Piston Clearance

[bogwheel]

Can i please have advise on the required piston clearence for a M199 (will now be 340cc!!)

thanks

[monoped]

Hi Bogwheel,

Rebore tolerance as qouted from the manual "the clearance between a new cylinder and a new piston, or a fully reconditioned assembly should be 0.0015 inch (0.038mm). Will need to run in for approx 2 hours at less than 2/3rd throttle opening

[lee harris]

Is that from a bulto with an iron bore? That seems VERY small. I go for 0,035mm when rebuilding a fantic but that uses Nikasil. I though Iron needed more, say 0,055mm?

[monoped]

Is that from a bulto with an iron bore? That seems VERY small. I go for 0,035mm when rebuilding a fantic but that uses Nikasil. I though Iron needed more, say 0,055mm?

Lee, I certainly hope so, as I have just had my 198B rebored to that tollerance. In fact they rang me under an hour ago to say the job was done and ready for collection!! The figures that I quoted are from a Haynes manual, I hope they are correct.

Any thoughts John Collins & Big John?

PS Lee I think all the Sherpa models had steel liners.

[isaacely]

Hi,

I recently had my villiers engine with an iron bore rebored and was advised to have it bored to 4-5 thousandths of an inch which I think is 0.102mm - 0.127mm.

I have done a couple of trials on it since the rebore and it seems fine. If you have it too tight I think it will seize when it gets hot.

[stork955]

Gday, bore clearance is one of those things people get confused about. Often too much clearance will cause a nip-up as the added clearance does not allow the piston to transfer heat to the bore wall. The piston relies on this close tolerance as the only other way to pass heat on is through the rings which are pretty thin by comparison and by radiation into the combustion chamber, which is pretty ineffectual. The best bet is to go with the manufacturers specs every time. 1 1/2 thou is pretty normal, 4 thou is getting pretty large and will probably be a bit slappy when cold.

Cheers,

Stork

[bogwheel]

Thanks for all your theories - i have been told 0.002" and to make sure i warm up the motor properly before riding.

What do you think?

I am located in a warm part of Australia if that has any bearing on the size.

[jse]

Gday, bore clearance is one of those things people get confused about. Often too much clearance will cause a nip-up as the added clearance does not allow the piston to transfer heat to the bore wall. The piston relies on this close tolerance as the only other way to pass heat on is through the rings which are pretty thin by comparison and by radiation into the combustion chamber, which is pretty ineffectual. The best bet is to go with the manufacturers specs every time. 1 1/2 thou is pretty normal, 4 thou is getting pretty large and will probably be a bit slappy when cold.

Cheers,

Stork

It's been a while since I had my Bultaco, but I'm with Stork on this. .0015" is a somewhat standard spec., providing the cylinder is prepped correctly (specifically 15 degree port edge chamfering and correct cross-hatch patterning). Since you are going 340cc's, a consideration will be the type of piston used as a forged piston will expand a little more than a cast one (proper break-in is a little more important with a forged piston).

I like to run the engine through a couple of heat/cool cycles and check all the fasteners. Then, with a tad more rich premix ratio, run the bike through an easy section (low RPM and no hard pulls) to help seat the rings and bed-in the piston a little. The on/off, light pull throttle applications, keeping an eye on engine heat, are important.

I just did some prep work on a new 2010 280 and the clearance miked out to .0015" using a forged Vertex piston.

Jon

[copemech]

It's been a while since I had my Bultaco, but I'm with Stork on this. .0015" is a somewhat standard spec., providing the cylinder is prepped correctly (specifically 15 degree port edge chamfering and correct cross-hatch patterning). Since you are going 340cc's, a consideration will be the type of piston used as a forged piston will expand a little more than a cast one (proper break-in is a little more important with a forged piston).

I like to run the engine through a couple of heat/cool cycles and check all the fasteners. Then, with a tad more rich premix ratio, run the bike through an easy section (low RPM and no hard pulls) to help seat the rings and bed-in the piston a little. The on/off, light pull throttle applications, keeping an eye on engine heat, are important.

I just did some prep work on a new 2010 280 and the clearance miked out to .0015" using a forged Vertex piston.

Jon

Agreed with the others, as you should normally keep them around the .0015 spec. Give it a decent run in period and thing should be fine for a long time.. as I would only back off to .002 for those that wanted to immediatly jump back to the MX track back in the day!

[feetupfun]

Remember that there is a reason for the clearance between the piston and the bore. It is to allow for the differential in expansion rates of the piston and the bore with temperature rise between ambient and with the motor at full running temperature.

For this reason, the diameter of the piston and the materials that the bore and piston are made of will determine the ideal clearance. The piston Bogwheel is using is much bigger than the piston that would be used in a M198 and so the clearance should also be bigger. One point five thou might be OK on a 238 or 250 with a forged piston, but I think it would not be enough for a Bultaco 325 or 340 piston, and even more clearance would be required if it is a high expansion rate alloy piston (like a Wiseco).

[sherpa325]

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.

Edited April 21, 2010 by sherpa325

[stork955]

Gday, if the piston you are using is a wiseco or similar, it will come with an instruction sheet specifying bore clearance. 2 thou is pretty right on the money as a rule of thumb. Living in Aus wont make any difference apart from the rest of the world building their engines upside-down in comparison to us. A larger piston wont necessarily need a larger clearance - my ship engineer mate uses similar specs on the big boat motors he works on (that make horsepower in the thousands...)Also very important is to chamfer the ports as Jon mentioned. I do this with a small file. This prevents the rings from catching in the edge of the port and is possibly more important than getting the exact clearance to within 1/2 a thou.

Cheers,

Stork

[bogwheel]

Piston is NOS Mahle - so will go with standard 0.0015" and carefull run in.

Thank you all.

[feetupfun]

Gday, if the piston you are using is a wiseco or similar, it will come with an instruction sheet specifying bore clearance. 2 thou is pretty right on the money as a rule of thumb. Living in Aus wont make any difference apart from the rest of the world building their engines upside-down in comparison to us. A larger piston wont necessarily need a larger clearance - my ship engineer mate uses similar specs on the big boat motors he works on (that make horsepower in the thousands...)Also very important is to chamfer the ports as Jon mentioned. I do this with a small file. This prevents the rings from catching in the edge of the port and is possibly more important than getting the exact clearance to within 1/2 a thou.

Cheers,

Stork

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.

[sherpa325]

Your next problem is to find a good machinist who can work to close tolerances. I had one of my 325 sherpas bored back in the 70's and after puting it back together was delighted to hear almost as much piston slap as before the rebuild, not happy, and that was the local importer. You dont need much wear in the larger bores for them to become noisy, which is why bultaco added the rubber bungs to the cylinders in the mid seventies. The smaller 250's can tolerate more clearance before they become noisy. I dont think the extra clearance detracts too much from the performance, I just find it annoying particularly after you have just spent your time and money on the rebuild.