my H1, RD350, TS250 & Puch 50]

How often have they been riden and how many miles have been clocked up? When was the last time those motorcycles were actually riden?

Fish oil was used to increase there life a bit more. The drain holes get blocked and athe moister is held inside and rots them out.  You will also probably notice the the majority of the ear 4-stroke Hondas actually in regular use had the mufflers or the whole exhaust system replaced. England and a few other countries use salt in the winter. Luckily we mostly use gravel grit. Climate is also a factor if it is a dry desert or temperature controlled emviroment of course they will last longer.

I've been riding Honda 4-strokes for a loooong time and know a lot issue with a lot of models.

Original 2-stroke smufflers of that era tend to rust from the outside if left to sit for a long time without any attention.

[Don R]

 My 500 exhaust rotted out in 2 seasons but I rode in the cold on short trips.  Any new exhaust I get now gets coated on the inside and baked to cure it before it's ever installed.

[caluser2000]

My 500 exhaust rotted out in 2 seasons but I rode in the cold on short trips.  Any new exhaust I get now gets coated on the inside and baked to cure it before it's ever installed.

What do you coat it with Don R?

[andy750]

The '70s exhausts didn't last that long at all before showing signs of rust out from the inside.-3 to 5years.

Simply not true. I have multiple '70s original exhaust on my bikes including CB750, CB550F, CB450 & SS50.
 
[have original exhaust on my H1, RD350, TS250 & Puch 50]

+1. I have 2 CB750s - one with HM341s and one with HM300s. Had both bikes for 20 yrs with the exhausts - in that time they have been ridden well - the HM341 bike has done 64, 000 miles and the HM300 bike 36,000 miles. One HM300 had to have a hole patched at the very end with JB Weld and its held for years. The HM300s originally came from Germany. I ride from March to December here in the Northeast. I park them in the garage for winter but do nothing special with the exhausts.

 

[Don R]

 The local Honda shop put 2x4's along a wall and hung all of the stock take off 4-4 pipes on the rack, when the rack went all around the shop, they got worried about the weight and scrapped them all. My buddy hauled pickup loads of them away.

[grcamna2]

I'll have a look to see what it'll cost to restore the HM300s

I recently had some Kawasaki pipes rechromed, I considered it a deal at $425 for three pipes.

Most shops wont chrome exhaust as they dont want to contaminate their tanks.

If your pipes have rust holes it may not be worth restoring them, most repairs that I've seen did not look good.

Steve,the pipes complete with mufflers $425 ?? That's a Huge Deal ! 

[PeWe]

More pipes, Lotus Root variant
http://forums.sohc4.net/index.php/topic,166498.0.html

Car stock exhausts became much better when the steel were aluminized on both sides. Before that, end silencer rottened in a few years.


It's a pity that these expensive pipes have no rust protection.

[Stev-o]

The '70s exhausts didn't last that long at all before showing signs of rust out from the inside.-3 to 5years.

Simply not true. I have multiple '70s original exhaust on my bikes including CB750, CB550F, CB450 & SS50. 
[have original exhaust on my H1, RD350, TS250 & Puch 50]

How often have they been riden and how many miles have been clocked up? When was the last time those motorcycles were actually riden?

1967 CB450....21,180 miles, last ridden 3/26/21
1976 CB550....14,198 miles, last ridden 3/19/21
1974 CB750.....9,868 miles shown, NOM, last ridden 1/29/21
1972 SS50.......TMU.......last ridden 3/25/21
[I generally ride one of my bikes every Friday]

Any other questions?

[caluser2000]

Good for you.

I just love the passive aggressive types.

Do the ladies really agree that everything is bigger in Texas?

[Don R]

 I was always told short rides in cold weather cause condensation.  Your mileage may vary.

[caluser2000]

I was always told short rides in cold weather cause condensation.  Your mileage may vary.

It does. After a while the drain holes at the bottem of the mufflers get clogged and if not clear rot the mufflers from the inside. Are where there are sulfur (like volcanoes, natural steam pools and the like) and a lot of salt on the air it is worse.

[stayathomedadwithnokids]

New member here, 2nd owner since 1994: '71 CB750 K1
thought I'd pass on my purchase experience buying mint condition new old stock HM300 pipes, still sealed in plastic bags in Honda boxes.

6/9/13.  26,357 miles
Purchased new mint condition OEM 4-into-4 HM300 exhaust system for $2694.70 from JTMARKS.   Perfectly sealed in original wrapping/protection/and boxes.  Sold old 1972 HM341 exhaust system good but not perfect condition on eBay for $850, fyi.  Bought those 4 HM341 in 3/1994 from Fremont Cycle Salvage for $200..to replace the Sanderson drag pipes that came on the bike from the original owner.

Thought I'd share some pricing perspective.  100% happy with the HM300's on my K1.

[caluser2000]

New member here, 2nd owner since 1994: '71 CB750 K1
thought I'd pass on my purchase experience buying mint condition new old stock HM300 pipes, still sealed in plastic bags in Honda boxes.

6/9/13.  26,357 miles
Purchased new mint condition OEM 4-into-4 HM300 exhaust system for $2694.70 from JTMARKS.   Perfectly sealed in original wrapping/protection/and boxes.  Sold old 1972 HM341 exhaust system good but not perfect condition on eBay for $850, fyi.  Bought those 4 HM341 in 3/1994 from Fremont Cycle Salvage for $200..to replace the Sanderson drag pipes that came on the bike from the original owner.

Thought I'd share some pricing perspective.  100% happy with the HM300's on my K1.

Looking good.

Welcome to the Honda SOHC forums stayathomedadwithnokids.

[grcamna2]

New member here, 2nd owner since 1994: '71 CB750 K1
thought I'd pass on my purchase experience buying mint condition new old stock HM300 pipes, still sealed in plastic bags in Honda boxes.

6/9/13.  26,357 miles
Purchased new mint condition OEM 4-into-4 HM300 exhaust system for $2694.70 from JTMARKS.   Perfectly sealed in original wrapping/protection/and boxes.  Sold old 1972 HM341 exhaust system good but not perfect condition on eBay for $850, fyi.  Bought those 4 HM341 in 3/1994 from Fremont Cycle Salvage for $200..to replace the Sanderson drag pipes that came on the bike from the original owner.

Thought I'd share some pricing perspective.  100% happy with the HM300's on my K1.

Looking good.

Welcome to the Honda SOHC forums stayathomedadwithnokids.

Welcome

[Don R]

My 500 exhaust rotted out in 2 seasons but I rode in the cold on short trips.  Any new exhaust I get now gets coated on the inside and baked to cure it before it's ever installed.

What do you coat it with Don R?

This, https://www.eastwood.com/eastwood-hi-temp-internal-exhaust-coating-w-nozzle.html

 When you look inside to see if it's covered you spill silver paint on your shoes. Plenty of paint. Now I tape the ends and clean out the drain holes when done. New pipes only, I don't think it would stick in used ones.

[caluser2000]

My 500 exhaust rotted out in 2 seasons but I rode in the cold on short trips.  Any new exhaust I get now gets coated on the inside and baked to cure it before it's ever installed.

What do you coat it with Don R?

This, https://www.eastwood.com/eastwood-hi-temp-internal-exhaust-coating-w-nozzle.html

When you look inside to see if it's covered you spill silver paint on your shoes. Plenty of paint. Now I tape the ends and clean out the drain holes when done. New pipes only, I don't think it would stick in used ones.

Yeah cleaning out used pipes would be a huge pain in the rectum. And right messy as well. Putting fish oil from new was a thing back in the olden days to extend steel made muffler life. Some folk used deisol to coat them as well.

[BenelliSEI]

Regarding the drain holes (in the mufflers, bottom, right where they join the head pipe):
On all of my older, still in nice shape, pipes I use a sharp 1/8” drill to keep them clear. I suspect that when new these holes only went through the outer shell (the muffler overlaps the thick wall head pipe at this junction).. I’m not shy. I drill right through everything. Each Fall I make sure I poke up through the hole and keep them clear. Give every pipe a good shot of WD40 (using the straw, in the hole and down the baffle). Seems to help.

I also never use the old bikes/pipes for short rides. If it gets used, it’s a decent length ride, with lots of twist. Blow out the crap, and get them hot!

[KasperK/Denmark]

Nothing rots an exhaust more and quicker than short trips. The exhausts rot up from the inside due to condensation. Long trips and long stretches heatens up the pipes and prevents condense.

[PeWe]

Living close to salt sea will help the rust too?
Some guys here lives where the climate is very humid.

I remember how much more humid it was in South Carolina (Greenville/Spartanburg) end of October, beginning of November when I was there for 2 weeks 2001.
This compared to central Sweden 100km west of Stockholm.

I suppose that Denmark gets humid air from Northern Sea.

[caluser2000]

Living close to salt sea will help the rust too?
Some guys here lives where the climate is very humid.

I remember how much more humid it was in South Carolina (Greenville/Spartanburg) end of October, beginning of November when I was there for 2 weeks 2001.
This compared to central Sweden 100km west of Stockholm.

I suppose that Denmark gets humid air from Northern Sea.

Yes salt does speed up corrosion. Some cuontries use salt on their roads during when ice gets formed on the road.

[ekpent]

Mouse piss is the worst !! Plug those holes when in storage. Always fun to see what shoots out exhaust pipes,especially 4 to 1's when you start up old dormant project bikes for the first time.