Still trying to figure out what is happening in the UL motor

Some information and questions:

The deck height is at .017 using a copper head gasket that compresses down to .020 which sets the head clearance at .037. The top ring stops at approximately .125 from the lowest point of the relieving but the piston does go up above the relived area a bit. On the area of the piston that goes up above the relieved area there are scores on the set that ran for 2,000 miles and on a set that were run for 10,000 miles, that area has scores and definite heat damage. I thought the damage on the set that ran 10,000 was caused by detonation that was going on at the end of last year but not sure now. It looks as though that area on the piston is either getting extremely hot and expanding enough (unlikely) to hit the cylinder and getting scraped as it passes the relieved area or something is pushing the piston over to the valve side of the cylinder. Whatever the cause it is defiantly hitting. On the front cylinder (the one that did not have a piston problem) there is scoring on the right (valve) side of the cylinder and little if any anywhere else. The scoring is in the entire ring travel area on the right side. Ring gap was correct, rods are straight and in the center, flywheel end play is OK.

Question:
Would it be a good idea to chamfer the top of the piston starting just below the depth of the reliefs? The chamfer would be done similar to the drawing of the Corvair TRW piston in Panic's 80 book. Other thoughts would be a radius cut on the edge of the piston dome or to turn the area of the piston that is above the relived area smaller so that it would not be scrapped. Or a heavier radius on the edge of the reliefs where they enter the cylinder.

Or am I missing something here?

Jim

[Pa]

Jim...Are you certain the main case bearing bores are square to the case half split lines ? One maybe high or left or right of center ? Just a thought...Pa

[Admin]

jim, i'll take a stab at this. i would say that the exposed area of the piston above the relief is definitely getting hotter at that section and expanding more than the rest of the piston above the top ring. i have seen pistons do all sorts of strange things for one reason or another. i would say that could happen especiallly on the flathead engine. beings that the air/fuel mixture is being compressed to that side and the spark plug is also over there, that side of the combustion chamber and piston are exposed to more heat at the time of igniton. any part of the piston that is exposed over the top of the cylinder is going to get a direct shot of hot expanding gases. instead of the gases just going right over the top of the piston, they are basically hitting a restriction right there and probably overheating that section of the piston for a brief instant every firing stroke.

on overhead valve engines if you have a rough spot on the head or piston sticking out in the combustion chamber, that spot or point can glow or hold heat which in turn causes premature ignition (detonation) and damage. that small spot on your pistons that is sticking out may be holding enough heat or glowing to cause detonation and damage. you may have a combined problem of detonation and the piston just getting too hot right there which will cause that one section the expand abnormally and rub the cylinder.

i don't care for cylinder relieving at all and think any mods done to help airflow on the flatheads should be done above the deck of the cylinder and in the head. i think the decks of all cylinders should be round and square. i would recommend some sort of piston relieving that matches the cylinder so there is nothing sticking out in the flame path. you can cut the deck of the piston as long as it doesn't get too thin right above the top ring. i would say just don't put a little bevel or cut in it, but to run it out and blend it in along a good portion of the piston. also personally, i would like to see a little more deck clearance that what you have. i have seen pistons that have rocked and stretched enough in the bores at that clearance that have touched the head. i try to stay with no less than .050", preferably .060" clearance on anything i fool with. i know it will kill some of the compression but it is much safer.

I agree...what can we really expect HIGH-performance wise from a STREET Flathead. If you set it up stock then you can enjoy MANY fun and trouble free miles out of it.

Correction on above post. The top ring is down .250

Pa,
Yes they are square and in line.

Admin,
The bevel or chamfer on the piston I am refering to would be put on the edge of the piston on the relief side. If a center line were drawn though piston front to back, the chamfer would start and end on the line. Starting at the line the chamfer would gradually increase towards the top ring as it headed to the relief area and then gradually decrease as it headed back to the line on the other side. It would only be about 1/3rd of the distance to the top ring. Would also make it as long as possible towards the center of the piston. I will probably incerease the piston to head clearence a bit more by .010 to .015. Right now I am going to have to sleeve the cylinders unless I can find a boreable rear cylinder. But funds are low so it's going to be a while.

The weird thing is that the setup was the same last year other than piston size and I ran it for 10,000 mile and I don't baby it. I don't beat on it either. There was some noticable heat damage on the top ring land when I took it apart buy other wise the piston and bore were OK.

Jim

[Admin]

jim, you really can't compare last year to this year as the setup wasn't the same if you bored the engine which i am assuming you did. if that's the case, you had a smaller bore, less heat and power, dimensions a little different, timing not "exactly" like before and so on. it may have been close to what you have now but not the same. sounds like your idea on the piston should work o.k., hope it does. hope i helped a little bit, let me know what you come up with or if you need any more suggestions.

Admin,

Yes your info has been helpful.

I agree with you on it not being the same as last year seeing as how I did bore it but I would think the differences would be minimal at best. I am also thinking about what you said about the relieving but not sure if I want to attempt to change it by some type of welding or brazing out near the bore then re-shape to get it above the piston. They need to be sleeved so it would be best done first. I know that there has been different degrees of relieving from mild to radical done on fatties over the years and have never heard of the problem I have had. Also would not a motor that was designed or changed to a popup piston and relived have the piston go positive on the deck exposing it to the combustion chamber with no adverse effects? Have never been able to find any info with drawings or pictures on popup.

If all else fails, I guess I'll have to take my wife to Foxwoods and see if she can hit big enough to get a new set of FHP cylinders and heads.

Jim

[Admin]

jim, other than the dome on a piston in any engine, i'm not much at all for the piston to stick out above the deck. i don't like the side of the piston to be exposed to the flame travel like that, but to each his own i guess. i agree with indianut, how much can you really expect from a flathead on the street? i say build it with sound and proven techniques and get out and enjoy it.

Jim,

You said -
"Also would not a motor that was designed or changed to a popup piston and relieved have the piston go positive on the deck exposing it to the combustion chamber with no adverse effects? Have never been able to find any info with drawings or pictures on popup"

The Jerry Branch article (June 1965 issue of Cycle World) on KR preparation at Eric's Beauty of Speed web site has excellent pictures of how a pop-up piston must be shaped to work correctly in a KR (provide maximum power while avoiding overheating problems). As you can see from the picture in the article the correctly shaped piston has no sharp edges and pretty well shields the piston circumference, in the vicinity of the relieving, from the flame front. I believe a similar shape would be appropriate for your situation.

http://www.beautyofspeed.com/data/doc_cycle-world_jerry-branch/index.htm

If so, why didn't it happen to every Chief Bonneville, Sport Scout Bonneville, Chief Blackhawk, 1948-52 WR, 1952-56 K/KH and all KR through 1967?

Above was me again, forgot to log in.

Jim

None of them do.

Damn I have to remember to turn off auto spell check it changed brazing in my above posts. Hell I ain't talking about cooking.

Panic thanks.

44Dwarf,
I'll look into BRIT-TECH but are you suggesting skirt coating or ceramic dome coating? The pistons I ran last year were skirt coated by Bore Tech.

So I am still at somewhat of a loss on the problem. So if none of the models listed by Panic have a ramp or chamfer on them and there are plenty of other people running pistons that go into the combustion chamber with out any problems then I will have to figure it this way.

Seeing as how both bores were scored from front center to rear center through the entire right (valve) side and through the entire ring travel including the oil ring and the left side was OK, I will assume a number of things are taking place.

Piston may be expanding in the area exposed to the chamber. But no visible piston damage. So no piston debris could cause it.

Right side of piston and rings getting hotter than the rest due to the exposed section getting hot.

Pistons may have been made from old rotted Coke cans and rings were made from old window weights.

Lousy piston rings, but I think that would cause scoring all around the bore not just half.


Possibly to much of the intake charge being washed down the right side of the cylinder washing the oil away when it hits the exposed part of the piston.

Chamber in the head not shaped correctly causing the charge to flow into the cylinder to close to the right side.

Possibly and I mean possibly oil being washed away caused from Ethanol mix washing down the cylinder caused by possible wrong chamber shape.

A combination of all of the above.

Jim

Sounds like the trouble that comes with a bent rod or a bad wrist pin bore in the piston. Was the wrist pin on the new piston slightly bigger, maybe too tight in the bushing? Did the pin show a color change? Your problem may have come not from combustion heat, but friction if the top of the piston was being forced to the valve side for some reason.

If it's front to rear it can't be a bent rod. The piston above the rings is about .030" smaller than the bore - for it to touch it must expand 10X its normal amount.

Jim said the entire valve side of the bore was scored. I was just thinking that if something kept the piston assembly to the far right, that friction may have been the source of the extreme heat.

[Admin]

one thing you have to remember about the flathead is since the exhaust heat is routed thru the cylinder and not the head as on an overhead valve motor, that one side of the cylinder is subject to more heat than the rest of it and therefore could exhibit abnormal wear patterns compared to more modern engines. it's very possible since you have lot's more mass to the cylinder around the valves on the right side, the bore will not stay round as the engine heats up. the right side of the cylinders will stay hotter and could possibly burn off the oil film quicker around the piston and bore at that spot and also contribute to weird wear patterns.

amklyde,

The rods are not bent, they were checked before they were assembled. The wrist pin bushings are fine, honed with my Sunnen and checked with my Sunnen gage.

Panic,

The scoring was mostly on the right side of both cylinders with less towards the front and rear and only within the ring travel and not a mark on the side of the pistons. There is a few very light mark on the skirts which would be expected since there would be some metal moving around in there. The rings were not tight.


Going to be looking closely at the head chamber shape, re-thinking the relief shape and a few other things. Will probably pull the entire motor apart again. There isn't much difference than last year other than the bore and the fuel change so nothing is jumping out at me. When the Ethanol was added to the fuel out here I needed to richen up the carb a bit to compensate. It was about a month or so after I had the bike out that they changed over. The motor was running fine before the change and seamed to start having problems shortly after. I am not saying that the Ethanol is to blame but it might be a contributing factor.

Jim

quote

When the Ethanol was added to the fuel out here I needed to richen up the carb a bit to compensate. It was about a month or so after I had the bike out that they changed over. The motor was running fine before the change and seamed to start having problems shortly after. I am not saying that the Ethanol is to blame but it might be a contributing factor.

i tried telling you
the ethanal washes away the oil in the cyl
talk to any 2 stroke wrench and ask to see his pile of wasted cyl's out back
they will look identical to yours
lack of lube will create more heat then anything in a motor as i'm sure you realize

also you need to check for the rods being straight after assembly in the cases
whose to say even though the races are true
the rods are sr8 that one race isnt 10 lower in the case then the other or that both holes in the rods ends are perfectly true to eachother?
it takes very little to make a rod push one way or the other

Jim, I wasn't meaning to imply any sloppy wrench work on your part. When tackling a mystery like this you can't overlook anything. I am sure it is unlikely that anyone tipped your motor over when it was on the bench, but big flat rods bend easily. If your wrist pin to bushing fit is sufficient and the pistons are square to the deck, that more or less eliminates that being the problem. How does the bottom end feel? If you have a failure with bearings, race or crankpin, very strange things happen. I have seen some gouged out washboard looking bores as the result of the pin or female races going away. Eventually the weird gyrations will force the wristpin retainers to fail. You may have mentioned this earlier, but what was the ring to land clearance? I have had a couple of sets of Chinese made pistons with just under .002. Not enough for a big flat. Your answer is out there somewhere.