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Thread: 1979 DT400 ? build

  1. #61
    Registered User ls400x's Avatar
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    I lowered the bottom edge of the inlet port to bring back the duration to roughly the same as standard including the piston skirt windows. The duration is probably a little bit less, I'll have a look through my notes. I'm quite sure the inlet port is fully covered at BDC with the standard piston and cylinder. I'll double check on my spare engine.

    Edit: This from earlier in the thread - (inlet port) duration 296 degrees with the standard window piston or 260 degrees with the MX400 wiseco piston.

    This was measured inclusive of the piston windows so the inlet port is fully blocked with either piston type.
    Last edited by ls400x; 18-01-16 at 04:37 PM.

  2. #62
    Registered User ls400x's Avatar
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    DT being used in anger, doing what it does, crushing humans. Will return with a better clutch.












    Last edited by ls400x; 10-04-16 at 06:07 PM.

  3. #63
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    Great shots, wtf is that in the last picture? A helmet?

  4. #64
    Registered User MVLUV's Avatar
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    Quote Originally Posted by Rube View Post
    Great shots, wtf is that in the last picture? A helmet?
    I would say yes with a person connected to it.

  5. #65
    Registered User ls400x's Avatar
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    not a scratch (on my bike)

  6. #66
    needs boost... morerevsm3's Avatar
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    awesome!
    E36 M3 12.92 @ 108.64mph, N/A 3.0L

  7. #67
    Registered User ls400x's Avatar
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    Thanks '78 YZ400 cylinder and head arrived, bolts straight on. Development continues.

  8. #68
    Registered User Synthian's Avatar
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    '79 DT400! Was my first bike on my L's & on the road (had 250 stickers / plastics)

    This build takes me back to all the nights in the old mans shed, tinkering & rebuilding :thumbsup:

  9. #69
    Registered User ls400x's Avatar
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    ^ thanks man

    Death Trap 400 had it's only outing since last Action Park day today. It was running really nice and making a bunch of power, much faster than last time with the clutch slipping issue fixed. I probably got about 8 or so runs in before a stripped head bolt let me down and the rear tyre spun on the rim even though there was 2 rim locks. Heaps of fun was had and I was grinning like an idiot all day.

    Last edited by ls400x; 14-08-16 at 09:54 PM.

  10. #70
    Registered User ls400x's Avatar
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    New DT400D project.


  11. #71
    Registered User ls400x's Avatar
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    So to fill in the gaps on the $175 bike above I got given a free one that had been sitting under a house for 20+ years. It was in very bad shape though, everything very rusty. It did have an air box and fuel tank that I needed. Luckily the rusty bike had been fitted with fork boots and they had protected the fork chrome from any rust which was a relief as both other sets of forks I had were rusty in the working area.







    It looks to me like the $175 dollar one had not done too much work as there were no frame cracks and the engine cases hadn't worn away at the mounts. The free one though had a crack in the headstock gusset area and the engine was very beaten up.

  12. #72
    Registered User ls400x's Avatar
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    The first job a tackled was the forks. I had 3 sets of forks to make into one good set. Unfortunately the lowers on the free bike that had the good chrome had some stripped and stretched studs. I ended up using the good inners from the free bike and the lowers from the $175 bike as I thought that those lowers would have the least wear based on the condition of the rest of the bike. They were fully stripped, cleaned and re-sealed.



    Last edited by ls400x; 21-05-17 at 01:07 PM.

  13. #73
    Registered User ls400x's Avatar
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    Whilst the engine that came with the $175 bike had decent cases but it had some other problems. The piston was stuck in the cylinder and I couldn't get it to budge even after I tried to weaken it out. I ended up having to cut the rod with a hacksaw. I got the cases split and the crank looked a bit secondhand but the gearbox and clutch was very good.







    Last edited by ls400x; 18-05-17 at 10:14 PM.

  14. #74
    Registered User ls400x's Avatar
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    As luck would have it I came across another engine bottom end, freshly bored 0.5mm oversize cylinder and Wossner piston kit as well as a new reproduction wiring loom and a few boxes of assorted parts including a spare crank all for $300 at a swap meet. This was handy as the crank out of the original bike was too rusty to re-use. Unfortunately the spare crank I got from the swap was rusty in the seal area too so I had to split the cases on another engine just for the crank.






  15. #75
    Registered User ls400x's Avatar
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    This rod had about 1mm side to side play rocking without it touching the thrust washers but I could feel a tiny bit of radial play. Suzuki RM250 specified 3mm side to side rock is allowable as does Maico. It's probably borderline usable but I wanted to try doing my own crank so I bought a rod kit and 20t press. I decided to use the cases and gearbox that match the original bike so I stripped them completely and bead blasted them so they look less like pieces of coral. The outer case around the oil pump was badly corroded but it won't affect function of appearance once it's all together. I also picked up new main and out bearings as well as seals from the local bearing shop for about half what Partzilla sell OEM for and about a third of what the local Yamaha dealer sell them for. $690 invested excluding the press.



















    Edit: I just had a head scratching moment when I realised this isn't the correct rod for the DT400 which is the "498" Yamaha designation. This is the "5x5" Yamaha designation but should interchange no worries, just the OD of the little end is 23mm instead of 22mm, both are 18mm pin. I'll just have to remember to buy the right wrist pin bearing for the 5x5 rod if I even need to. Hopefully putting it here will remind me later.

    Main bearings are 6306 C3 and the trans output is 6305N or NR C3.
    Last edited by ls400x; 20-05-17 at 09:58 PM.

  16. #76
    Registered User ls400x's Avatar
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    My last 2 crank rebuilds that I had other people do have been bad experiences so i bought a press and had a crack at it myself. I haven't trued it yet but it's together at least and with the right side clearance.




  17. #77
    Registered User womblesti's Avatar
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    This thread brings back a few memories!

    I have a 79 YZ400F set up to do dirt track (lowered etc). Haven't ridden it for 15 years and it's currently in my brothers shed 200km away. Might drag it home next month.

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