What Causes Piston Slap &/or galled cylinders ?

[SteveAdventure]

I toasted the front aluninum piston on my 28 JD. The front of the cylinder is galled and aluminum melted and transferred to the front wall of the cylinder. The top piston ring also fused to the piston and not the other 2. I’d like to take a shot at my own question! I suspect that the piston OD is too small allowing the piston to pivot and drag on the front cylinder wall. Piston specifications were offered in another post but this is looking at cause and effect of the problem. I live in KC and will be back working on the bike on Tuesday in Topeka 40 miles away. I’d like to get my ducks in a row so i make sure we order the correct size with all the information to make the decision.

Am I close? Should the replacement pistons be larger for my Cannonball? Thanks for all your help! Steve

[harleytoprock]

Piston slap is refered to when there is too much clearance between piston and cylinder bore. The piston will rock and rattle in the bore creating noise. The noise will be greater when the the motor is cold. As the piston expands from heat, the noise will lessen or disappear depending on how loose it is. Motors will run ,with piston slap for many many miles with no problems. The problem with your motor is the opposite, too much heat toasted the piston. It could be for lots of reasons. Here are some: Bent connecting rod, too little piston wall clearance, too lean fuel mixture, wet sumped crank case, lack of lubrication, seized piston pin, too little ring end gap.

[Tommo]

The first question I would ask is; were there oil control rings fitted to the pistons?
From your description it appears that the thrust face of the piston overheated and my bet would be that it occured because of lack of lubrication.
Too much oil removed from the cylinder wall.

[sirhrmechanic]

Harleytoprock has the usual suspects down cold.

The 'slap' is usually caused by the piston being too small in the bore, but while noisy, that is generally not fatal, especially if lubrication is adequate.

More than likely, you lacked sufficient piston clearance combined with inadequate lubrication.

As I mentioned in an earlier post, the piston fitting to the bore is critical. And it varies by piston material and design. Your engine builder should know what it is and when the cylinder is cut and final honed, it should provide the correct clearance not only for the piston COLD, but for the piston that is going to heat up at a rate significantly greater than the cast iron cylinder.

Afraid I have not been in a JD engine, but have done more pre-war babbitt car engines than I can count and we always sweat the piston clearance. Measure many times before cutting. There are also factors such as: Is the piston split skirt? Is it cam ground (ie some pistons are not round!)? Material. Ring configuration, etc. My earlier post outlines some of the clearance thoughts and Steve Slocombe was able to add to it nicely.

Finally, we have found a wide variance in the quality of pistons on the market. For example, Egge... don't bother. Others will have their opinions and I won't argue with them. But we won't fit egge pistons. Period. There are also pistons coming out of Australia that can be hit or miss... at least for the engines we work on. We have a brand and supplier (they don't do HD pistons) and we stick with it for predictibility. With your engine, I would work with a reputable engine builder and go with the pistons they recommend and the clearances they recommend. If you try and second-guess the experienced builder or cheap-out on materials, you'll end up rebuilding things again. As the saying goes "the quality remains long after the price is forgotten."

Again, just some thoughts.. I can post some pictures of 'squeaked' pistons if anyone wants to see them. But 'loose' pistons almost never seize up an engine. They just rattle around as long as they get lubricated. Noisy and annoying, but not fatal. Stop the lubrication (or have a piston that's too tight) or overheat the piston and melt some metal off... and you get galling (deposition of metal off the piston and onto the cylinder wall) and once that starts it's a vicious circle. The deposited metal is a lot harder than the piston and acts like a cutting tool. Your engine wrecks itself before you can do anything about it.

Problem is definitely reduced with cast iron pistons running in cast iron bores. The heavier pistons do not have the performance of aluminum pistons, certainly. But for dead-nuts reliability... those old cast iron pistons are hard to beat.

Cheers,

Sirhr

[DanM]

SteveAdventure- I fitted aluminum pistons from Competition Distributing in my '26- the clearance is .008" as recommended by them.

Steve- others have used aluminum UL pistons with the skirts shortened.

Tommo- you have raised an issue discussed many times without final resolution- are you suggesting leaving out the oil control ring and just run the top 2 rings?

I installed the oil rings- and they are the 3 piece scraper style- 2 thin rings with the convoluted spacer. There is of course the 1 piece oil ring style.

Do you suspect and engine would run cooler without the oil rings? I have been measuring the temps with an infrared gun type and the front is about 50 degrees f hotter than the rear in general.

[Jerry Wieland]

Dan The people I have talked to say to put a compression ring in instead of an oil scraper. Jerry

SteveAdventure- I fitted aluminum pistons from Competition Distributing in my '26- the clearance is .008" as recommended by them.

Tommo- you have raised an issue discussed many times without final resolution- are you suggesting leaving out the oil control ring and just run the top 2 rings?

I installed the oil rings- and they are the 3 piece scraper style- 2 thin rings with the convoluted spacer. There is of course the 1 piece oil ring style.

Do you suspect and engine would run cooler without the oil rings? I have been measuring the temps with an infrared gun type and the front is about 50 degrees f hotter than the rear in general.

[Tommo]

Dan,
If you look at an original engine you will find it has no oil control rings fitted so you ask yourself why?
All the oil pump on a total loss Harley does is mechanically keep the oil in the crankcase at the prefered level.
All other oiling in the motor is done by splash or oil vapour feed. There is no positive feed anywhere.
The top end of the motor totally relys on oil getting past the rings to lubricate it and if you start pulling the oil off the cylinder walls that was designed to get past the rings and lubricate the top end you will have trouble.
Read the Riders Instruction Manual and you will find that it tells you that you should see a "Light blue haze" coming from the exhaust following downhill decents or periods of coasting with a trailing throttle.
When you fit oil control rings you introduce a more modern technology to an engine that was designed to operate with an older system.
There will be people that disagree with what I've just said but this is how we here in NZ set up our bikes and all the old ones we bought to the USA in 2003 were set up like this and gave no trouble what-so-ever.
They've all got mileages in excess of 20.000 now and are still going strong. Just remember that with the exception of one they all haul sidecars as well.
Oil is another issue and if you set a bike up like I've outlined you need to try and use an oil that has no friction modifiers or detergents in it so that you restrict the coke buildup in the top end of the motor.
I use a 50 weight Circulating oil that has no addittives what-so-ever and consequently have no carbon buildup in my cylinders.
And to answer your question we just put compression rings in the gaps where the oil control rings would have been fitted.
We use Bedford truck pistons that have a ring below the gudgeon pin just like the original cast pistons and this controls any piston slap that you get with pistons that don't have any rings below the gudgeon pin.

[DanM]

Thanks to Jerry and Tommo for their insight. I'll have to consider pulling the cylinders!!

Any recommendations on a US available circulating oil- no additives? I have been using Valvoline 50 wt racing oil.

[Tommo]

This is what I use it was known as Mobil BB and you will find it reccommended in many 1920's and 30's riders manuals
It is now known as DTE 220 but I have attached a photo of the label on the container so there can be no confusion

[exeric]

Thanks Peter; that's the best reasoning I've heard for omitting the oil control rings in a J motor.

[harleytoprock]

I have to respectfully disagree with those recommending not using an oil control ring. The practice of not using an oil control ring is old school technology which promotes high carbon build up. Carbon is detrimental to top ends as it is abrasive and creates premature wear on components such as valves, guides, rings, spark plugs etc. A modern harley with oil control rings can go 100k miles and more before needing any top end work. Top end and carbon removal jobs were regularly performed every 4K miles on old bikes in their day. My JD was fitted with Eastern MC aluminum flathead pistons that are cam ground, steel struted and have
.004 piston wall clearance. The pistons use modern thin rings which seal better than antique thick ones. They also use oil control rings and I am not running any crank case baffles. The bike does not burn any oil. The exhaust is clear and the bikes does not overheat. I've ridden with other modern harley riders and have been clocked doing 75 MPH many times. The only problen is that since I am not burning or leaking any oil, I am not loosing any oil on a total loss system. Therefore, I drain the oil daily after a typical 200 mile day so that the crankcase oil does not over accumulate and wet sump. Two fresh pumps of oil into the empty crankcase in the morning and I'm good for the day. With this method, my oil is always fresh clean and the oil doesn't build up in the crankcase. Too much oil in the crank case creates problems. If you are really concerned about valve lubrication some people will add a little 2 cycle oil to the fuel supply. 2 cycle oil burns more clean, carbon free than 4 cycle motor oil leaking past the rings. We have learned a lot in the past 80 years about motor techonolgy, why repeat yesteryears mistakes?

[Randy STL]

I would like to suggest that we continue the oil ring discussion in a post by Tom Lovejoy in the Engine-Transmission Rebuilding section titled "Oil control rings on total loss oil systems, use them or not?"

[SteveAdventure]

We believe the problem was that the pistons were too tight and/or not getting enough oil. We consulted the great folks at Topeka HD and Steve Sackman at the Antique Motorcycle Company in St Louis, Mo. Steve recommends the flat top aluminum cast piston from Competition Distributing and leaving out the thick bottom third ring exposing the ten 1/8” (est) holes. The flat head develops less compression and is easier on the old iron and the holes drilled through the bottom ring position will allow oil to come through. Thanks for all your input; I hope to meet you on the road, hopefully on Fisherman's Wharf! Steve

[Tom Lovejoy]

Steve, good luck with the JD. Hope you have solved the problem and that I get to see ya roar by on the Cannonball.

[Randy STL]

Steve,

The Competition Distributing piston is not a true flat top like the original JD or an 80” UL replacement. The piston on the left is an original JD flat top, the center piston is the Comp Dist mild dome, on the right is a Knucklehead domed piston.

Notice that the sides of the Comp Dist piston is lower the than top of original JD flat top, but the top of the dome is higher. I don’t know what the net difference is, higher or lower compression. Maybe Lonnie can shed light on why they chose this design.

Randy
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