Honda BF 100 Head Gasket Replacement

I am replacing the head gasket on my pre 79’ BF100. I ordered a new honda oem head gasket off of a website and it is an exact match. I cleaned out alot of broken impeller pieces from the water jackets and passages. (Also replaced impeller). Reason for head gasket replacement was that the exhaust valves were bent I am guessing from a broken timing belt from the previous owner. I lapped in the new valves and am putting this old girl back together. My question is Should I put any kind of sealant on the head gasket? There doesn’t seem to be very detailed instructions in the honda oem repair manual I found online other than 18 ft-lb of torque on the head bolts and don’t forget the O-ring. The old head gasket seemed to have a thin clear rubbery compound on it so I wasn’t sure if I should use something other than just the gasket. I have never used any rtv or anything on a head gasket before but the substance on the old gasket makes me wonder. I also didn’t use any compound on the oil pump to head gasket as the old gasket didn’t seem to have any. Any help is appreciated.

Hi,

I bet you are right about the timing belt breaking or jumping.

My Honda shop manual lists the head bolt torque as 16 lb.ft. not 18.

Also, the valve seat angle is 45° and the width is 0.030".

I hope the guides are good.

The head gasket goes on dry and yes, DON'T FORGET the ORING!

There's no recommendation for the oil pump gasket that I could find. It's not unheard of though for a thin layer of grease to be used on those "types" (paper) of bolted flange gaskets and I doubt that using RTV would hurt anything either. A matter of preference in my opinion.

Hope she runs great when you're finished!

Thinking about this further, it's been awhile but I remember that there are 6 head bolts having 5 different lengths. I have found that "previous hands" had placed the lengths in the wrong positions. Hopefully your manual has the little pictorial chart of what length belongs where. If not, let me know and I will try my best to explain it using my chart.

I found this to also be true with the powerhead to oil case bolt lengths should you ever go there.

Good luck.

Hey jgmo, If you could post a picture of the head bolt lengths and explain that would be great! I don’t have a service manual and have been trying to use the service manuals that I have found online. The pics are a bit grainy.

Woodsguy,

You don't need a picture. Actually, the photo in the manual is grainy too and after downsizing the file to post it here it would not come out clear at all.

It's really very simple though if you measure the length of the six 8mm diameter bolts and put them in the correct hole in the head as follows:

Looking down on the head with the intake side on your left the three bolts on the right side, from top to bottom are; 94mm...50mm and 55mm

The three bolts on the left from top to bottom are; 70mm...65mm and 65mm.

I also suggest torqueing them in three steps. First to 8 ft. lbs. Then to 12 ft. lbs. then finish with 16 ft. lbs.

Torque the center bolts first then go to the corner bolts alternately (criss-cross) to get an even clamp.

Also, I don't know if these heads and blocks are prone to warping but a check with a quality straight edge might be a good idea.

Good luck.

jgmo said:

Woodsguy,

You don't need a picture. Actually, the photo in the manual is grainy too and after downsizing the file to post it here it would not come out clear at all.

It's really very simple though if you measure the length of the six 8mm diameter bolts and put them in the correct hole in the head as follows:

Looking down on the head with the intake side on your left the three bolts on the right side, from top to bottom are; 94mm...50mm and 55mm

The three bolts on the left from top to bottom are; 70mm...65mm and 65mm.

I also suggest torqueing them in three steps. First to 8 ft. lbs. Then to 12 ft. lbs. then finish with 16 ft. lbs.

Torque the center bolts first then go to the corner bolts alternately (criss-cross) to get an even clamp.

Also, I don't know if these heads and blocks are prone to warping but a check with a quality straight edge might be a good idea.

Good luck.

Click to expand...

Hey JMGO

Thanks for your help on this. I got everything back together, cold compression is about 90psi. Also removed carb and cleaned it a bit although it was actually pretty clean. I couldn't get the primer bulb in the fuel line to work and pump gas so I dumped a bit of gas in the carb and was able to get it started- Yay! Ordered a new fuel line and will get it installed in the next few weeks, We are in the middle of moving...

Well, 90 psi compression is fairly low but obviously it will run on that.

Are you holding the throttle wide open with both spark plugs removed?

The more rpm you can generate with your pull the better.

The compression may come up some as the valves seat in.

Doing a "wet" compression test or a cylinder leakdown would help identify where the leakage is.

Glad you got her going.

Good luck.