How to tell if an engine is good or not.

[nuclearchrist]

Engine 1:I have a engine with the front end torn apart. After riding with muffler loose, it quit running on me. Just by looking at it, I cant tell what's good on it or not. Can anyone tell me just by looking at the pictures?

Engine 2: I also have a second engine that I tore apart from a '09 Peace Sports scooter. I don't know any thing about it other than that the thing was ran with a rusty gas tank and the oil was gunked up inside.

Here are the links to the pictures.

Engine one. 50cc taotao imgur.com/gallery/FomWT

Engine 2. Peace power sports. 50cc imgur.com/gallery/UV9Ib

[KSR Moto]

Well, if I were you I would rebuild that 1st engine which has top end disasembled,due to exhaust leak it probably run too lean and overheated,but I would buy 47mm bbk because if you already have to replace top end, then why not upgrade it to bigger bore😉

For engine num 2, if the oil is really in that bad condition I wouldnt even repair anything on it since it will soon need crankcase bearings, and potentialy crankshaft may have some play in it too, along with new top end beause someone obviously run that thing without servicing till it died, and I know that it takes a lot of time to get oil that thick especially on a petrol engine

My advice(what I would do)
Rebuild that first engine with bbk(47mm because that bore really last it is the perfect ratio of power to reliability)
And as far as the second engine is concerned i would inspect it and see if there are any good parts which can be reused

Rusty gas tank can only gum up your carb, the problem is with that oil that was in😬

[nuclearchrist]

If I go put a bbk on that wngine, I'll have to upgrade the transmission and carb. I don't have the money at the moment for that.

[FrankenMech]

Keep them both around for pieces-parts.

[KSR Moto]

You dont need to upgrade transmission, just dont drive on wot all thr time,and as fas as carb is concerned just put a bigger jet

[stoneforth]

You could get a piston and ring kit and a small 3 stone hone to hone the cylinder. Not the best hone but it worked well for me , or you could get a new top end kit that's already honed.

Do a leakdown test on the valves while you're at it. They could probably use a clean and lap

[GrumpyUnk]

The first piston has some scoring, but is serviceable. You can still see the machining marks (going round the piston) from the factory. The cylinder does not look to have scoring that I can see.
The second piston also has some scoring, but is again usable. Ditto on the cylinder wall. From the pics I cannot see any scoring.
The piston rings on both seem to be free to move, and have not appreciably lost their tension, or they would not expand out so much, and would stay 'closer' to the piston.
Were I fiddling, I'd clean the piston, rings and cylinder with a solvent, and then soap and water using a scrub brush. Do NOT let them 'air dry' if you wash with soap & water. You must dry the rings & cylinder or they'll rust, and coat the bare iron bits with oil to prevent rusting.
Once cleaned you can tell if the cylinder wall & piston are more damaged than can be determined from these pictures.
I think that both could be put back together, have the valves looked at, and they'll run. I'd take the valves out, and inspect the looseness of the valve stem in the valve guide, and the condition of the valve seating area on the valve tulip and the seat. If they are not pitted or eroded, I'd use some fine valve grinding compount, and a manual valve tool with suction cup to clean up both seating areas until I had a nice grey all the way around both seats. Clean all ALL the compound out, lube the guide, and PBT. Re-assemble and set the clearance, and I bet they'd both run.
The second machine did not have the same level of service work as the first. The color of the metal indicates oil changes were left a bit too long. Given that the crankshaft has ball bearings, if it spins nicely, then it is likely undamaged. The color and deposits don't hurt anything, but you'll likely have new oil get 'dirty' looking pretty quickly. Or not. The only oil under pressure is the lube that feeds the rocker arms/cam followers, and if they move freely without any slop, they are find. Inspect the rubbing area of the followers and the cam lobes for any scoring. You may see a bit of wear, but if there is scoring, they were run w/o any lube or minimal lube, or run too hot.
Some used parts go together better than new parts, and can produce more power than new due to less internal friction. If the rings and valves seal, an old and new piston still displace the same volume, and should produce similar power less the friction difference. If the wrist pins fit nicely into the piston, and the connecting rod small end without play, and the big end of the connecting rod on the crankshaft journal does not have 'up and down' motion, it should be ok. If it will wobble, that's not good. If it rotates with 'rough spots', that's not good, and the bearings are likely on their way out.
If you don't have a lot of money, you can replace the bearings on the crank and rod, but it will take some effort. The bearings are not real expensive, and a gasket set is pretty low cost, so you can 'refurb' for not much money.
It is pretty surprising what engines will tolerate as far as havin worn parts, and still running pretty well. The main thing is non-failing bearings and decent sealing of valves & rings. They'll run with that and some lube in the sump.
tom

added:
 As far as using a hone or glaze breaker on the cylinder, there is a discussion topic about that indicating there is not much if any difference whether a cylinder is honed or not when replacing rings.  The ring moves (rotates) in the piston groove, and gets 'matched' to the cylinder wall.  If you just remove, clean  and re-assemble, you can free up stuck rings, and regain compression.  The rings should seal as  well or better(after being freed) without the need for any honing or glaze breaking.
 It is not absolutely necessary, more or less, is the opinion.  And, it leaves the surface the same as the rings are 'used to'.
tom

[cagiva4ever]

Those valves with the "mercedes" looking symbol, are apparently the worst ever 139qmb valve's. iirc very soft or poorly machined for "seating properly", etc valve seat issues... i have notes....

if you ever replace stock 49cc valves on any 139qmb 49cc head, into another stock 49cc valve's, look for Taiwanise made "VTC" symbol marked valves. most likley they are also counterfeit cloned by chinese...

[mrhooper]

Engine number 1 looks pretty clean. It seems like it has been ran a lot , the slight scoring on the piston looks ok and no scoring on the cylinders. Clean the piston ,cylinder , rocker arms and cam with mineral spirits. Make sure you blow out the bearings on the cam . Once clean check for major scores and damage if all is good you may be able to just hone and re ring.

[GrumpyUnk]

For an interesting(if so inclined) read about honing cylinders to remove glaze:

www.turbododge.com/forums/f11/f69/245224-cylinder-glaze-myth.html

I have not checked back in a while, but last I looked it was still controversial. If I could read a foreign language, I might ponder the instructions that come with sets of rings(as if). Seems even the markings on the two compression rings are inscrutable at times.
Vaguely, in general, if the cylinder wall is scored, the ring will not conform well, and there'll be leakage. The amount and depth of the score marks will be the determining factor.
Of note is to realize that piston rings rotate in their grooves when the piston is used, unless they are gummed in place by poor maintenance, lack of oil changes allowing deposits to build up. The cylinder wall in all 360 is 'familiar' to the piston ring, and it should be able to be re-installed and work as well or better than when removed. If the varnish and other deposits are cleaned from the piston & rings, they will function adequately as long as the cylinder is not damaged AND the rings have not lost tension due to overheating. I would not re-use old rings if I were installing any sort of 'enhanced' breathing, such as supercharger or turbo. In short, if they are clean, the lands and grooves are clean, and the cylinder wall intact(maybe even see the original diagonal hone marks), they should work ok if used.
tom