Author Topic: motor rebuild thread (Read 34036 times)

kidder86 · « **on:** May 23, 2014, 04:09:50 PM »

Just had my trx blow a hole through the piston

so now I am tearing down the motor to rebuild it. My plans are to ask for help deciding what to do to this motor. I plan on leaving the cylinder stock and the rod stock. I don't race just play in the trails so large power isn't as big of a concern as reliability. I plan on getting the crank rebuilt with a long rod ( 87 +) and new crank bearings. I am a bit on the fence on what year of piston and cylinder to use as I have both styles. Check out the one pic, there seems like something is going on in the bottom of the crank case, any ideas?? Not sure why it leaned out and blew a hole in the piston. running a pj38 175 main dgh needle mid clip and 52 pilot jet. Feel free to chime in and give your opinion and ask questions.

kidder86 · « **Reply #1 on:** May 23, 2014, 04:44:06 PM »

it was there from the previous rebuild as the grind marks are not from me. should I epoxy over it or just leave it?

kb250r · « **Reply #2 on:** May 23, 2014, 05:58:11 PM »

Well that looks bad, I'm interested in hearing the technical comments on this thread...Thats one of the worst looking pistons ive ever seen...

thesmith87250r · « **Reply #3 on:** May 23, 2014, 06:27:42 PM »

im thinking blown head gasket. or detonation . need some info what compression and what fuel octane . its hard to tell for me any way . seen a head gasket issue eat a hole in a piston in the same spot as yours. and fried edges of piston from detonating. or bad electrical system

kidder86 · « **Reply #4 on:** May 23, 2014, 07:00:52 PM »

last compression check was 150psi was using regular gas 87 octane. new build is getting a 2002 cr250 ignition.

thesmith87250r · « **Reply #5 on:** May 23, 2014, 07:20:43 PM »

150 is pretty low I would have rebuilt it at that point. how was your coolant level .

kidder86 · « **Reply #6 on:** May 23, 2014, 08:14:06 PM »

coolant level was good, there was lots in the engine when I tore it down

thesmith87250r · « **Reply #7 on:** May 23, 2014, 09:47:17 PM »

was it breaking up and you kept riding? those pics look worse than when i seen them on my phone.could have been from low compression not burning the fuel and when it finally had enough in there and lit off it went boom. this is a wild guess

udontknowme · « **Reply #8 on:** May 23, 2014, 09:50:21 PM »

if the crank bearings slid right out of the bores then thats not good.

thesmith87250r · « **Reply #9 on:** May 23, 2014, 10:05:48 PM »

http://www.smellofdeath.com/lloydy/images/piston25.jpg BURNT-OUT BLOW-HOLE
This piston was overheated so badly that a hole melted through the crown and collapsed the ring grooves on the exhaust side. Normally the piston temperature is higher on the exhaust side so catastrophic problems will appear there first. There are several reasons for a failure like this, here are the most common; air-leak at the magneto side crankshaft seal, too lean carb jetting, too far advanced ignition timing or faulty igniter box, too hot of a spark plug range, too high of a compression ratio, too low octane fuel.

GO OVRIT · « **Reply #10 on:** May 23, 2014, 10:47:18 PM »

It looks like it was long over due for a rebuild. The color of the skirts looks like it's been blowing by the rings for a long time probably because of too much piston to wall clearance. Hopefully Neil or Jerry or one of the other really smart guys will chime in. Whatever the cause, you're in for a full rebuild. You may need new or repaired cases too. It looks like the crank bearing was spinning inside its bore.

kidder86 · « **Reply #11 on:** May 24, 2014, 11:13:59 AM »

the crank bearings didn't slide out of the cases, they were pretty difficult to get out. the magneto side was the easiest and the other side was a bit easier. I will take better pics of the cases where the bearings were sitting. the bearings were not seized, the magneto side was a bit chunks as there were piston bits in there.

kidder86 · « **Reply #12 on:** May 24, 2014, 11:35:46 AM »

I am hoping someone here can guide me into what year cylinder/head is the best to use with what year crank rod and piston manufacturer, ect. Is it better to get a new rod or get a new crank. I have a good local machine to do all the work.

kidder86 · « **Reply #13 on:** May 24, 2014, 11:55:04 AM »

here are some more pics I took today of the cases and cylinder. the 86 cylinder has a bit of porting and has a bore around 66.25mm but needs a bore and the 87 cylinder is stock and a bore of 66mm.

udontknowme · « **Reply #14 on:** May 24, 2014, 02:04:41 PM »

Quote from: kidder86;34719

the crank bearings didn't slide out of the cases, they were pretty difficult to get out. the magneto side was the easiest and the other side was a bit easier. I will take better pics of the cases where the bearings were sitting. the bearings were not seized, the magneto side was a bit chunks as there were piston bits in there.

ok so they werent still attached to the crankshaft when you pulled the cases apart ? thats good if they didnt stay attached to the crank

i dont want to tell you how to do it since people have their own methods but i dont recomend beating or pressing the bearings out of the bores, especially with unheated cases. pop the cases in the oven at 200* and they should be rather easy to remove. might have to go up to 250* but just play it by ear and see what happens. i like to start at 175-200 and if i have problems then i go up alittle higher if need be. ill usually have the new bearings in the freezer from the day before and ready to go in when the old ones are out so i dont have to keep reheating the cases