# Strange Pistons **URL:** \ **Category:** Factual Questions\ **Created:** [April 30, 2003, 5:37am UTC](https://boards.straightdope.com/t/strange-pistons/172088 "2003-04-30T05:37:48Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [April 30, 2003, 5:37am UTC](https://boards.straightdope.com/t/strange-pistons/172088/1 "2003-04-30T05:37:48Z")
I’m currently temping at a tractor parts warehouse and one of the things we ship out is what looks like a head gasket for a four cylinder head. The odd thing is that the holes for the pistons aren’t round but ‘D’ shaped. I’ve never heard of an engine which used anything but round pistons, and yet here’s this head gasket for what appears to be an engine that has ‘D’ shaped pistons. I’ve asked around, but no one has a clue and the documentation which accompanies the gasket isn’t informative (another series of parts that we ship out are simply labeled “Spare Parts” evidently, they’re what supposed to be left over when you put the thing back together), so I’m wondering if anyone knows anything about this and what the advantages might be. ---
**Author:** ![kferr](https://avatars.discourse-cdn.com/v4/letter/k/71e660/32.png) [@kferr](https://boards.straightdope.com/u/kferr)\ **Post date:** [April 30, 2003, 6:11am UTC](https://boards.straightdope.com/t/strange-pistons/172088/2 "2003-04-30T06:11:16Z")
My WAG is that the engine is similar in design to a Briggs & Stratton lawn mower engine, with the valves in the block instead of the head. I think this is called a side valve configuration. The hole in the gasket of the mower engine goes around both the piston and the valves. So with your D shaped hole, the round side would be where the piston is, and the flat part would surround the valves. ---
**Author:** ![Barty](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Barty](https://boards.straightdope.com/u/Barty)\ **Post date:** [April 30, 2003, 6:11am UTC](https://boards.straightdope.com/t/strange-pistons/172088/3 "2003-04-30T06:11:16Z")
I think that you are looking at a head gasket for a side valve or flathead engine. On these type of engines the valves are in the engine block, not the cylinder head. The gasket is shaped to encompass the inlet and exhaust valve and the piston. The pistion is round as you would expect in a modern overhead valve engine [www.cmcengine.com/cmcengine/FlatHeads.htm](http://www.cmcengine.com/cmcengine/FlatHeads.htm) has a picture of a flat head that shows the cutouts for the valves and gives you an idea of what the head gasket shape should be like. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [April 30, 2003, 6:23am UTC](https://boards.straightdope.com/t/strange-pistons/172088/4 "2003-04-30T06:23:56Z")
Hmm. The head’s in your link look like they’d have an acorn shaped opening in them. This is a definate ‘D’ shape, not a kinda sorta one. But, it seems likely that they are for some sort of flathead engine as the tractor company supplies New Holland, which bought Ford’s tractor line, and Ford did make a flathead engine. Oh, and congrats on your first post **Barty** , hope you don’t wait another 2 years before you make your next one. ---
**Author:** ![herman\_and\_bill](https://avatars.discourse-cdn.com/v4/letter/h/f1d935/32.png) [@herman\_and\_bill](https://boards.straightdope.com/u/herman_and_bill)\ **Post date:** [April 30, 2003, 10:52am UTC](https://boards.straightdope.com/t/strange-pistons/172088/5 "2003-04-30T10:52:53Z")
Well, you said it “looks” like a head gasket. are you sure its not an exhaust manifold gasket? ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [May 1, 2003, 4:42am UTC](https://boards.straightdope.com/t/strange-pistons/172088/6 "2003-05-01T04:42:03Z")
> [@](#): > > \*Originally posted by herman\_and\_bill \* > \*\*Well, you said it “looks” like a head gasket. are you sure its not an exhaust manifold gasket? \*\* Yeah, all the exhaust manifold gaskets I’ve seen are made out of a lead-like material, this is made out the same kind of material as head gastket. ---
**Author:** ![herman\_and\_bill](https://avatars.discourse-cdn.com/v4/letter/h/f1d935/32.png) [@herman\_and\_bill](https://boards.straightdope.com/u/herman_and_bill)\ **Post date:** [May 1, 2003, 12:53pm UTC](https://boards.straightdope.com/t/strange-pistons/172088/7 "2003-05-01T12:53:56Z")
Well, can’t you do some kind of back-tracking on the part number? ---
**Author:** ![li\_l\_Dickie\_Dirtz](https://avatars.discourse-cdn.com/v4/letter/l/2bfe46/32.png) [@li\_l\_Dickie\_Dirtz](https://boards.straightdope.com/u/li_l_Dickie_Dirtz)\ **Post date:** [May 1, 2003, 1:58pm UTC](https://boards.straightdope.com/t/strange-pistons/172088/8 "2003-05-01T13:58:18Z")
Slightly off-topic: Back in the '80s, Honda was working on oval pistons and cylinders so they could cram more valves into the head. Don’t know what became of that. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [May 1, 2003, 3:00pm UTC](https://boards.straightdope.com/t/strange-pistons/172088/9 "2003-05-01T15:00:53Z")
Honda dropped the idea. The design produced more horsepower, but only at 20,000RPM[sup]+[/sup]. If it helps, I’ll ask an 8N fanatic what his head gaskets look like and reply tomorrow. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [May 1, 2003, 3:39pm UTC](https://boards.straightdope.com/t/strange-pistons/172088/10 "2003-05-01T15:39:29Z")
> [@](#): > > \*Originally posted by herman\_and\_bill \* > \*\*Well, can’t you do some kind of back-tracking on the part number? \*\* In theory, yes, but the computer system they use at work is, ah, primitive to say the least. And the times when I’ve tried to use their internet site to check part numbers, it’s been down. ---
**Author:** ![Ender\_Will](https://avatars.discourse-cdn.com/v4/letter/e/eb9ed0/32.png) [@Ender\_Will](https://boards.straightdope.com/u/Ender_Will)\ **Post date:** [May 1, 2003, 4:04pm UTC](https://boards.straightdope.com/t/strange-pistons/172088/11 "2003-05-01T16:04:59Z")
Some Formula 1 teams were working on oval shaped pistons, but the FIA added a rule saying pistons had to be round. Something about leveling out the playing field, as oval were more expensive. No cite, just something I recall reading, probably in Road and Track, or Formula 1 magazine or some such.