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25hp cam shaft vs 36hp cam shaft
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vintage fiberglass
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PostPosted: Wed Oct 13, 2021 5:18 pm    Post subject: 25hp cam shaft vs 36hp cam shaft Reply with quote

I am collecting the parts to build a 1950 25hp engine. I have a lot of NOS stuff but no cam shaft yet. I never had a cam shaft for this engine so I don't know what numbers need to be on it for a 1950 engine? I know they had the fiber gear which I do not want? Can someone help me? Tim
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PostPosted: Wed Oct 13, 2021 8:26 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

When rebuilding my 1950 25hp back to original, I used a new 36hp camshaft with aluminum gear and new 36hp cam followers. The number which appears on the end of the timing gear indicates the diameter of gear to establish proper backlash of the two timing gears when rotated. I'm not aware of any supplier that can currently provide a specific size cam gear, but have not had issues when checking backlash while rotating crankshaft and cam before and after bolting the case halves back together. The fibre gears were a continual problem until VW replaced them with aluminum.
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PostPosted: Thu Oct 14, 2021 4:17 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

first, a big thank you for the response and info. Greatly appreciated. Second, since I am not an engine guy, what do you mean by cam followers?
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PostPosted: Thu Oct 14, 2021 4:37 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Can followers: lifters. On the 25/36hp style, the pushrod is pressed into the lifter so they appear to be one piece.
Matt for Aircooled Research had some NOS 36 cams that I had traded him for some custom ground cams. Does anyone know what is happening to his inventory? Or I am pretty sure WW has a stock cam and lifters.
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PostPosted: Thu Oct 14, 2021 4:44 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Oh, I know what lifters are or pushrods. i think 25hpo and 36hp are the same right?
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PostPosted: Thu Oct 14, 2021 9:41 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

25hp & 36hp cams and cam followers are the same. Engine case, crankshaft, cylinders & pistons, cylinder heads and valves are not.
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PostPosted: Thu Oct 14, 2021 1:56 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Why do some cams say -1 or 0 or +2?
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PostPosted: Thu Oct 14, 2021 2:00 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

vintage fiberglass wrote:
Why do some cams say -1 or 0 or +2?


geneL3c wrote:
The number which appears on the end of the timing gear indicates the {relative} diameter of gear to establish proper backlash of the two timing gears when rotated.
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PostPosted: Thu Oct 14, 2021 2:27 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

geneL3c wrote:
25hp & 36hp cams and cam followers are the same. Engine case, crankshaft, cylinders & pistons, cylinder heads and valves are not.


Engine cases actually are the same for 25 Hp, 36 Hp and early 356.
Only difference are the cylinder size.

The gear / cam to look for is number 111 109 111.
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PostPosted: Thu Oct 14, 2021 3:36 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

You can also by a new aftermarket porsche 356 aluminum gear it's the same as the 25hp/36hp gear ....
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PostPosted: Thu Oct 14, 2021 8:01 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Engine cases for 25hp and 36hp are NOT the same. For one, the 25hp case was all aluminum, 36hp case was aluminum / magnesium alloy. Second, 36hp cylinders will not fit the 25hp engine case and cylinder heads without machining. The numbers, such as -1, 0, or +1, +2 are the diameter of the camshaft timing gear to set the backlash with the crankshaft drive gear. Try reading my earlier explanation, I'm just repeating myself !
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PostPosted: Thu Oct 14, 2021 8:09 pm    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Lots of 25hp cases were made from Elektron (magnesium alloy). 25hp ended December of 53, aluminum cases were discontinued sometime in 1950 or 1951
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PostPosted: Fri Oct 15, 2021 5:22 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

geneL3c wrote:
25hp & 36hp cams and cam followers are the same. Engine case, crankshaft, cylinders & pistons, cylinder heads and valves are not.



How are the cranks not the same? Same stroke, journal sizes, dowel pins. What is the difference?

The 25 hp and 36hp shared the same exhaust valves.
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PostPosted: Fri Oct 15, 2021 5:34 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

geneL3c wrote:
Engine cases for 25hp and 36hp are NOT the same. For one, the 25hp case was all aluminum, 36hp case was aluminum / magnesium alloy. Second, 36hp cylinders will not fit the 25hp engine case and cylinder heads without machining. The numbers, such as -1, 0, or +1, +2 are the diameter of the camshaft timing gear to set the backlash with the crankshaft drive gear. Try reading my earlier explanation, I'm just repeating myself !


As Mr OG Paint pointed out, only the early 25 hp engines were aluminum so you are wrong on that detail.

While you are technically correct about the case, the case casting is the same. The only difference is the size of the opening for the cylinder. There are plenty of 25hp engines that got bored for 36hp cylinders along with the heads back when it was harder to find 25hp cylinders. You can also take a 36 hp case, cylinders and 25hp heads and build an engine that is a cross between the two. All that needs to be done is to flycut the heads for the larger cylinders and change the bottom cylinder studs. There are a couple of details that changed on the case but all of the internals are interchangeable with no machining.
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PostPosted: Fri Oct 15, 2021 6:53 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

I stand corrected, ( typing too fast ! ), crankshafts are the same. The original question was about a 1950 25hp engine, which has an aluminum case, which were, as pointed out, discontinued sometime after 1950. The point I was making was that most 36hp parts are not directly interchangeable without modifying / machining the engine case and cylinder heads. It is true that cases and heads WERE modified to use 36hp cylinders and cyl heads, but in so doing, the originality of the engine is permanently lost. When rebuilding my 1950 25hp engine I could not find a useable standard size cylinder matched to the other 3 cylinders and used new first oversize pistons so as not to have to permanently modify the engine case and heads. 25hp and 36hp exhaust valves are the same, intake valves are not, however 36hp intake valves can be cut down to work. Since the aluminum 1950 engine case is so hard to find, I considered it almost sacrilege to modify a near perfect original to the car, serial numbered ( according to the birth certificate ) engine case. I tried my best to bring the engine back to as close to correct as possible. I realize that not everyone is so concerned about originality and sometimes just getting back on the road with a dependable engine is more important. I was answering a question about a specific engine for a specific year, which I think I have.
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PostPosted: Mon Oct 18, 2021 3:42 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Bob at BBT is making new 25/36hp cam shafts. He is also making +1,0,and -1 gears.
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PostPosted: Mon Oct 18, 2021 5:47 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Wow! Lots of good info here. So to break it down, 36hp cranks, pushrods, exhaust valves, are the same as 36hp? Good to know. Intake valves are different. I think the connecting rods are the same right? But pistons and cylinders are not. Thanks everyone for info

Now I have a question about the thermostat assembly and rings for a 1950 25hp?? I have the seamed fan shroud and the thermostat bracket but missing the flap assembly with the spring and the ring. Same as 36hp or different?
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PostPosted: Mon Oct 18, 2021 10:59 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

The connecting rods are the same.

The intake valves on a 25hp engine are the same size as the exhaust valves, 28.6mm. They wisely enlarged the intakes on the 36hp to 30mm. You can use a 25/36hp exhaust valve for an intake on a 25hp head, but you cannot use a 25hp intake for an exhaust valve. They will fit but the exhaust valves are sodium filled and designed for the extreme heat in the exhaust position. If you use 25hp intake valve as an exhaust, it will fail in short order.

The intakes are all marked with a S on the face, At least the factory VW ones are. Never install a valve with the S in the exhaust position.

At some point VW stopped supplying the 28.6mm intake valves for the 25hp. Instead they sold only the exhaust valves for use as intake or exhaust. I have an early 70s parts manual and it lists the 28.6mm exhaust valve as exhaust for the 25 and 36hp and as intake for 25hp. That part number is 111 109 611A

The cooling ring mounting bracket should fit. There are detail differences, but functionally they are the same.
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PostPosted: Tue Oct 19, 2021 12:33 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

Nobody mentioned here that the cam itself is different although one can use the 36hp cam on a 30hp engine. The lobes are different. The ones for the 36hp have a more curved track over its complete lenght, the 30hp are looking more as if they have two diffenerent radiuses connected by two linear tracks.

Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Tue Oct 19, 2021 8:50 am    Post subject: Re: 25hp cam shaft vs 36hp cam shaft Reply with quote

pastellgreen wrote:
Nobody mentioned here that the cam itself is different although one can use the 36hp cam on a 30hp engine. The lobes are different. The ones for the 36hp have a more curved track over its complete lenght, the 30hp are looking more as if they have two diffenerent radiuses connected by two linear tracks.

Image may have been reduced in size. Click image to view fullscreen.


The one on the left looks more like 30, and the saggy one on the right 60.
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