Author Topic: 350/420 teardown part2  (Read 2310 times)

nuke em

  • Sr. Member
  • ****
  • Posts: 369
  • Karma: 0
    • View Profile
350/420 teardown part2
« on: November 09, 2006, 04:51:28 PM »
I finally got around to pulling apart my engine this morning after it lost compression a few weeks back limping it back to the pits at the Deer Field race grounds. I knocked a tire off the bead around mile 6 of an 8 mile race.

The piston top looked good and rich with the small wash areas to the intake side. Plug also looked good. I never noticed my EGT getting out of the normal running range. I did find 2 loose/backed out reed bolts and alot of carb fuel leakage making a big mess all over the bottom end. The track was very rough with lots of jumps, maybe that caused the overflows to splash everywhere. The base gasket was still shiney with everything above and below very dirty. I may have got a base gasket leak. I guess I should have done a leakdown test before opening it up, but thats too late now. I'll take some better close up pictures later this weekend.

I'll know more about the jug once I get it cleaned up too. It looks like a muric acid and hone job should clean it back up. I'll mic the bore for out of round later too.

Gary

PilotHawK

  • Hero Member
  • *****
  • Posts: 616
  • Karma: 0
    • View Profile
350/420 teardown part2
« Reply #1 on: November 10, 2006, 08:17:55 PM »
Well it definately got hot enough to melt and splatter in the exhaust port. Not enough squish band?

Ozpilot

  • Jr. Member
  • **
  • Posts: 92
  • Karma: 0
    • View Profile
350/420 teardown part2
« Reply #2 on: November 11, 2006, 04:25:12 PM »
It may be that two things went wrong (although one could have led to the other).  The way the top corner of the piston has melted looks (to me) like it must have been done by hot gasses.  I had pistons melt in a Drak (600 reed engine) that way.  I have not worked out why it happened for sure but I took one of the pistons to the guy tbat runs the Crank Shop in VT and he said that it was definitely detonation.  He was very certain of that.  I suspect the guy before me raised the compression and didn't tell me (when I rebuid I will use a different base gasket and I won't use pump gas unless I can be sure the compression is OK for it).  It makes sense that it is hot exhaust gas because of the location of it and how it melted.  

It has happened to me twice (in two 600 reed engines) - the first time it just had no compression when I went to start it so it obviously happened on the last part of the last run before it was shut down.  The second time it happened on a long straight run - it just lost compression and quit.  I don't know much more about how it felt because I wasn't driving either time that it happened.

PilotHawK

  • Hero Member
  • *****
  • Posts: 616
  • Karma: 0
    • View Profile
350/420 teardown part2
« Reply #3 on: November 11, 2006, 04:42:21 PM »
Hey guys check this out. Scroll down the page a ways and you will see what I am talking about. Nuke'em, you've had had gasket issues from day one with this engine. This may be the problem.

http://www.apexleisure.co.uk/pistondiagnosis.htm

Ozpilot

  • Jr. Member
  • **
  • Posts: 92
  • Karma: 0
    • View Profile
350/420 teardown part2
« Reply #4 on: November 11, 2006, 07:04:26 PM »
Hawk, that's a great diagnosis page - excellent pictures.

Looking more closely, the 350/420 piston shows some chipping (to the left in the picture) whereas mine did not.  The head gasket problem seems to match up pretty much spot on.  Where the piston in the 420 looks burnt away rather than chipped, it may have started out chipped then burnt away.  It all happens real fast once it starts!  That would also match in with little or no high EGTs.  It would also match in with the fact that there looks to be very little aluminum in the exhaust port.  Mine was absolutely coated in aluninium but the 420 has less.  It may be that a lot of the material departed in chunks (from "chipping" and only a small amount burnt away.

nuke em

  • Sr. Member
  • ****
  • Posts: 369
  • Karma: 0
    • View Profile
350/420 teardown part2
« Reply #5 on: November 18, 2006, 11:27:25 AM »
Great stuff guys, Thanks. I am debating whether to just modify another YZ490 piston and still have the unbalanced piston issue or get crazy with it. I am considering boring the crank/rod pin hole to fit a cr500 rod to the crank. Make a base spacer plate to match the longer rod. That all sounds good in theory, but we all know it is never that easy.

PilotHawK

  • Hero Member
  • *****
  • Posts: 616
  • Karma: 0
    • View Profile
350/420 teardown part2
« Reply #6 on: November 18, 2006, 09:43:17 PM »
The only way to do anything like that would be to make sure you don't change the strok any when you change the pin out to the larger CR500 pin. Any change in stroke length will only create a ton of headaches, especially with that cylinder.

Without going back through all the 420 threads is there any way to modify the piston without creating the balance issues? Instead of cutting a huge eyebrow is it possible to "window" the piston like the pilot?

I'm sure that if you spend enough on this thing you'll get it where YOU want it :)