Piston Clearance

[stork955]

Stork the piston clearance on your mates large ship engines is not a good example to use because the pistons and cylinders on them are made of materials with matched rates of thermal expansion, so there is no requirement to allow for expansion from cold to hot. The Bultaco motor has cast iron sleeve and aluminium alloy piston which have quite different rates of expansion.

Gday feetup, most bike engines are like that as you know. A quick example - Honda XR 400 max piston to bore clearance = 0.004 inch (0.10 mm). Similar engine in that it is air cooled, Cast Sleeve in an alloy barrell. Most pistons in use these days use struts cast in to control expansion, and are cam ground so the piston expands into a perfect (reasonably anyway) circle. God knows what boat engines are made from...

Bogwheel - Also, I just remembered, it is important to check your ring end clearances as well before fitting up. Rule of thumb is 4 thou per inch of bore size. Use the piston crown to set the ring square into the bore and watch out for the ports. Too much is better than not enough. You can carefully file the end as needed but be gentle as rings can break easily. I set them in a vice with soft jaws and go slowly.

Cheers,

Stork

[jse]

Hi , I have to agree with Jon , I have two 199A's that have been bored to 340 and both were done with 1.5 thou clearance. I took great care running them in, ie heat up and cool down several times using an oil rich mixture. Both bikes are very quiet and run perfectly. My experience is that the larger bore motors can develop noisy piston slap with clearances that wouldn't cause noticable noise in a 250. My advice is to keep clearances on the small side and be careful running in and you will enjoy a quiet motor and long lasting rebuild.

I spoke with a retired machinist friend who has several top-notch Bultacos (stock, modified frame and a Sammy Miller framed example) and is an expert on rebuilding the Bul engines. He said he sets up shirt clearance at .0015" and has never had a problem. I really trust his judgement so this is why I passed on that recommendation.

On the subject of port chamfering, I often see the port edge cut sharply at a thin, 45 degree angle (just enough to "break the edge", as some machinists say), which will keep the ring edge from catching, but promotes damaging harmonics and wear by slamming the ring back into the land, and in a place where the ring experiences the most heat. In my experience, a better way (using an exhaust port for an example) is to use a 15-18 degree angle with the chamfer about 2-2.5mm in width in the top/center of the port and tapering out to the edges. This way the ring is eased back into the ring land, rather than slammed back when it catches the center of the 45 degree slope. This type of chamfering is mandatory in a roadrace engine, for instance, with big ports, high RPM and high heat, but I've found it promotes longer ring life in any two-stroke. Intake ports are a little less critical as they are usually narrower and have the cooling effect of the intake charge, but I think you'll get the idea.

Jon

Edited April 23, 2010 by JSE

[bultacosteve]

I have the factory Bultaco service manual and it states that for a new 326cc engine that the clearance should be .002 . With a 360cc engine the clearance should be .0035 Now one thing that has not been mentioned is when using a new Mahle piston you will find a letter and number stamped on top of the piston. It may be T2+ or T2-. Pay strict attention to this numbering system when boring and honing the cylinder to the correct size. These figures when used in conjunction with the Bultaco Manual will give you optimal piston clearance when reboring. The pistons were carefully measured when new and the clearance numbers were stamped on top of the crown for this reason. Pay attention to them. Bultaco knew what they were doing when they manufactured these bikes. Their way of proper setup should be accepted as gospel truth. Its fine that other people have bored and honed cylinders to whatever clearance they wanted and have "never had any problems". The problem with that is they weren't the ones who designed the engine and won't know what the thermal expansion rate of that engine is or why it is. Honestly Bultaco knew what they were doing. Piston, and piston ring and cylinder design were one of Bultacos' fortes that is sorely overlooked. Also ask your machinist when they take their clearance measurement. Is it directly after reboring? or do they wait..... If they do it right away I'd ask for my cylinder back and never use their services again. Proper clearance can only be done when the cylinder is at room temp!!!. There are many things to consider when reboring and fitment. I strongly suggest getting the proper manual and adhering to it. And make sure your machinist adheres to it too. The above only applies to original Mahle parts. When using seizco pistons I'd run the clearance at .004 thou cold to make sure a seizure doesn't occur. In 18 years of being a professional motorcycle mechanic every Wiseco 2 stroke piston that I have ever seen in an engine has seized. They're total junk. Stick with Mahle if you can .

Edited April 25, 2010 by BultacoSteve

[bogwheel]

oops! too late - the guru engineer doing the rebore has talked me into 1.5th. (think i'll trust his forty years experience)

thanks - over and out

[jse]

When using seizco pistons I'd run the clearance at .004 thou cold to make sure a seizure doesn't occur. In 18 years of being a professional motorcycle mechanic every Wiseco 2 stroke piston that I have ever seen in an engine has seized. They're total junk. Stick with Mahle if you can .

Actually, over the years I've had pretty good results with the Wiseco pistons, provided proper set-up and break-in proceedures are adhered to.

Then again, I find it hard to believe a piston company established in 1941 could remain successful today by making "total junk".

Jon

[bultacosteve]

Wiseco pistons are widely available and have been around for years no question. Bare in mind that this forum has a broad membership of enthusiasts, most with some mechanical back ground. And most are aware that engine break in is critical. But in 18 years of professional wrenching all wiseco pistons I have ever seen in 2 stroke engines have seized . Under the right break in conditions and a knowledgeable owner they do perform well. Their lightness means less reciprocating mass which in turn gives more power. But to the average person riding a 2stroke dirt bike out there they are a poor choice period. The use of a wiseco 4 stroke piston is a different matter. they actually perform well and are quite durable.

As a professional mechanic I cannot stress enough the importance to all motorcycle owners on this forum or that come into the dealership that I work at that your factory service manual and owners manual should always be the last word in what is right and what is wrong!!!. Now given that Bultaco is not around any longer many theories can abound about what should or shouldn't be done but the bottom line is the Bultaco factory service manual should always be adhered to. The haynes manul or clymer manual that are widely available simply aren't good enough when it comes to the finer details of engine assembly and setup. Also it doesn't matter how many years experience the local machinist has. His or her experience won't come close to the engineers and people who designed the engine in question. Always follow manufacturers spec vintage or modern. I see more problems that are due to "my buddy said this" or "the internet said that". The last thing most people do is actually open the proper manual and follow its instructions. Read them, know them, understand them, your bike will be better off for it!!!

[jse]

Wiseco pistons are widely available and have been around for years no question. Bare in mind that this forum has a broad membership of enthusiasts, most with some mechanical back ground. And most are aware that engine break in is critical. But in 18 years of professional wrenching all wiseco pistons I have ever seen in 2 stroke engines have seized . Under the right break in conditions and a knowledgeable owner they do perform well. Their lightness means less reciprocating mass which in turn gives more power. But to the average person riding a 2stroke dirt bike out there they are a poor choice period. The use of a wiseco 4 stroke piston is a different matter. they actually perform well and are quite durable.

As a professional mechanic I cannot stress enough the importance to all motorcycle owners on this forum or that come into the dealership that I work at that your factory service manual and owners manual should always be the last word in what is right and what is wrong!!!. Now given that Bultaco is not around any longer many theories can abound about what should or shouldn't be done but the bottom line is the Bultaco factory service manual should always be adhered to. The haynes manul or clymer manual that are widely available simply aren't good enough when it comes to the finer details of engine assembly and setup. Also it doesn't matter how many years experience the local machinist has. His or her experience won't come close to the engineers and people who designed the engine in question. Always follow manufacturers spec vintage or modern. I see more problems that are due to "my buddy said this" or "the internet said that". The last thing most people do is actually open the proper manual and follow its instructions. Read them, know them, understand them, your bike will be better off for it!!!

I surely can agree with that. I imagine that it's a "guy thing" and we hate to ask for (or read) directions.

Jon