When I rebuilt my transmission a few weeks ago I had to
re-use my head gasket. I knew I had another one in my parts supply, but I
couldn't locate it at 1:30 in the morning when I was on a tear rebuilding my
engine. Reusing the head gasket was the only way I was going to get to the GouldMicrocar rally, so back on it went.
Last week I found the new head gasket so I decided to install
it. First I had to determine if I had to remove the engine to complete the
procedure. I asked three Heinkel Kabine
experts; two told me I could, and one said no, so I went with the majority
opinion and decided that to replace it with the engine in the car My fallback position would be to remove the engine if necessary.
I use the head gaskets with a crushable copper ring.
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| Heinkel Head Gasket with Copper Ring |
It is not part of the standard gasket set you purchase from
the clubs. I use these because I blew through a standard head gasket on my
Heinkel Tourist scooter. When you tighten the head the copper ring gets crushed
into place and provides an excellent seal.
Copper is soft, but Aluminum is softer, and the head is made
of aluminum alloy. When I took off my head gasket I noticed my head had a semi-circular
indentation from the gasket. When I reused the gasket it was already crushed,
and when I torqued it down during the rebuild the only thing that could give
was the aluminum in the head. Another expensive lesson to learn!
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| Location of Indent on Heinkel Head |
I went to my local machine shop and had the head skimmed to
remove the indentation. I was lucky- I have a great relationship with my machinist
and he had it done in an hour on a Saturday morning! So, time to reassemble.
I’ve written before about finding Top Dead Center with a
simple indicator. I received several complaints telling me this was a “flawed”
approach since it didn’t account for the dwell at the top of the piston arc.
So, while I had the head removed, I decided to use a runout dial indicator to precisely
measure TDC. You can see my setup in the picture below.
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| Finding Heinkel TDC With a Gauge |
The idea here is to turn the crank in the direction of the engine
(clockwise) until you reach TDC and make a note of the reading with a mark on
the flywheel. Then, turn the engine in the opposite direction until you reach
the same dial measurement. Mark that on the case as well. Half the distance
between the two marks is TDC. In my case the both line up in the same place,
which was exactly where I had marked TDC before with my simple indicator. I
repeated this several times to make sure I didn’t make an error.
Whenever you skim a head you need to make sure the clearance
between the piston and the valves has not been compromised. The best way to do
this is to do a dry fit without the gasket. The Heinkel head gasket is about
2mm thick. To get a good measurement I took some Play-doh modeling compound and
laid it across the piston, then dry fit the head. I turned the fan by hand to
run the engine through one revolution, then took the head off and examined the
Play-doh. The thought here is that the valves will make an impression in the
Play-doh. You can measure the clearance and determine if it is sufficient. In
my case everything was fine, so I had the clearance to go ahead with the
rebuild.
After setting the valve clearance I re-tested the compression
to make sure everything was ok. I took it for a spin and it still seems a
little pokey.
Did I have to remove the engine- No!
Cruiser News Review - March to May 2003
ReplyDeleteHeinkel Engine Compression Ratios
I was chatting to Ted Miller about his 198cc engine rebuild a few weeks ago. He told me that he
had bought an unused 66mm Mahle piston from the little engineering shop that he uses. His first
comment was that according to wife Shirley’s scales it was 30 grams heavier than the piston
previously fitted. This he suggested must surely affect the balance of the engine. His second
comment was that it would make the engine’s capacity 210cc and this would increase the
compression ratio. This got me thinking and checking the data in the Workshop Manual.
For the 408 B-0 engines, as far as I know only ever fitted to early four-wheel cars, it states the bore
is 65mm and the stroke 61.5mm, which gives a swept volume, or cubic capacity of approximately
204cc. It also states the clearance volume; the volume above the piston when it is at top dead
centre is 35.2cc, this, as the manual says, gives a compression ratio of 1 : 6.8.
My understanding was that the Heinkel engineers thought the 204cc engine was over stressed so
reduced the bore to 64mm giving a cubic capacity of 198cc. This configuration was used in the
408 B-1 engine fitted to later four-wheel cars and the 408 B-2 engine fitted to three wheel cars. As
far as I know there were no changes to the cylinder head so the clearance volume was the same.
But as Ted suspected the compression ration was reduced to about 1 : 6.6 by fitting the smaller
piston. While this is only about a three per cent reduction from the 204 to 198cc engines it
becomes a six percent increase over standard when a 66mm piston is fitted in a 198cc engine.
The main effect of increasing the compression ratio is to increase power at high revs but it will
also produce more heat, another effect is to narrow the power band, reducing power at lower revs,
making the car less drivable.
The head on the left has been skimmed at least twice following blown head gaskets while that on
the right may be original, the difference can clearly be seen by how far the face extends beyond the
finned section.
When restoring engines I have both the head and barrel skimmed so that I know they are both flat.
The effect of this is to reduce the clearance volume and thereby increase the compression ration
further. As some of you will recall from the Newsletter I was suffering from an overheating
engine late last year, which finally resulted in the head gasket failing between the cylinder and
push rod tubes. I was surprised to find how much metal had been removed from the head, the one
on the left, when I got it back from the engineering shop. I estimate that about 1.25mm in total has
now been removed. When you add to this the amount taken of the barrel when it was surfaced the
total reduction in cleared height is approaching 1.5mm. This reduces the cleared volume by nearly
5cc to around 30.5cc. This in conjunction with the larger piston increases the compression ratio
from an original 1 : 6.6 to nearly 1 : 8! An increase of nearly 25%.
The simple solution, suggested to me by Garry Longstaff, was to fit additional cylinder base
gaskets. The base gaskets supplied by the Club are 1mm thick, so I have fitted three to my engine,
which should give me the standard compression ration. I will let you know hoe I get on. –Ed.
The above article (which is missing it's original accompanying photograph) was previously published in the UK's Heinkel Trojan Club archives.
ReplyDeleteVery interesting, if somewhat technical, and suggests a useful remedy for correcting over skimmed Cylinder Heads and/or Cylinder Barrels, & might just help prevent Head Gasket failures due to excessively increased Compression Ratio.
Regards,
Jon Way, Somerset, England.