I have a 68 XLH that came with a burned up rear piston. Front piston looks fine but rear has a hole burned through it. So I am going through the engine trying to find a cause and fix anything that looks too wrong.

Former owner had just put new PB cams in it. Also big valves with a 1 3/4" for the exhaust and 1 15/16" for the intake.

I have never timed the cams before on a Harley. I am a little perplexed. I have the factory manual. It says to make sure the timing mark on the crank for the advanced-fired position for #1 cylinder shows in the center of the timing hole. When lined up - the front piston should be 11/16" BTDC. So I lined up the mark and checked piston position and all is fine.
Problem is - I cannot get all the cam timing marks to line up perfectly no matter what I do. It seems the gear that sits on splines on the crankshaft cannot be fine tuned. So when I have all the cams with the marks pointing to each other as they should - including the crank-gear mark - the front piston is 1" BTDC and not 11/16" BTDC. Is this normal? I see no way to change it. It seems it is the crank-gear that determines all this.

One other question. My Harley manual shows only one timing mark on the crankshaft. Mine had three. Maybe a newer crank was installed? Mine has a one-line mark for advanced position for the front cylinder, a dimple mark for TDC for the front cylinder, and two lines I guess for advanced on the rear cylinder? No mention of any marks except one in my 1968 tech manual.

 

First you must be reading the manual incorrectly. There is absolutely no relationship between cam timing and ignition timing. They should not have used the word timing for the cams as it has now for many decades created this confusion. So, completely ignore the ignition timing mark in the timing hole when setting up the cams. Here is the correct process...

The procedure to re-install the cams may be a bit tedious, but it is simple and easy to do ...

Do not use the ignition timing hole mark. Proceed as follows ...

1. Position the pinion gear mark at approximately the 11:00 position by rotating the engine using either the rear wheel or the kicker
2. Install cam #2 so that the mark lines up with the pinion gear mark. This may require moving the pinion gear mark a bit so they match perfectly
3. Temporarily, partially uninstall #2 while you install #1
4. Install #3, then #4 and finally the generator idler gear

When checking for the marks matching get down low and look straight on. Looking down at an angle may result in marks appearing to be correctly aligned when they are not.

The idea that installing cam gears is difficult or risky or to be feared even is an internet myth. It may be based on installing cam gear #1 first simply because it is #1. It is important to start with #2 so that it and the pinion gear can be lined up before installing any other cam gears.

 

Bigger valves in an IronHead engine are a mistake made by guys who think that they must be better simply because they are bigger. The stock valves are already oversize. Nothing to be gained from bigger valves. I do not know what if any problems they may create.

 

For setting ignition timing...

First you need to know what is the TDC mark absolutely, disregarding what any book or any knowledgeable person may say. So do this...

1. remove both spark plugs
2. rotate the engine using the rear wheel or the kicker
3. determine the compression stroke - place your thumb firmly over the front cylinder spark plug hole; when you feel the pressure trying to dislodge your thumb you are on the compression stroke
4. determine what is the TDC mark - when the front cylinder piston is precisely at the top look in the timing hole

Note: to be precise, if you go past the mark you should really keep going rather than rotate backward to get it centered.

Every IronHead engine has all of the marks you see. That section of the manual is probably specific to normal ignition timing only.

 

There are a few different conditions that can burn a hole in a piston. I am not familiar with them. You should research this more before proceeding with more engine work. Some are...

• ignition timing
• lean fuel mixture
• ???

 

This thread explains the crankshaft timing marks you see >>> http://xlforum.net/forums/showpost.p...70&postcount=5

Like Mick has already stated, do NOT use these crankshaft marks for your CAM timing.

 

Thanks for the info. As to the hole in the rear piston? I realize, in general, what conditions can cause it to happen. What I do not know is how it can happen to only one piston and not the other. Front piston and cylinder is fine. Rear cylinder a mess. Besides a hole in the piston, the entire combustion chamber was coated with the remnants of molten aluminum. One valve was stuck open due to some of that aluminum stuck on the valve stem and guide. Subsequently, I am looking for what can cause just one cylinder to be messed up. Carb is on an intake manifold common to both cylinders so I figure it cannot be a fuel problem.

I have been an auto-mechanic for 50 years. No experience with Harleys until now. I am a little surprised at how poorly the Harley tech manual is - or at least - incomplete. I also got the 1968 Harley factory parts manual and am also surprised at how many parts are left out of the book for the 1968 model.

I got this from a guy near 90 years old. He can barely walk, yet he tells me when he rides he only rides "hard." So when he got this bike, it had just had an engine rebuild - all done stock with .020" over pistons. He said he wanted more power - so he had his Harley expert friend "hot rod" it for him. After that, it never ran right and burned up the rear piston.

So that is all the history I have. I tore it apart to look over. Has a 38 MM Mikuni carb that was missing the little clip that locks the needle to the slide. Certainly an issue but seems it should of affected both cylinders equally.

He told me he threw the stock cams our and put in new PB cams. I took it apart and checked and all four are marked PB as he said.

Old heads got chucks and this has somewhat newer heads. Have numbers 16681-57 and 16683-57. Been modified for two plugs per head and larger valves put in. Intake valves are 1 15/16" and exhaust valves are 1 3/4". One valve was ruined but I found new ones and just reseated the valves and put the heads back together.

I wonder now if maybe the ignition timing was somehow messed up so the rear cylinder ran too advanced?

Thanks again for the help. All my cam timing marks line up as the book shows with the piston at 1" BTDC and no timing marks on the crank visible. I will assume this is correct.

Ignition still has a single set of points. Two coils with dual outputs each. I'd like to convert to breakerless but have not found a kit yet that shows it works on this 1968.

Here is how the cams look when marks are in line.

 

Thanks for all the info. I posted a reply this morning and I do not see it here. So I apologize if it shows up twice.

I have been an auto-mechanic for 50 years. No Harley experience though. All my motorcycle experience is with British bikes.

I have the 1968 Harley tech manual and it is the way it is organized that threw me off. On the page that describes how to install the cams and timing gears - it shows having the crankshaft in the position so the advanced ignition timing mark shows (11/16" BTDC). Since that is what Harley shows on page 3D-16 of the factory tech manual - I assumed that was where the crank needs to be when all the cam timing marks are in their proper place. I read it over several times and the manual still does not make complete sense to me. Glad to get some help here. The manual says to get the advanced mark showing in the middle of the timing hole. Then get the oil-breather hole in the oil-pump gear facing out. Next it shows all the cams with all the timing marks lined up. Maybe just bad editing from the book guys at Harley?

I just wanted to be sure the cams are timed correctly. I was getting read to put a degree wheel on the engine and actually measure cam timing myself but that is a lot of work. All the timing marks align as shown in the book when the front piston is 1" before top dead center and no mark of any kind shows in the timing window. I will now assume this is correct.

I am still trying to come up with what could cause one piston to melt and the other to be fine. I cannot see how it can be a fuel issue since there is on carb on an intake manifold that feeds both cylinders. This is why I was checking cam timing. Seems these problems started after the previous owner installed a set of new PB cams.

I wish I had stock heads for it with the smaller valves but guess I will live with what I have for now. The valves in it are huge for this engine. Intakes are 1 15/16" and exhausts are 1 3/4".

 

Marks lined up at 1" BTDC
Here is what the marks look like with the piston at 1" before top dead center. From what I am told here, I guess the timing is fine.

 

Maybe this photo is better.