Author Topic: Flat head trouble  (Read 6420 times)

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Offline offroadwhatever8

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Flat head trouble
« on: January 25, 2007, 07:16:14 pm »
As you all know, i just bought a mini bike. It came with a 3 hp horizontal Briggs and Stratton engine. Well any ways, since I'm gonna complete restore the bike, i have to take the motor out and get it running good and cleaned up. So i get it out of the frame and take the head off. I notice that there were a bunch of dings in the head and on top of the piston. Almost looks like some one went in there with a hammer and a screw driver and start beating it all over the place. The cylinder wall has a couple scratches that are a little deep. My guess is that some one rode it with know air filter and a rock or something got in the carb and it to the engine. But this engine was running last night when i went out for a drive yesterday. It still has compression. Will i be alright running it like this? I can probably fix the head when it gets shaved. Is it possible to shave the top of a position?, wouldn't that give it more compression since the rings would be closer to the head? The only pic i got was this one



ill have to get better pics when i get home tomarrow because you cant really see any damage...

PS. since George for forbidded me to put my mini bike in the Other Interesting Builds Section  :eyebrow: :mad2: :hmmm: :grumpy: ::censored:: >:(  :roll:

....You can just visit the build at http://www.oldminibikes.com/forum/showthread.php?t=289

But back to the Briggs motor, any of you have any answers  :confused:

Offline septemberrein

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Re: Flat head trouble
« Reply #1 on: January 25, 2007, 07:23:15 pm »
I would pull it apart rest of the way . A broken ring would hammer the head like that .Seen quite a few times on snowmobiles and atvs .
Sarah

BSMRA

Offline Tom Fox

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Re: Flat head trouble
« Reply #2 on: January 25, 2007, 07:37:44 pm »
Can't tell much from the picture.......Just do a complete rebuild on the engine...Even with a unfocused shot of your engine you can see the score marks on the cyl. wall.

Get a new piston, rings, gaskets.....

How deep are the score marks? Maybe a hone job may get you going again...but I we can see them that good...it will need to be resurfaced via a bore job.

Either way...with score marks it could be a cracked ring...so disassembly would be needed for further evaluation.

By shaving a piston, you loose chamber volume that gives you less compression.

As for the HAMMERING marks...it may be detonation. If a "STONE" came in..it would of had to make it past the intake valve, causing possible valve damage as well! Maybe a wrong plug was installed sometime in the engines life as well. Could of been a little to long, or broke....so many possibilities!
Tom Fox       
Brookfield, CT

Offline cycloneracer

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Re: Flat head trouble
« Reply #3 on: January 25, 2007, 07:42:39 pm »
With your description and the pic.  That engine is shot!  Scratches in the cylinder will require a new piston and a bore job.  Here is a test.  Run your finger nail across the cylinder wall, if you "catch" your nail on the scratches they are huge.  Any catch at all, if you can feel the scratches it needs bored. Espically in that small of bore.  If you leave it this way and shave the head it will only make it have more blow by, and will puke oil.
So tear it down and rebuild it, it is your only good option if you want it to run right.

I've said it before and I'll say it again.  You can't race lawnmowers for free!

But in this case it is a bit different.  You can't work on engines for free!
Later
Paul
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Offline offroadwhatever8

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Re: Flat head trouble
« Reply #4 on: January 25, 2007, 07:44:58 pm »
thanks

I know about the pic, I'm going to get better ones tomarrow, i was in a hurray because my family was waiting to eat dinner. But remember, it was running last night and was still running good when i shut it off, so it must have been like that for a while.

Offline offroadwhatever8

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Re: Flat head trouble
« Reply #5 on: January 25, 2007, 07:45:51 pm »
sorry cyclone, you posted before i did. Do you think i could get alway with honing it?

Offline offroadwhatever8

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Re: Flat head trouble
« Reply #6 on: January 25, 2007, 07:47:45 pm »
This motor also didnt smoke either.... :confused:

Offline money89tractors

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Re: Flat head trouble
« Reply #7 on: January 25, 2007, 08:02:13 pm »
your really gunna want to fix the scratch. it more than likely has a broken ring. you MAY get by with honing, but from the looks, its probably going to need a bore.

personally, ive ran engine before with pretty good scratchs in the wall. not sayin its the right thing to do, but ive ran them with just fine results.

if your wanting to shave the head, then you will want to get the scratch fixed bacause it will start blowing by from the increased compression.

if the engine ran fine, and didnt smoke, then theres not much gettin by the rings. as for rings, you may want to go ahead and pull the old set and put new ones in just to make sure you dont get a worst scratch. that is, of course, if one of them is broken.

-Phil
..."I used that motto to keep going, keep pushing, keep digging to
find a way around, or through to find a hole, some kind of break that
would allow some space. Once something broke thru, I could be there to
take advantage. Then I would finish 7th instead of 10th." - Dubba G

Offline offroadwhatever8

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Re: Flat head trouble
« Reply #8 on: January 25, 2007, 08:41:03 pm »
I have a few more questions

Are the olny difference between 3.5 and 3hp's the carb jetting. Also, where can i get a 10 or 20 piston for it?

Offline money89tractors

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..."I used that motto to keep going, keep pushing, keep digging to
find a way around, or through to find a hole, some kind of break that
would allow some space. Once something broke thru, I could be there to
take advantage. Then I would finish 7th instead of 10th." - Dubba G

Offline offroadwhatever8

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Re: Flat head trouble
« Reply #10 on: January 25, 2007, 08:55:47 pm »
Thanks Phil  :+1:

Offline cycloneracer

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Re: Flat head trouble
« Reply #11 on: January 26, 2007, 11:02:15 am »
The only problem with hitting it with a hone is you make they cylinder bigger.  The rings and the piston are going to stay the same.  This will increase the end gap in the rings and give blow by, also if the side clearance on the piston is to big the piston will flop around in the bore, and that will rob power.  Forgot to ask, how bad is the ring ridge?  If you can feel it, a bore is your best option.  That will tell you how wore your engine is.  Just because it don't smoke don't mean it is shot.  Remember that piston is small so everything else needs to be taken in that proportion.  A small amount of oil won't smoke that much.
So if it was me, I'd rebuild it or look for something else.  Or you can just run it till it blows, but I would not shave the head, it will only compound your problem.
Later
Paul
Paul Krueger

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eddiehenjr

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Re: Flat head trouble
« Reply #12 on: January 26, 2007, 05:44:02 pm »
If you shave the piston, won't that give less compression?  I know a guy that had his motor seize up because he left the spark plug out for a while so he stuck a screwdriver end in the plug hole and beat it to free it up :tard: and he got it running.

Dude, bore it .50 over, shave the head, run it on alcohol.  Briggs 3.5hp + alcohol = :burnout:

eddiehenjr

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Re: Flat head trouble
« Reply #13 on: January 26, 2007, 05:45:20 pm »
Oh yeah and aluminum rod :twothumbsup:

Offline offroadwhatever8

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Re: Flat head trouble
« Reply #14 on: January 26, 2007, 07:35:31 pm »








Got the pics guys  :D