Hawk 250 New piston and rings

[zach3697]

Jug is off

[Megadan]

Use this chance to measure some things while you have it off. Wrist pin diameter especially. This will come in handy for ordering replacement bits. If you have bore gauges, you could measure for any out of round. If that cylinder is merely scuffed, but the bore is not out of round, you could likely get away with a new piston and rings with a quick cylinder hone.

[zach3697]

I don't have a set of bore gauges right now. But I do have a set of calipers I can use. How would you recommend going about determining if the cylinder is off round?

[Megadan]

There is a little evidence of some debris scuffing going on, and maybe a little evidence of some mild overheating as well, but the fact that the cross hatch is still visible in the cylinder wall where the scuffing occourred tells me that this wear isn't that severe and shouldn't contribute to such a drastic loss in compression. The rings look to be in decent shape as well.

What interests me more is the evidence of poor valve sealing, especially on the exhaust valve.

If it were me, and I didn't have the proper gauges to check the cylinder, then I would likely take the cylinder to a local shop, have them check and hone the cylinder. Either get a new piston and rings, or clean up the current piston and get new rings - the difference in cost is pretty small between the two.

The last thing I would do would be to clean up the head, check the head to ensure it isn't warped, and lap the valves.

The poor valve sealing doesn't surprise me. The head I bought and ported showed very poor valve to seat contact from the factory, and I had to lap the valves starting with a coarse grit to get a full seat contact, and then finished with a fine grit compound. This is a good affordable little kit of tungsten carbide lapping compound, comes with a small amount of 100 and 240 grit, which is still way more than needed for this head (you could probably do 50+) https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Beyond that, new base and head gaskets obviously.

[zach3697]

Originally Posted by Megadan

There is a little evidence of some debris scuffing going on, and maybe a little evidence of some mild overheating as well, but the fact that the cross hatch is still visible in the cylinder wall where the scuffing occourred tells me that this wear isn't that severe and shouldn't contribute to such a drastic loss in compression. The rings look to be in decent shape as well.

What interests me more is the evidence of poor valve sealing, especially on the exhaust valve.

If it were me, and I didn't have the proper gauges to check the cylinder, then I would likely take the cylinder to a local shop, have them check and hone the cylinder. Either get a new piston and rings, or clean up the current piston and get new rings - the difference in cost is pretty small between the two.

The last thing I would do would be to clean up the head, check the head to ensure it isn't warped, and lap the valves.

The poor valve sealing doesn't surprise me. The head I bought and ported showed very poor valve to seat contact from the factory, and I had to lap the valves starting with a coarse grit to get a full seat contact, and then finished with a fine grit compound. This is a good affordable little kit of tungsten carbide lapping compound, comes with a small amount of 100 and 240 grit, which is still way more than needed for this head (you could probably do 50+) https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Beyond that, new base and head gaskets obviously.

Perfect! I have gaskets and a new piston and rings on order. Got lapping compound from a local store and im going to lap the valves tonight.

The only thing im concerned about it the warped head. If it is warped can it be fixed myself or would a machine shop have to get involved?

Excited to get this motor running!

here are some pictures of my cylinder wall

[JerryHawk250]

You can resurface the head by placing either a fine emery cloth sandpaper or wet sandpaper on a flat surface like a thick piece of glass or plate steel. With the head in hand, sand in a back and forth method and rotate every so often. Once all the high spots are gone sand in a circular motion. There's tons of youtube videos on how to do it. I've used this method tons of times on rebuilding outboard motors.

[zach3697]

Originally Posted by JerryHawk250

You can resurface the head by placing either a fine emery cloth sandpaper or wet sandpaper on a flat surface like a thin piece of glass or plate steel. With the head in hand, sand in a back and forth method and rotate every so often. Once all the high spots are gone sand in a circular motion. There's tons of youtube videos on how to do it. I've used this method tons of times on rebuilding outboard motors.

Thanks for the info! Ill check to see if the cylinder is warped tonight with a straight edge and report back my findings

[JerryHawk250]

Originally Posted by zach3697

Thanks for the info! Ill check to see if the cylinder is warped tonight with a straight edge and report back my findings

It would be a good idea to do it even if it's not warped just to clean it up.

[Megadan]

I agree. It' worth doing just to ensure a smooth clean surface. If the head is not warped at all, then simply a little time with 100 or 120 grit wet/dry paper soaked with a little WD40 or light oil will give a nice clean surface and not remove any significant amount of material.

I would suggest a thicker piece of glass supported underneath by a table to ensure it does not bend at all under weight. You can simply duct tape the edges down to ensure it does not move, just make sure you get it as tight as you can so the paper doesn't curl up under the head when it is moving. These heads are so small that simply using 1 sheet of sandpaper should be enough to get the job done.

I too have used this method before with great success. My only added advice would be to not press down on the head, simply focus on moving it laterally and let the weight of the head do the work. It will take longer this way, but also ensures a better result.

If you find the head is warped and don't trust yourself with this method, then take it to a good machine shop. A quick mill on a head this small would likely cost less than 40 bucks, maybe even less than 30 depending on the shop.

[zach3697]

Originally Posted by Megadan

I agree. It' worth doing just to ensure a smooth clean surface. If the head is not warped at all, then simply a little time with 100 or 120 grit wet/dry paper soaked with a little WD40 or light oil will give a nice clean surface and not remove any significant amount of material.

I would suggest a thicker piece of glass supported underneath by a table to ensure it does not bend at all under weight. You can simply duct tape the edges down to ensure it does not move, just make sure you get it as tight as you can so the paper doesn't curl up under the head when it is moving. These heads are so small that simply using 1 sheet of sandpaper should be enough to get the job done.

I too have used this method before with great success. My only added advice would be to not press down on the head, simply focus on moving it laterally and let the weight of the head do the work. It will take longer this way, but also ensures a better result.

If you find the head is warped and don't trust yourself with this method, then take it to a good machine shop. A quick mill on a head this small would likely cost less than 40 bucks, maybe even less than 30 depending on the shop.

Since we are talking about the surface of the head, what about the cylinder body. does that ever tend to warp?

[Megadan]

Originally Posted by zach3697

Since we are talking about the surface of the head, what about the cylinder body. does that ever tend to warp?

It is always a possibility, but much less likely. You can always check it, and if necessary do the same thing as recommended with the head.

[zach3697]

So today I went ahead and honed the cylinder. Took some degreaser and cleaned the whole head, then used some wd40 and lubed the cylinder and honed the cylinder.

Here are some pics!

[JerryHawk250]

Looks pretty good. Can still see a few lines. Maybe hone a little more. As long as the cylinder is round you should be good to go. When you get the new rings in you can slip one in there to see if it is out of round.

[zach3697]

Originally Posted by JerryHawk250

Looks pretty good. Can still see a few lines. Maybe hone a little more. As long as the cylinder is round you should be good to go. When you get the new rings in you can slip one in there to see if it is out of round.

Awesome! Ill be sure todo that when they come in. For now I went ahead and used my calipers to measure the cylinder diameter at multiple point to check for out of round and the cylinder is reading 67mm at every location. I also measured the top and bottom of the cylinder to check for taper and they were both again at 67mm

Would that be a good way to check for those problems?

[JerryHawk250]

Originally Posted by zach3697

Awesome! Ill be sure todo that when they come in. For now I went ahead and used my calipers to measure the cylinder diameter at multiple point to check for out of round and the cylinder is reading 67mm at every location. I also measured the top and bottom of the cylinder to check for taper and they were both again at 67mm

Would that be a good way to check for those problems?

Yes. I didn't think it would be out of round as it didn't look like it had any wear on it.