1942 knucklehead rear head hits top of frame

[DANPATCH]

I have a 1942 knuckle motor . I have had it in three frames.Rear head hits top of the frame . Gas tank is to close to the head.I measured from the top of the rear mount to the rear of the backbone and it is correct. Has repo cylinders but I took the motor apart and measured the rear cylinder and it is correct . Any help would be welcomed .

[46EL]

Has the Frame been fooled with? I remember building my 42 and getting the Frame back from the repair shop. I then put the lower end in with the Cylinders installed. Then I went to install the rear head and the back edge of the rear exhaust tin hit the seat post tube. The Frame guy admitted he did not check the clearance as he did not have any mockup parts available? Jp

[Lonnie]

Several things to consider:

1> Are the frames you are using 1941 or later? Those frames have a minor depression near the seatpost at the bottom of the frame backbone to accommodate the longer cylinder length.
2> Even with the right frame it is very tight at the seatpost. If the frame mounts have been monkeyed with to the point that the engine is tilted back even slightly the head could hit the seatpost, usually it smushes the drain tube if that is the case.
3> On my 42FL the left gas tank is quite close to the head, closer than on my '41EL, to the point that the choke lever is almost hitting the tank. FYI, this frame was actually repaired by the factory and the tank was NOS. Otherwise there are no other issues with this engine/frame combo.

Sorry I cannot offer any other ideas.

[DANPATCH]

the frame does have the flat spot on the bottom of the backbone thanks

[46EL]

Twenty Five years ago I bought a 39 Knuckle. The Motor was in either (really can't remember) a 41 to 45 or 46-47 Frame/Chassis. I remember how someone took a Grinder and took a pretty good chunk out of the backbone so the motor could fit in.

[Steve Little]

Some of the following issues may affect engine fittment:

Harley Davidson re-machined the decks of completed frames after heat treatment and quenching of a completed frame.
The quench was done from brazing temprature... so you can imagine a bit of movement happened to the tubes when it hit the water.

Depending on how much the engine mounts moved during this treatment, is relevant to how much needed to be machined off the decks to bring them back into alignment.
If you take a vernia and measure the thickness of your frame engine decks from the washer area underneath, "front or rear engine mount" you should find a difference from side to side.
The washer area underneath is the constant and was not re-cut.

When the engine mount was being machined by HD as a new sepperate part, the engine deck and front trans deck were machined.
Then the part was flipped over onto the freshly machined engine deck surface, and clamped down while all the washer flats were machined with an endmill to the same depth.

I have seen many frames with a difference from .010 to 030" from side to side. Particularly on 1936 to 1945 brazed frames.
You will still find a difference on the 1945 and later welded frames but it is usually not as much. I dont think they were quenched after assembly.

When the rear deck of a frame is machined down, the engine sits lower in the frame and the rear head gets closer to the seatpost, which is running in at an angle toward the back of the engine.

This is why some of the early "original untouched" frames had the rear tins touching the seatpost...depending on how much distortion took place during quenching and what was taken off the decks by HD.

Possiblity 2
Over the years, many cases have been decked.
The owner takes a set of cases to any machine shop and says clean these up.
The machinist probably does a great job of cleaning or matching decks, but does not know the critical relevance that 1 degree out of spec at the engine mount will make up at the rear head.
1 degree at the deck would tilt the engine back ..."I cant be bothered working it out with cos sin tan at the moment but a guess would be 1/16" of an inch.

Possiblity 3
Many old frames have bounced through the old pot holed dirt roads of the 40's and later.
The evidence is the flattened tubes under the front engine mount back to the seatpost.
When the tube is flattened "even marginally" the frame decks will not be in spec when the engine is removed.

Possiblitty 4.
Aftermarket tins will foul on the seat post a close frame.

There are more reasons, but that gives you a start.

[Robert Luland]

Steve, Does any thing keep you from slightly shimming the engine higher in the mounts to enquire more clearance at the head? Bob

[Steve Little]

Hi Bob.
Appologies for the late reply.
I have been away at a swap meet all weekend and have been trying to get up to speed at work this morning.

I think shimming is OK if it is done right. The shim needs to be the same width and depth as the contact area of the engine and frame pads.

There are two ways to shim. You can put shims under the front and rear decks and shim the engine straight up, if there is room above the rear inlet tin

Or you can use tapered shims on the front and rear decks.
To do this you tip the engine forward to the point of clearance that is needed and measure the gap at the rear edge of the rear deck with a feeler gauge. If you get .020" with a feeler gauge in the back edge of the rear deck then this is the fat edge. Once you have established the fat edge your away.
eg use 020'' steel shim material and drill the engine mount holes in it.
Then put the shim on the surface grinder magnetic table and clock to .005" on one side and .020" on the fat edge and grind away the excess on both front and rear shims.
This will tip the engine forward and still have a full contact footprint from the engine deck to the frame deck.

One of our mills is set up to put frames on.
We cut the decks after manufacture of our frames or after restoration work.
The head of this mill also rotates, so I have had situations were I have rotated the head 30 minutes (1/2 degree) and cut the front and rear decks. This tips the engine forward and away from the seatpost but does not lift the engine toward the backbone.
Regards Steve