Difference between flathead and OHV
- Thread starter madbike
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Flathead engine Flathead engine - Wikipedia, the free encyclopedia
OHV Overhead valve - Wikipedia, the free encyclopedia
It might fit but you will probably need some machine work
OHV Overhead valve - Wikipedia, the free encyclopedia
It might fit but you will probably need some machine work
hurricane jeep engines are half and half 
ut:
Willys Hurricane engine - Wikipedia, the free encyclopedia
ut:Willys Hurricane engine - Wikipedia, the free encyclopedia
1.) IMO, yes.....a lot.
2.) Depends on what is expected of the engine. I wish my truck had a pushrod engine instead of OHC, especially since I'll be using my weekend to change the head gasket. :censure: For high-revving, high-HP engines like my truck does not have, OHC has proven to be an advantage.
I lump "OHV" and OHC together regarding valve position. I never understood how OHV came to describe pushrod engines exclusively when OHV means "overhead valve" layout, which both pushrod and OHC engines have.
2.) Depends on what is expected of the engine. I wish my truck had a pushrod engine instead of OHC, especially since I'll be using my weekend to change the head gasket. :censure: For high-revving, high-HP engines like my truck does not have, OHC has proven to be an advantage.
I lump "OHV" and OHC together regarding valve position. I never understood how OHV came to describe pushrod engines exclusively when OHV means "overhead valve" layout, which both pushrod and OHC engines have.
Here is how I look at the various configurations of Otto cycle engines as it applies to our hobby:
If air cooled [such as our MiniBike engines].
-Flatheads cool the best. The hottest part, the exhaust valve can easily have a cooling 'gap' between the valve and the cylinder.
-OHV [pushrod] engines severely limit head cooling because of the rocker box and most importantly the exhaust valve area. [VWs come with oil coolers stock because oil is used to cool the heads].
-OHC. The entire head is diabolically denied cooling. The entire head is covered by cam boxes.
Weight differences.
It is easy to see who wins again. The Flathead.
Reliability.
-The flathead has only a lifter to actuate a valve. Valve adjustment rarely required [from 1932 till early 1954 flathead Fords and Mercs did not have an adjustment]. The valve gear is light. Oiling is adequate by splash in the crankcase.
-The OHV has [minimum] added a push rod, and a rocker with an adjustment device. Also removable exterior parts to allow adjustment. These added parts limit the RPM of the engine unless higher valve spring pressure is used to control the greater mass of the valve train. Several more wear surfaces are required in the valve train and subsequent wear makes valve adjustment considerably more frequent. Oiling the OHV configuration is a real challenge. An oil pump is necessary. The OHV Briggs, Honda, Clone and etc. engines get their lube by fumes from the crankcase!
-OHC. What a mess.
The flathead is the most reliable. There can be no argument here. The fewest parts, better cooling, lighter valve gear, no 'marginal' oiling method like the OHV engines.
Cost.
No need to belabor the issue. Flathead wins. [Other than the stock clones which many people can't bring themselves to install in a vintage minibike].
Weight.
-The Flathead is the lightest and has the lowest center of gravity which is an advantage on a two-wheeler.
Package size.
-The flathead has the smallest footprint. The lowest height.
-OHV engines are an eternal hassle finding a way to relocate gas tanks and their added length makes them hard to fit into some applications.
Flathead wins again.
A mindless quest for the 'latest' in technology is not always a good idea.
If air cooled [such as our MiniBike engines].
-Flatheads cool the best. The hottest part, the exhaust valve can easily have a cooling 'gap' between the valve and the cylinder.
-OHV [pushrod] engines severely limit head cooling because of the rocker box and most importantly the exhaust valve area. [VWs come with oil coolers stock because oil is used to cool the heads].
-OHC. The entire head is diabolically denied cooling. The entire head is covered by cam boxes.
Weight differences.
It is easy to see who wins again. The Flathead.
Reliability.
-The flathead has only a lifter to actuate a valve. Valve adjustment rarely required [from 1932 till early 1954 flathead Fords and Mercs did not have an adjustment]. The valve gear is light. Oiling is adequate by splash in the crankcase.
-The OHV has [minimum] added a push rod, and a rocker with an adjustment device. Also removable exterior parts to allow adjustment. These added parts limit the RPM of the engine unless higher valve spring pressure is used to control the greater mass of the valve train. Several more wear surfaces are required in the valve train and subsequent wear makes valve adjustment considerably more frequent. Oiling the OHV configuration is a real challenge. An oil pump is necessary. The OHV Briggs, Honda, Clone and etc. engines get their lube by fumes from the crankcase!
-OHC. What a mess.
The flathead is the most reliable. There can be no argument here. The fewest parts, better cooling, lighter valve gear, no 'marginal' oiling method like the OHV engines.
Cost.
No need to belabor the issue. Flathead wins. [Other than the stock clones which many people can't bring themselves to install in a vintage minibike].
Weight.
-The Flathead is the lightest and has the lowest center of gravity which is an advantage on a two-wheeler.
Package size.
-The flathead has the smallest footprint. The lowest height.
-OHV engines are an eternal hassle finding a way to relocate gas tanks and their added length makes them hard to fit into some applications.
Flathead wins again.
A mindless quest for the 'latest' in technology is not always a good idea.
Many flathead utility engines still manage to crack the block around the exhaust port or pull out the head bolt next to it. Iron Kohlers come to mind and Briggs to a lesser extent. Also, flatheads have a big chamber that sends a lot of combustion heat into the cooling "system," making them more thermally inefficient than an OHV. They have an inherently large chamber volume as well as the valves being on the far side it, both of which conspire to decrease volumetric efficiency, burn quality, and, dare I say, increase exhaust emissions. They do have advantages, but I don't think they exceed the advantages of an OHV.
I think it's interesting that pushrod designs may actually predate flatheads in the evolution of the ICE and that they obviously superseded the flathead as well.
I think it's interesting that pushrod designs may actually predate flatheads in the evolution of the ICE and that they obviously superseded the flathead as well.
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My purpose in stating the advantages of a flathead was to attempt to wake up a few people to the fact that your 'average' [a huge majority of 99%] minibike engine is not improved by going to a OHV and certainly, as long as we are talking air cooled, forget about a OHC being more reliable than a flattie or in any way a better engine for the application. The Manx comes to mind as a cammer that was air cooled [in fact a desmo] and also the Ducati 250s and 350s singles back in the old days had OHCs and were unreliable as hell. The super hot area around the exhaust valve on a flatmotor [sitting next to the cylinder bore] is no more problematical than having the exhaust port inside a bolt-on head as with the OHV. Flatheads never seem to have head gasket problems...with OHV engines it is legendary.
Again, we live in a world where the majority mindlessly seek the latest technology and seemingly don't want to believe there are going to drawbacks and tradeoffs to any design that is more complicated. Folks should buy and use anything they want but they should do it without illusions.
My purpose in stating the advantages of a flathead was to attempt to wake up a few people to the fact that your 'average' [a huge majority of 99%] minibike engine is not improved by going to a OHV and certainly, as long as we are talking air cooled, forget about a OHC being more reliable than a flattie or in any way a better engine for the application. The Manx comes to mind as a cammer that was air cooled [in fact a desmo] and also the Ducati 250s and 350s singles back in the old days had OHCs and were unreliable as hell. The super hot area around the exhaust valve on a flatmotor [sitting next to the cylinder bore] is no more problematical than having the exhaust port inside a bolt-on head as with the OHV. Flatheads never seem to have head gasket problems...with OHV engines it is legendary.
Again, we live in a world where the majority mindlessly seek the latest technology and seemingly don't want to believe there are going to drawbacks and tradeoffs to any design that is more complicated. Folks should buy and use anything they want but they should do it without illusions.
I am firmly in the OHV camp since my introduction to the clone.
I would not waste the moisture in my mouth to spit on another 5 hp briggs when I can get a clone that starts and runs better without all the screwing around.
Yes 13 cube brigs is simple but its always demanding some fidling about.
Not realy an easy starting thing and hard to pull over.
A Clone starts good, burn less fuel, and stays more or less on track untill it burns oil an dies.
Without a side by side test spanning years I can't say for sure if I could nurse a clone along as far as a 13 cube briggs on say a water pump. The fact you can fidle around with the old flat head to keep it running is an advantage because nothing short of the valve on the clone is adjustable.
The flat head will also burn just about anything provided its clean and the engine ia allowed to warm up before you dump the last of the diesel fuel and outboard motor gas into it.
Still I rather buy clean gas and pour it in a clone in the long run its simply more costs effective.
I would not waste the moisture in my mouth to spit on another 5 hp briggs when I can get a clone that starts and runs better without all the screwing around.
Yes 13 cube brigs is simple but its always demanding some fidling about.
Not realy an easy starting thing and hard to pull over.
A Clone starts good, burn less fuel, and stays more or less on track untill it burns oil an dies.
Without a side by side test spanning years I can't say for sure if I could nurse a clone along as far as a 13 cube briggs on say a water pump. The fact you can fidle around with the old flat head to keep it running is an advantage because nothing short of the valve on the clone is adjustable.
The flat head will also burn just about anything provided its clean and the engine ia allowed to warm up before you dump the last of the diesel fuel and outboard motor gas into it.
Still I rather buy clean gas and pour it in a clone in the long run its simply more costs effective.
You hit the nail on the head. The only thing that made the OHV engines work at all was the advancements in metallurgy years ago. Ballonium and Uydaisum and, hopefully, Un-obtainable-ium will certainly change things to make them clones reliable...sometime in the future. Another nice thing about a flathead: If you lose a valve keeper or spring or a valve insert or cam gear [or just good 'ol valve float] the valve will not hit the piston and grenade the engine. The attached picture is an early Briggs OHV design that had adequate head cooling because it did not have valve covers that largely eliminate head cooling.
I am wondering if the majority of folks that are sold on clones have ever [in recent times] bought a new Briggs 5 HP flattie. I keep getting the idea that the comparisons between the 6.5 C.H. Ina engine an the Briggs is often being made based a brand new 6.5 and a collection of experiences with old, well used, 3, 4, and possibly 5 HP Briggs flatties taken off worn out rototillers and lawnmowers. My experience has been that the Briggs is hard to beat even a more than twice the price.
I am wondering if the majority of folks that are sold on clones have ever [in recent times] bought a new Briggs 5 HP flattie. I keep getting the idea that the comparisons between the 6.5 C.H. Ina engine an the Briggs is often being made based a brand new 6.5 and a collection of experiences with old, well used, 3, 4, and possibly 5 HP Briggs flatties taken off worn out rototillers and lawnmowers. My experience has been that the Briggs is hard to beat even a more than twice the price.
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