Thai Mechanic’s Fix for Head Gasket Oil Leak – Is it Correct?

[Paul1212]

About 10 months ago, my 2003 xr250 Baja started leaking oil at the head gasket (not a lot, but more than just a film of oil on the fins). Since I was due for general engine maintenance inspection and timing chain replacement, I asked my Thai mechanic (I live in Thailand) to see if he could fix the oil leak as well. He adjusted the valves, and put in a new timing chain, piston rings, valve stem seals, cylinder head gasket, spark plug, and also a metal cylinder related to the piston (not sure what it was, but it measured about 3 inches long and 1 inch in diameter). This stopped the leak for a couple of months, but then it returned, and got progressively worse. My mechanic checked the engine again, and decided that the distance between the top of the cylinder and the top of the cylinder head was too small - in other words, that the top of the cylinder had worn down. So he had a local lathe shop grind down the top of the cylinder head a little bit. There is a spec mentioned in the manual (I have both the Baja manual in Japanese and a manual for the xr250r in English) that refers to the maximum allowance for warpage on the top of the cylinder head (0.10 mm), but my mechanic took this to refer to the maximum distance between the top of the cylinder and the top of the cylinder head. When I explained to him that that spec was for warpage, he said that Thai mechanics he had talked with took that spec to refer to the maximum distance between the top of the cylinder and the top of the cylinder head! I’m not very knowledgeable about motorcycle engines, and didn’t realize his mistake until after the repair was made. This fix stopped the oil leak at the head gasket for a few weeks, and then it returned. So next, he had the local lathe shop make a new cylinder for the engine, so that the top of the cylinder would be a greater distance above the top of the cylinder head. He said that the only other alternative would be to buy an entire new Japanese-made engine for about $1500, so I let him make that repair. (It only cost $50 for the new cylinder, and another $20 for 2 gaskets; labor in Thailand is very inexpensive.) I wasn’t sure that this was the right way to go, but as the engine was leaking quite a bit of oil at the head gasket, and as the Baja is my only means of transportation, I wasn't able to wait to research the problem. Now, the surprising thing is that, after almost 2 months (and an initial 1000 km break-in period where I tried not to drive the bike hard), not only is there no more oil leaking at the head gasket, but the engine has more power than before. (It also seems to work a little harder - judging by the sound of the engine at higher revs - and run a little hotter than before as well, but as I’ve recently switched to a full-synthetic oil, I’m not too concerned about the extra heat.) So my questions, for those with experience in this area, are:

1. (main question) Is my Thai mechanic correct in his belief that 0.10 mm is the correct maximum distance between the top of the cylinder and the top of the cylinder head? Does anyone know the correct distance for this spec?

2. I’m concerned about problems developing if the new cylinder wasn’t made to exact specifications. What problems would I notice if the cylinder isn’t exactly the correct size, or if the kind of metal used isn’t of the same high quality as the original?

3. Since my bike doesn’t have a tachometer, how can I tell if I’m running the engine too hard? (especially re. higher speeds in 6th gear; in the past, at speeds over 110 kph, the engine would sound like it was working hard; now it sounds like the engine’s working hard at speeds over 105 - not a huge difference.) Can I judge this by the engine temperature? (I have an Xrs Only temperature gauge that is used in place of the oil dipstick.)

Thanks for any help anyone can give with my questions. Qualified xr mechanics are very hard to find here in Thailand.

[kfngrn]

Well I am not sure what he is measuring, but the top of the clynider/jug the gasket surface should be flat with a straight edge. The bottom of the head should be flat with a straight edge. When you put them to gether and they both have a flat surface there sould be no gap. I asume that they made a new cly. sleeve that they pressed into the "jug" that most likely is not the same material as the original. By them "grinding down the head" this gave the bike more compression, how much, depends on how much they took off the head. You may need to run higher octane fuel.

[Baja trail Rider]

In the USA you can buy head gaskets that are thicker,for this problem.Too much comp. is not good on a XR also cam chain spacing.If it rund good and cam chain is adjusted right your good to go.

[150ron]

What is the highest temperature you see on your XR'S only oil temp dip stick?

[Baja trail Rider]

I have not been able to use the Temp sick in years.Because of the Scotts bracket..When I had it normal 185-200,two up deep sand,hot outside,riding like a madman got up to 220.Thing is my 440s are set up to run cold(oil-jetting-low comp.pipe muffler ect)High comp.air cooled xr not the hot ticket for Baja.

[keenxxx]

Milling the head will increase the compression ratio (smaller combustion chamber area). Milling the cylinder changes the piston to deck height - not such a good thing depending on the original deck height specification. Anyway you slice it it's out of spec. You could trying a thicker headgasket as mentioned.

[Paul1212]

Thanks for all the replies so far.

To 150ron - before the cylinder replacement, I would see temperatures up to 275 F., and on one 90 mile trip where I was pushing the bike hard (between 105-112 kph [65-70 mph]), it went up to 285. I've communicated with a Castrol rep by email, who recommended that I switch from the semi-synthetic oil I was using to a full synthetic, which I've done now. (He said the semi-synthetic was good for temps up to 265.) After switching to the full synthetic oil, temperatures seem to be lower (265-270 max), but I haven't pushed the bike hard yet since the oil change.

To keenxxx and kfngrn - I'm confused about the reason for the increased compression (which seems to be the case, since the bike has more power). From what I understand, the top of the cylinder rim sits a little bit above the top of the cylinder head; I assume that's because the gasket covers the head but not the center part of the head where the cylinder is. I think my mechanic had the head milled down, but not the top of the cylinder rim - in other words, the part of the head around the rim of the cylinder. Since combustion takes place inside the cylinder, it seems to me that milling down the head around the cylinder wouldn't increase compression, but rather it's probably because the new cylinder is probably a little lower (shorter) than the original; a slightly shorter cylinder would thus increase compression. I'll check with my mechanic about my understanding of this. As I said, I'm not highly knowledgable about motorcycle engines, so I may have some of this wrong.

To Baja trail Rider - I think my mechanic just cut a thicker piece of cork to adjust for the increased gap due to the head being milled down (he said it was milled just a very small amount). But I'm not sure about that. One person here told me that the Baja uses a metal head gasket, but I haven't checked on that, yet.

[pjk_xr4]

I put head gasket on my xr250r a while ago, and it was metal. I asked a mechanic friend about the metal / cork ( or other) material for head gasket and he laughed, saying only metal for that, as there's explosions going on there, not just oil like other places.

If you have a cork head gasket, that may be why it was leaking around that mating surface. But you say it's not leaking now, so that's good, but I would be concerned about it breaking down before long. Maybe that's why it was not leaking for awhile, but then continuing again.

[keenxxx]

Thanks for all the replies so far.

To keenxxx and kfngrn - I'm confused about the reason for the increased compression (which seems to be the case, since the bike has more power). From what I understand, the top of the cylinder rim sits a little bit above the top of the cylinder head; I assume that's because the gasket covers the head but not the center part of the head where the cylinder is. I think my mechanic had the head milled down, but not the top of the cylinder rim - in other words, the part of the head around the rim of the cylinder. Since combustion takes place inside the cylinder, it seems to me that milling down the head around the cylinder wouldn't increase compression, but rather it's probably because the new cylinder is probably a little lower (shorter) than the original; a slightly shorter cylinder would thus increase compression. I'll check with my mechanic about my understanding of this. As I said, I'm not highly knowledgable about motorcycle engines, so I may have some of this wrong.

To Baja trail Rider - I think my mechanic just cut a thicker piece of cork to adjust for the increased gap due to the head being milled down (he said it was milled just a very small amount). But I'm not sure about that. One person here told me that the Baja uses a metal head gasket, but I haven't checked on that, yet.

If you mill the head's mounting surface the combustion chamber size deceases. Milling the cylinder gives the same effect but changes the piston to deck height. If available a thicker gasket may remedy the issue.

[pjk_xr4]

I think a thicker base gasket may be the ticket. If there's any chance of the piston hitting a valve, or the piston reaching up to the the head surface, I'd increase the base gasket, as there's no explosion going on there. I've heard of people putting 2 base gaskets in.

[Paul1212]

Thanks to everyone who responded to my questions. I'm going to need to do some study and checking on some issues, like what kind of head gasket my mechanic put in, and whether using a thicker base gasket might help.