Planet Minis

dax · 12-17-2007, 01:43 AM

I have a 110cc Honda (maybe going bigger). I assume the combustion chamber in a Lifan BVH is much bigger than the one in 6v Honda 70 head. So if I swith to Lifan head and use the same piston, the compression ratio will be very low?
Is it possible to mill the head / cylinder enough to get decent (maybe 11 - 12:1) compression ratio? How much milling would that be? Then there will be some issues with the chain tensioner ? Cam timing I can handle.

And one more question, how high the bvh combustion chamber is (gasket surface to top of the chamber)?

Or would it be better to stick with the 25/23 head instead to switching to the BVH. I think my engine will be 119cc.

fatcaaat · 12-17-2007, 05:58 AM

Actually, you will find that the combustion chamber sizes are similiar in total volume, but different in design. And, you will have to mill a 6v style of piston slightly in the valve pockets to keep the valves from hitting

firepower354 · 12-17-2007, 06:55 AM

The old 70 ports are actually pretty good looking, size and shape-wise. The steel chamber and non-bearing cam are odd, but the head seems like it should be able to be massaged for power. I'm tuning one up right now. DrATV has some decent pistons and cams for them.

Mock up the BVH with no gaskets and see what happens. If nothing hits, add some clay to see if a mill job's viable. .5mm takes the chamfer off the 140 head.

Milling the head or jug 1mm only retards .5 degrees. We drill and tap the tensioner botom bolt and and thread a proper length stud in from the inside, slot the exposed portion and jamb nut it to convert to manual, or at least use it as a spring/oil backup. Most don't have problems with a little milling, but we like to keep things close and where they belong. I'm cutting a slot in a pair of valves right now, to braze a cutter on to for a clean&quick valve pocket tool.

Akunar offers pistons with BVH domes in lots'o sizes. We've used Kitaco SE, Tak Superhead and R head pistons with BVH too. The Akunars need the least amount of molestation, but are taller comp height than some of the others.

Toss the stock valve springs for some TB, Kibblewhite or ATC125M ones depending on how big a cam you'll run. The stock inners coil bind with any more lift than a stock cam.

Do a little bowl blend and get rid of the sharp edge on the short side of the port, and it will breathe great.

The BVH has a much taller intake port so manifold and carb might get shoved up where there are hard parts. It's pretty cozy on the stock framed 50's we did.

12-17-2007, 01:43 AM	#1 (permalink)
dax 1st Gear Member Join Date: Jun 2006 Posts: 156	old 70 head to Lifan BVH, loss of compression I have a 110cc Honda (maybe going bigger). I assume the combustion chamber in a Lifan BVH is much bigger than the one in 6v Honda 70 head. So if I swith to Lifan head and use the same piston, the compression ratio will be very low? Is it possible to mill the head / cylinder enough to get decent (maybe 11 - 12:1) compression ratio? How much milling would that be? Then there will be some issues with the chain tensioner ? Cam timing I can handle. And one more question, how high the bvh combustion chamber is (gasket surface to top of the chamber)? Or would it be better to stick with the 25/23 head instead to switching to the BVH. I think my engine will be 119cc. Last edited by dax; 12-17-2007 at 04:11 AM.

12-17-2007, 05:58 AM	#2 (permalink)
fatcaaat 3rd Gear Member Join Date: Sep 2004 Location: Arlington, VA Posts: 3,778	Re: old 70 head to Lifan BVH, loss of compression Actually, you will find that the combustion chamber sizes are similiar in total volume, but different in design. And, you will have to mill a 6v style of piston slightly in the valve pockets to keep the valves from hitting

12-17-2007, 06:55 AM	#3 (permalink)
firepower354 Advertiser Join Date: Oct 2006 Location: Flint MI Posts: 10,989 My Mood:	Re: old 70 head to Lifan BVH, loss of compression The old 70 ports are actually pretty good looking, size and shape-wise. The steel chamber and non-bearing cam are odd, but the head seems like it should be able to be massaged for power. I'm tuning one up right now. DrATV has some decent pistons and cams for them. Mock up the BVH with no gaskets and see what happens. If nothing hits, add some clay to see if a mill job's viable. .5mm takes the chamfer off the 140 head. Milling the head or jug 1mm only retards .5 degrees. We drill and tap the tensioner botom bolt and and thread a proper length stud in from the inside, slot the exposed portion and jamb nut it to convert to manual, or at least use it as a spring/oil backup. Most don't have problems with a little milling, but we like to keep things close and where they belong. I'm cutting a slot in a pair of valves right now, to braze a cutter on to for a clean&quick valve pocket tool. Akunar offers pistons with BVH domes in lots'o sizes. We've used Kitaco SE, Tak Superhead and R head pistons with BVH too. The Akunars need the least amount of molestation, but are taller comp height than some of the others. Toss the stock valve springs for some TB, Kibblewhite or ATC125M ones depending on how big a cam you'll run. The stock inners coil bind with any more lift than a stock cam. Do a little bowl blend and get rid of the sharp edge on the short side of the port, and it will breathe great. The BVH has a much taller intake port so manifold and carb might get shoved up where there are hard parts. It's pretty cozy on the stock framed 50's we did. __________________ If it has my name on it I did it;) FirepowerMinis.com Custom FPM parts and engines. Parts and service for all makes. Honda Trail Bikes parts and accessories. Pitster/Piranha/Thumpstar bikes, parts, engines and accessories.

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