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Stock Exhaust System Diameters for 1974 L18 620

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I was under the impression they were 1.5" for the entire system.


I found an exhaust shop to bend up a stainless system, but they thought the exhaust pipe was 1.75 inches.


I went to carpartsmanual and found the aft clamp (20643-B5000) for the exhaust pipe and found a reference that it is a 45mm clamp:  (45mm = 1.75")



There is a tail pipe clamp that is not a u-clamp (20641-B5000) which I cannot find any info on.  The tailpipe and muffler appear to be a single unit so there is no proper clamp between the muffler and tailpipe.


I suspect the tailpipe is 1.5 inches.


If I had a Walker Exhaust Diagram for the 74 620, I could probably ascertain the sizes from the clamps called out.


I need this info to proceed with my stainless exhaust fabrication.


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My stainless exhaust job is now waiting on this information and have my rotted off tailpipe attached to my SS muffler with a rolled up peaches in heavy syrup steel can and 2 hose clamps.


Trying to determine diameters from part numbers is difficult since specs of parts are usually not given.  So I figure only... hey wait a minute, the folks that make the exhaust flanges might know...  well, you get the idea, I believe my only course is to find someone who actually knows...


I think I've listed on all relevant forums (datsunminitruckin, nicoclub, and datsun1200) but only the "Datsun 620 Owners" Facebook group. 


Looking for suggestions of where else to look, but for now, I'm going to scour FB for Datsun groups to post on.


If I find the information, I will post it.



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I have to laugh.  There is one forum that you have to prove you are a human and they ask "What will fit in the trunk of a 510" and there is this list to choose from (I know I'm forgetting a few):


Elephant, Groceries, Parts, Uranium, Lead Bricks, Gravel, Fuel, ...?


Clearly, the ONLY one which won't fit is an Elephant, but that is a Wrong Answer.  



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Some exhaust systems use a smaller outlet from the 'muffler' and tail pipe so a clamp from back there may not be right.


Does it really matter if you have a 1.75" system but it came with 1.5"? Make it 2" if you want.

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I know I'm being completely OCD about this, but I just want this 620 to be as original as possible.  


I was all ready to pull the trigger on a 1.5 system but what is messing with me is the discovery that the parts manual calls out a 45mm (1.75") clamp at the muffler.  (at least that is what a Russian site says)


I wonder if a Nissan dealer may have more information?

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Better for me would be more mpg and not more HP.


I had a C10 w/396 and put a Mile-a-More Crane cam in it and got over 13mpg around town.  Less cam = more mpg.


So I'm thinking bigger exhaust is equivalent to bigger cam OR bigger displacement engine - both of which hurt fuel economy.


But these are only simple theories applied to a complex problem.  I do know that exhaust tube diameter vs hp tables show that 1.5 is good for 75 hp which is exactly what I have.


Another factor is that a given size exhaust matches with a specific rpm range.  Bigger exhaust, higher rpm matching, and usually loss of lower end torque.  So again, I'm not eager to give up my "grunting around" torque - sometimes I shift from 1 to 4 or pull out from slow roll in 2nd.


Listing of my shop interviews:


*) "Can't quote a price because studs break..."  - FAIL

*) Wants to weld SS to crappy OE steel flange - FAIL

*)  Uses steel MIG wire for SS - FAIL

*)  Guy is a perfectionist, uses "mix" wire, when asked for guestimate he asked "are you going to get the tubing or do you want me to?" - PASS  (this guy is so busy, I think he would prefer I get the tubing to save him doing dog work.  his rate is $70/hr and still quoted the lowest price - like over 25% lower that these other journeymen)


COMMENT:  If it wasn't for everyone doing 620 customs, I'll bet there would be someone making stock exhausts.  Of course Datsun screwed the pooch having both a twin and single port exhaust manifold outlet for the 620's.  They sure wanted the 4-2-1 setup.

Edited by Cardinal Grammeter
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If looking for more mileage it's unlikely you will be running at full throttle. Exhaust size is more about running the minimum you can get away with so it costs less to make. Running a larger diameter pipe will reduce resistance to flow (back pressure) and this will upset the emptying of the cylinder of residual exhaust. This can be accounted for with tuning but often isn't. On an EFI car it's done automatically. More efficient means less work pumping air and more mileage and HP. If just going for more HP then mileage probably will suffer. One and a half inch is probably fine for an L16 if strictly limited to stretching the mileage. Once you start stepping down on the throttle, the exhaust pipe becomes crowded. A one and  three quarters pipe is 36% larger, more than enough for an L16

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I know that reversion really kills fuel economy and makes an engine pig rich when that happens.  It is common at WOT low rpm - as rpm increases, the "reversion cloud" gets sucked into the intake thus eliminating the extra fuel being introduced at the venturies during reverse flow.


At low rpm and low throttle, I'm not so sure what happens.


When cruising, the engine is at low throttle settings and producing nowhere near its max horsepower rating.  So for that condition, the 1.5" exhaust is quite large.  


I also suspect that the higher velocities of the 1.5 might be more effective during overlap during cruise.  But "more effective" might be bad for fuel economy.  Too much overlap scavenging might be possible and might harm FE - note the two IF's in that statement.


Might post on economodder which I joined many years ago when I was running a 92 Geo Metro and an 87 Chevy Turbo Sprint (which I totaled, R.I.P., murdered one of the last Unicorns.)

Edited by Cardinal Grammeter
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8 hours ago, Cardinal Grammeter said:



When cruising, the engine is at low throttle settings and producing nowhere near its max horsepower rating.  So for that condition, the 1.5" exhaust is quite large.  




Yes, the same is true of intake ports and valves.

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  • 2 weeks later...

The size isn't the problem.  I found 1.5 304 16ga at Summit w/free shipping since over $100.


At first, I didn't realize there were issues bending thin wall (16ga) stainless - especially 2.5 and up.  So I have no idea what these shops are doing since they don't have mandrel benders and I doubt of they have that ribbed die that lets you bend 4" on a normal bender.  

The larger the tubing the more likely it will wrinkle.  The 1.5 16ga will probably bend, but none of the shops know for sure.  No one will do the job if it wrinkles.  I've seen you tubes of 2" SS wrinkles and they are not that bad.  1.5" should not be a problem since no big angle bends in the exhaust pipe.


Not knowing if 1.5" SS bends means these shops really are not experts.  And turning down a "vehicle inspection class job" because of the possibility of a tiny wrinkle doesn't make any sense.  


The shops that will do the job are quoting prices too low for welded mandrel pieced systems.  


PITA #1:  I'm going to have to contact the full list again and specifically find out how they plan to make the bends.


And the one shop that looked like the goto place has had some kind of family emergency and I haven't been able to get a response from them in 10 days.  (What ELSE can go wrong????)


PITA #2:  I'm now looking into 12ga 1.5" steel tube to the SS muffler and a simple SS tailpipe.  I'll air dry Cerakote the exhaust pipe if I go this route.


ALSO found 2 OEM exhaust pipes (!)  One was brand new but laid out in the weather for 20 years, another is very rusty.  People universally underestimate rust damage and considering shipping from CA to PA, it's a lot of work for something that may only last a couple years.


THE SMART THING might be to change the exhaust manifold to the single outlet.  Then any shop could make exhaust pipe cheaply - for the single outlet, I'd have no issue with aluminized steel.  This whole stainless thing is because of the y-pipe fabrication cost.  Changing the manifold will take Time and Money, so maybe have a stainless (I already have the SS flange) stubby y-pipe made with a single outlet flange.  What is really annoying is that I could fab the stubby SS Y but I'm so jammed with the car crash and injuries and backlogs of chores, I really need to pay someone to do it.

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Here is a bending demonstration comparing Compression bending w/ and w/o the Huth Control Wrinkling Die, but check out the 2" pipe comparison @ 2:22 time:



all bends to "100 and 125 degrees" according to the narrative but you can see what they did


2"OD tube:

.065 wall aluminized steel just fine

.049 wall SS had just a few wrinkles in it (@ 2:22 in the video) 


Why .049?  Because .065 would NOT wrinkle at the 100* bend.

The .049 would not wrinkle up to some lower bend angle - that is not established.


But even so, the amount of .049" wrinkling is more than acceptable for maintenance and inspection work.  Those kinks are strictly cosmetic and would have negligible effect on system performance.


And that was for 2" OD.


Increasing from .049 to .065 would kink less and bend more.


decreasing to 1.75" OD would kink even less and bend even more


decreasing to 1.5" OD would kink still less and bend still more.


From this video it is clear that 1.5"OD .065" wall 304 can be bent for just about any exhaust application and probably bend to the max compression bender limit of about 135 degrees.


...but what do I know....


NOTE:  These Wrinkle Die's would allow a shop to bend BIG SS tubing and not have to weld fab using mandrel bends.  If I was in business I would get this tech ASAP.

Edited by Cardinal Grammeter
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You want to keep as original as possible yet are using SS pipe and thinking about replacing the exhaust manifold to single to get rid of the dual down pipe and joining them. The L20B is a single down pipe but is 1 3/4" or 1 7/8" I think. Take the path of least resistance. As everything is being replaced anyway, you can use what ever pipe size is available in soft steel and match a muffler to it. Cheaper, easier to bend, done.

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I never liked the L20B manifold swap idea, but it was on the table.  I was focusing on the time and money spent just to change a perfectly good manifold - the sort of thing I'm really against.  While it would be possible to fab a 1.5" system on that LB manifold, the 4-1 config would be different than the 4-2-1 of the l18 and I think there would be a loss of low end torque.  So the manifold swap is really not an option.  Too many negatives - the only positive is eliminating the Y.


It's no so much I want to keep the system original, it's really I don't want to modify it.  It's a stock, daily driver, work truck, and I want it to stay that way.


Regarding stainless, for a little extra money it can save a lot of money in the long run because the primary expense in a custom exhaust is the labor, not the materials.  SS should be the last exhaust system I put on this truck.  NOTE:  I believe I can order 1.5 12ga in steel which is .109 wall.  That should last far better than 16ga.  And with a SS muffler and tailpipe, the whole system should have good life.



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3 hours ago, datzenmike said:

It'll outlive us both.


LOL, that's funny but I'm seriously  taking that into consideration.  I'm 68 and have really started "slowing down" since 65.  So the writing is on the wall.  As long as I'm driving, I plan to keep the 620 as my 2nd vehicle.  ...and I never want to deal with exhaust again after this job.    

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Ordered the 1.5 304 tubes from Summit.  There was an issue that although "free" shipping over $100, they still wanted to charge for Oversize on the tubing.  However, Stainless Works which is the manufacturer of that tubing, ships for free, so Summit matched that.  Cost me 1 hour on the phone to save $40.  $40/hr is not a bad rate (!)  haha



The Walker SS 22266 was 409 and the one on the truck maybe 3-4 years old was showing rust on the case.  I quickly gave up trying to find a 1.5" 304 muffler because it was hard enough finding a 1.5" muffler and decided since the 22266 is only $30, just buy another one for this job.  So the exhaust will be 100% new.



I found a shop that said $250-300 for the labor and I promised them I would loosen the manifold stud nuts.  Mike I like you idea that I'll be the one to break my studs if it comes to that.  Will be interesting since there are BRASS NUTS on the studs - only place I ever saw that was on Studebakers which did that from the factory. 


Oh, there is one other shop that I have not contacted yet that is still in the running, but unless things go swimmingly well with this other shop, I'm going with the shop I found.


BRASS NUTS:  (Oh yeah?  My truck is BadAss, it has Brass Nuts.  And 3 of them.  OK OK, I quit)

I'm thinking heat and paraffin.  I seem to recall that when removing brass nuts, their threads can pull out and stay on the studs.  Clearly the nuts should be room temp when trying to break free.  Would it be too much to hope for that anti-seize was used when they were installed?  (Is there a possibility that they are the original nuts???  



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The whole reason brass nuts are used on exhaust is so that they do not rust to the stud, so I would bet they come right off. I've never had a problem breaking them free. As a matter of fact, brass nuts are notorious for backing themselves off. I tend to use stake nuts on exhaust studs to keep them tight.

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I sure hope you're right but I see you are in CA and not the Rust Belt. 


I think it's the salt:  I know that with cars from WV where they do not use salt, you can remove brake line nuts like they were new even when severely rusted - in Pittsburgh where they use salt, rusted brake lines could never be R&R'd for a wheel cylinder replacement.  


The combo Studebaker used did not always work as intended.  I believe what happened was the stud still rusted and the rust would eat up the clearance between nut and stud and then begin to further expand "locking" the nut.  I do remember the studs pulling the threads out of the nuts.  My solution in early 70's was Never Seez and steel nuts after a friend of mine turned me on to the product - he used it when maintenancing water treatment plants and raved about it.


We'll know one way or another - I plan to do the job first of October when I can tolerate some downtime if things go sideways.



Edited by Cardinal Grammeter
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