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tech week 6-71 Blower rebuild

Discussion in 'The Hokey Ass Message Board' started by Rob Kozak, Jun 2, 2009.

  1. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    I'll Add Pics later Today.

    6-71 Detroit Diesel Supercharger to 6-71 Blower

    First thing you need to do is pull it apart make sure there are no deep scratches in the case or rotors. (Minor ones are OK. This isn’t a competition blower it’s a street/strip blower at best.) When you remove the gear cover you need to try and mark the orientation of the gears to the rotors so to keep them in time (phase) when you reassemble. (A punch mark is best.) When you try and remove the gears the best way is to use two pullers and alternate the pressure on them so to pull them off together. (This assures you not to chip or warp the gears.) Also mark the gears left and right and relate them to the rotors after you’ve pulled them out. SAVE ALL THE ORIGINAL PARTS TILL RE-ASSEMBLED

    Take a look at your rotors. You will notice they are stamped one with a U and one with an L. This is for upper and lower. (Remember this thing hung on its side on the Detroit Diesel it came from.) This will be important during re-assembly. Check the rotor "sealing lip" surface (marked X) and the rotor lobe side surfaces (marked O) for galling, scoring, or deep grooves if it has them forget it the rotors can't be repaired the rotors are junk. Most people look at the outer edge of the rotor lobe, but this is not where wear is first noticeable (the rotors will usually hit each other before they rub on the case wall, marked by the "O") the outer lip (marked "X") or flat raised edge of the rotor should be at least .080-.090-inch tall.

    Next, throw away the stock endplates. Keep the front cover. (The one without the funny foreskin type lip.) You can reuse the stock plates them with some work, but if you call Don Hampton He can set you up with quality parts. Front and rear plates 105.00 each cast finish. (I found him to be eager to help and his parts the most reasonably priced.)

    The two extra support bars on the inlet side can come out because the added strength is no longer needed since it's no longer hanging sideways on a big diesel engine.


    OK, your blower is apart now you need to cut the tab off the one side. A hack saw would do but if you have a Bridgeport available use it. Then use a belt sander to reshape the case where you cut off the tab.


    Now clean and sand blast the case. If you see any bubbles or holes in the case especially around the boltholes for the endplates leave them alone and paint the case black. If you try to polish the case these will get worse and welding cast is a bitch. The same goes for your front cover.>>
    Assuming you are going to polish your blower now is the time to order you parts. (I’m not going to go through polishing but I will tell you it is a lot of work but it’s worth it.)

    You will need new bearings, seals and gaskets. Both ebay stores of dean671 blowers motorpartsfanaticseller carry complete kits with bearings, seals and gaskets for about $80.00 shipped. Not sure if you get the shaft savers with the motorpartsfanaticseller kit but they do include extra rotor pins. (The shaft savers press onto the rotor shafts giving you a clean smooth surface for your seal to ride on.)
    You will also need to add extra pins to the blower rotor shafts to assure they don’t twist on you. (The rotors are hollow and the shafts are just stubs pressed into each end.) Again go to ebay to dean 671 blowers, 18 bucks plus shipping.

    If by some chance you have used blower that’s already been converted and need new bearings you will need 2 6505-2RS bearings and 2 5205 zz bearings. The 6205’s are a sealed high- speed single roller bearing. They are easy to find. Tractor Supply has them for 8 bucks. Call your local bearing store for the 5205’s. These are about 20 bucks each. They’re a little pricey compared to the 6205’s but still reasonable compared to what the “blower companies” charge.

    As for the seals, the best price I’ve found is Rick Dean’s dean 671 blowers on ebay 35 clams for all for seals and shaft savers. If you need standard seals contact either of the two ebay stores I’ve mentioned. I tried to buy the seals locally and they’re hard to find almost always have to be “special ordered” and the price goes up. The Detroit Diesel seal number is 5192438. NAPA or your local bearing supplier will be able to cross the number for you. The shaft saver number is 5192439.

    You will also need 12 freeze plugs for the ends of the rotors. These need to be replaced because when you drill for your new pins the rotor fills with the shavings. They are 15/16 part number I use is PC39 deep cup. Napa has them for 70 cents each.

    You’ll also need to order a pressure relief valve and if you want an oil sight glass. Best price I found is from Good Vibrations around 18 dollars for each. They also sell a bolt and washer kit for 20 bucks that has all the hardware you need to put this blower back together.
    You will also need to replace the tri-cover on the front cover. This is a matter of preference, but I don’t like to buy these from the “blower companies because they all have their logos on them and this isn’t their blower it’s yours. There is a guy who puts up finned ones on ebay from time to time for 25 bucks, you’ll have to search. They are made from billet but I sandblasted mine then polished the fins. Looks real cool.
    Total for parts: (never pay more than $350. You can get them all day from a local Diesel scrap yard for that price.)

    Blower: my latest was $200.00
    End Plates: 2 @ $105.00 ea
    Bearings, Seals and Gaskets: $80.00
    Pins: $18.00
    Freeze Plugs: 12 @ $0.70ea
    Blot and Washer Kit: $19.95
    EZ-Slide $6.95
    Total: $543.30

    Add about $75 bucks for shipping and taxes depending where you live and where you get your parts.

    Now if you can find a good used “converted” blower for under $650.00 go for it but compared to a new “rebuilt” GMC unit DIY is the best option for guys not wanting to fork out a big hunk of cash all at once. Yes you still need a snout, manifold, crank hub and pulleys but that’s what the HAMB classifieds and ebay are for. It can be done for under $750.00 keeping your total less than the cost of a new “rebuilt” GMC 6-71.

    OK, back to the parts you have in front of you. Clean the parts with quality aluminum cleaner then tape off the rotor shafts and bead blast the rotors and the case and cover.

    Take a look inside the case to see if there are any scratches that you didn’t see before you cleaned it. If there are some small scratches in the case use some fine sandpaper in long strokes to smooth them out.

    Next you’ll need to square up your case. You want the ends parallel and flat. This can be done by your local machine shop or if you have the time you can do it yourself. Just get a flat plate of steel or aluminum and bolt two handles to it then glue some sandpaper to the plate and lap all four surfaces of the blower case. Make sure you clamp the case firmly to your work area. Keep this lapping plate to use on your manifold later on.

    First thing to do to the rotors after cleaning and blasting is to remove the freeze plugs. Best way I found is to punch a hole in the three on one end on the rotor big enough to shove a long 3/8 extension down then pound out the plug on the other side. Then put a socket on the extension and pound out the ones you made holes in.

    Assuming your parts came in you can now put the extra pins in your rotors. You’ll notice the stock rotors only have one pin on each end these are placed 1.00 in from the end. Turn the rotor and on an opposite female lobe place a center-punch mark .750 in from the edge of the rotor. You’ll need to drill them with a 19/64-drill bit. Then lightly tap the pins in place and head over to a press to press them in. They are a tapered pin, so the fit is tight. Be careful to support your rotor and use plenty of rags to cushion the aluminum. Place two pins in each rotor. You are putting these pins in to assure the shafts do not spin at the higher RPM you’ll be running. (Remember these things only ran 2,500-RPM Max on a diesel.)

    Now blow out all debris from inside the rotor and replace the freeze plugs. I use ½ inch 3/8 drive impact socket and my trusty B.F.H.

    After you have your pins and freeze plugs in stand the rotor on end and slide the shaft savers on. You’ll need a deep well socket and a hammer. Carefully tap the bushings onto the shaft.

    Next grab your new Hampton Blowers endplates (That’s if you listened to me earlier.), seals and bearings. Remember the 6205’s go in the rear endplate and the 5205’s go in the front. You may need to press them in but an appropriate sized socket and a B.F.H will do. When you do the rear plate make sure to get the bearing against the lip in the plate. Next install the seals, once again a press would be best but a seal installer or large socket and a B.F.H will do. After you install your bearings and seals in your front plate you will need to bolt on the retaining plates. You should have saved these from your teardown. (DO NOT oil or grease the seals! It will ruin them. They are designed to run “dry”.)

    At this point you can coat the inside of your case and your rotors with EZ-Slide. It’s graphite paint available at Tractor Supply for under 7 dollars. This is not as good as having them hard anodized but it helps fill in any imperfections. This coating will wear off eventually but the scratches and pits should stay filled.

    Now it’s time to put this huffer back together.Before we go any further I should mention that a blower could be accidentally put together backwards. Match up your mounting bolts with your manifold to make sure of the bolt pattern. The bolt pattern is offset.

    One important tool you’ll need is a long set of feeler gauges. You can buy feeler stock in various sizes from the MCS book. This might save you some money or ask a machinist friend to come over and bring them for you to borrow. This option usually is the cheapest. (About 20 bucks for the beer.)

    The dimensions shown alphabetically are as shown are
    "A" is Rotor to case lower clearance
    "B" is Rotor to case upper clearance
    "C" is Front cover to case pin (to adjust A and B dimensions)
    "D" are rotor balance holes explained earlier
    "E" is Rotor to rotor clearance (also known as the "C and "CC" dimensions found in all Detroit Diesel repair manuals) every different model of blower 3-71, 4-71 6-71 etc has different dimensions) these dimensions ARE used to set up the blower. We change the other dimensions to turn the blower into a compressor.

    We need to “clearance” the blower. To do this we set the clearances between the back of the rotors and the face of the rear plate, between the front of the rotors and the front plate, between the edge of the rotor tip and the case wall at the outlet port (bottom of blower) and less importantly between the rotor tip and the inlet port (top of the blower) The front and back clearances are set by machining the ends of the case and the rotors and the rotor to case clearance are changed by realigning the end plates on the case, and then drilling the end plate alignment dowel locations and inserting larger dowels.

    Also reset is the rotor-to-rotor clearance "C" and "CC" clearances. You cannot reset the rotors closer together or farther apart the goal is to set the timing of the helical rotors so that the clearance between any two-rotor lobes is the same. The timing of the rotors is controlled by the meshing of the drive gears The way you set the timing is by machining the back of the gears the "rule of thumb" is every .003 thousandths machined off the back of the gears changes rotor timing .001 thousandth). The factory used shims behind the gears to set the timing but you don't want shims on a gas engine blower.

    The most important clearances to inspect and hold are the rotor to case clearance at the bottom of the blower followed by the front and back rotor to end plate clearance then the rotor-to-rotor clearance.

    So now the magic numbers for the clearances are front rotor to case clearance is .007 to .008 thousandths. To get that machine the larger diameter outside dimension shoulder where it butts up to the bearing.

    The over length of the rotors are compared to the case. The rotors are then inserted into the front cover. The rotors are set into the case with large serrations on the splined end of both rotors at 3:00.

    The case is then placed horizontally over the rotors and then a depth mic is used to measure from the case down the rotors you want .023 to .24 thousandths. Subtracting the .007-.008 clearance at the front that gives you a .016 clearance at the back. Machine the back face of the rotor to obtain this dimension. If the rotor is short already which it shouldn't be but if it is or a mistake was made just machine the case to bring this dimension in tolerance.

    Then assemble the blower rotors in the end plates, and the end plates to the case together with screws (finger tight). Now it's time to get to the inside clearances.

    Install spacers in place of the gears on the rotor shafts with the gear retaining bolts torqued to spec. To torque them place a piece of wood in between the rotors to hold them while you tighten.


    Then use the feeler gages to check the bottom rotor to case clearance. You want .004 thousandths here. To obtain this dimension tap the end plates around until this dimension is in tolerance along the whole length of the bottom of the case on both rotors.

    The top rotor to case clearance will be determined by the bottom clearance, you really can't do a thing about it but it isn't that important.

    Next rotor to case clearance is set, on both rotors the actual dimension isn't critical just make them equal on both sides of the rotors to the case. To do this you have to tap the end plates side to side which usually upsets the bottom tolerance so then you just start over and over till all these dimensions are located. (This can take some time)

    After these dimensions are correct tighten the end plate screws.

    Next remove the gear retaining bolts, and the spacers and install the drive gears and tighten the retaining bolts. The mesh of the drive gears determines the timing of the rotors. Remember both gears must be installed at the same time.

    Then we check the "C" and "CC" clearance from the Detroit Service Manual. CC and C dimensions are where the top (CC) and bottom (C) of the male rotor inserted into the female lobe. The clearances are achieved by machining the back of the gears as stated above. DO NOT USE THE STOCK SHIMS IN THE FINAL ASSEMBLY!!!! They can be used to determine how much to cut your gears but that’s it!


    After all is well redrill the dowel pin holes in the case and install the oversize pins 5/16's of an inch diameter. Add your front cover and drive and you’re done. Don’t forget to put oil in the front cover.

    Disclaimer : While I was researching how to rebuild a 6-71 I came across several articles on the net and in print. Each one lacked something that the others had. Not a one went into complete A to Z detail. Here I compiled what I felt was the best of all of them plus my own experiences to come up, with what I feel is, a pretty good guide to your rebuild. (Some pictures and text are not my own. I did not take pics of mine when I started, plus some of them are better than mine. As for the text cut and paste is easier than re typing.)

    sources: Street Supercharging by: Pat Ganahl
    several 60's through 80's Hot Rod, PHR and Drag mags
    Last edited: Jun 2, 2009
  2. Awesome tech!

    One point, the stock plates can be reinforced and used. The week spot is the boss around the bearing. A steel ring can be pressed over it to reinforce it.
  3. 38FLATTIE
    Joined: Oct 26, 2008
    Posts: 4,349

    from Colorado

    Awesome tech, but I think a few pics for some of us that are not all that bright would help!
    hipster likes this.
  4. 85-percent
    Joined: Apr 5, 2005
    Posts: 328



    All I can say is thank you very much for your generosity with all this wonderful info!

    I appreciate your efforts immensely!

    -90% jimmy

  5. Jethro
    Joined: Mar 5, 2001
    Posts: 1,902


    Thank you! well written article!
  6. t-town-track-t
    Joined: Jan 11, 2006
    Posts: 884

    from Tulsa

    I can not wait to see the pics that go along with this, I have mine aparts and blasted, and am ready to order parts to re-assemble it. You have no idea how long I have been waiting for this tech!! I know where my vote is going for tech week.
  7. Awesome....Im just about to do the rebuold on my GMC 6-71.

    What timing !

    Also score my bits from Rick on Ebay. Great guy and very prompt in shipping the bits I needed..

    Also waiting for the pix !

  8. I should be able to apply this to a 4-71 as well, true statement?
  9. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    sorry i had to do it this way i couldn't add any pics to my original post.
  10. Dirty2
    Joined: Jun 13, 2004
    Posts: 8,903


    Great tech !
  11. Iceberg460
    Joined: Jun 6, 2007
    Posts: 880


    Fucking A! Got my vote
    hipster likes this.
  12. Shaggy
    Joined: Mar 6, 2003
    Posts: 5,207

    from Sultan, WA

    Just what i needed!!!
  13. Shaggy
    Joined: Mar 6, 2003
    Posts: 5,207

    from Sultan, WA

    Why cant X be welded up and turned? i have a little groove on my 6v-71, and that's what i was going to do
  14. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    It can be done but it can go real bad if it's not a good weld.
  15. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    Your're correct about reusing the stock plates but for the price hampton has them I felt it wasn't worth mentioning.
  16. Ruiner
    Joined: May 17, 2004
    Posts: 4,141


    Damn man, thanks...I'm going to be going through this over the winter, so this should come in REALLY handy...any before and after pics of the tab that needs to be milled off?
  17. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    Yes, this does apply to a 4-71 but the C and CC tolerances are different. I have them somewhere; I'll look.
  18. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    The before picis the fourth pic of the article.
    Just make it look like the other side.
  19. Ruiner
    Joined: May 17, 2004
    Posts: 4,141


    gotcha...what kinda work goes into salvaging the stock front and rear plates? I'm a machinist by trade so that stuff doesn't scare me... (sitting at work typing this as my mill is making chips behind me)...
    Nocero and hipster like this.
  20. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    If you look at your stock bearing plates you'll notice that the bearing boss has three screws that buldge outside of the circle. you need to slightly turn those down and press on a steel ring about 3/16 thick around it to support the bearing.

    Also there are oil return holes in the lower corner of the plate that needs to be blocked off. Some guys used to use a penny but you can find small freeze plugs and loctite them in.
  21. ago
    Joined: Oct 12, 2005
    Posts: 2,199

    from pgh. pa.

    What kind of seals need to be installed dry? (neoprene)

  22. skwurl
    Joined: Aug 25, 2008
    Posts: 1,620


    Where can you find blower end plates?
  23. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    read the entire article i tell you where
    hipster likes this.
  24. Don Hampton. Awesome Tech. I have missed a lot being gone for four months.
  25. That is a great Tech. Thanks for taking the time to put it in writing. The photo sure help as well. Its got my vote.
  26. willysfun41
    Joined: Sep 4, 2009
    Posts: 1


    Great post, alot of info that was not in my Street Supercharging book. I am currently rebuilding an Isky 6-71 due to an leaking oil seal. I am having trouble finding the rear bearings. They are a Hyatt 1205TA roller bearing-oil lubricated not sealed. I have cross referenced the bearings since Hyatt went out of business 30 years ago but the lead time is 3 weeks. Any suggestions would be greatly appreciated. I had emailed Deans Blowers from Ebay with no response.
  27. rhpope
    Joined: Oct 22, 2007
    Posts: 73


    The rear bearings are plain old 6205 single row ball bearings. Most anyone should have them in stock or you should be able to get them by mail order or Ebay in less than a week. You can choose from open, sheilded, or sealed. Sealed would be best since they are already fully lubricated.

    The front bearings are plain old open 5205 double row angular contact bearings and anyone should have them as above with the 6205's.

    Hope this helps,
    hipster likes this.
  28. yule16met
    Joined: Dec 30, 2007
    Posts: 625

    from Hudson, WI

    I did a writeup awhile ago that no one noticed... Yours has more in depth information. I have ALOT more pictures. Even pictures that show the steel rings for using the stock end plates. Mine is a late 60's iski setup.

    Explain why you are against the shims? I am going to play with the timing a little before I install my blower.
    Stevie G likes this.
  29. Garyr
    Joined: Sep 20, 2006
    Posts: 75


    very good tech, I've built blowers for about 40 years and have very little that I'd do differently. A word of warning about using shims under the gears, they will beat out and you'll lose the timing. rather than shim cut the opposite gear the like amount. e-mail me at [email protected] for further instruction or explination. Also 90% of the blowers I find have good clearances from the factory so I would check the clearances before heading out to the bridgeport and lathe. I usually tig weld up the endplates to strengthen them rather than use aftermarket ones. Again great job!
    Last edited: Oct 8, 2009
    Stevie G and hipster like this.

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