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Domed piston into stock 36hp engine
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grailoc
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PostPosted: Mon Jan 23, 2017 4:26 pm    Post subject: Domed piston into stock 36hp engine Reply with quote

Hi everyone,

I am rebuilding an original 36hp engine with stock Crankshaft, Okrasa style camshaft and original heads and rockers.

I am looking into the option to install an NOS 77mm cylinder kit with domed pistons.
I wanted to know if with domed piston I needed to intall shims below the cylinder as well as doing either machining on the case or the heads to get the correct space and ratio into the combustion chamber?

I am not looking into getting fancy compression ratio and prefer avoiding any machining,
so just keep stock with domed pistons.

Any help or advise would be greattly appreciated.

Thanks for help.
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roachdub58
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PostPosted: Mon Jan 23, 2017 6:00 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

Well, the whole point of the domed pistons IS to raise compression. If all the other dimensions and pin height are identical to an original flat piston and you want to keep factory compression specs, then yes, you will have to shim and/or machine somewhere on the build.
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grailoc
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PostPosted: Mon Jan 23, 2017 6:16 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

roachdub58 wrote:
Well, the whole point of the domed pistons IS to raise compression. If all the other dimensions and pin height are identical to an original flat piston and you want to keep factory compression specs, then yes, you will have to shim and/or machine somewhere on the build.


raising compression might not be a bad idea I think, what I wanted to make sure with my question also was to figure out if domed piston would clear out the valves movement if I do not put neither shims nor do machining.
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PostPosted: Mon Jan 23, 2017 9:53 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

It depend of how doomed the pistons are, but VW used doomed pistons in T3 1500 and T1 1200 73-up some years.

You want higher compression, so no modifications needed unless you want higher compressions. I know that you can flycut the heads more than 3 mm without problems, with the flat pistons.
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grailoc
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PostPosted: Tue Jan 24, 2017 6:42 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

I am trying to find the answer to my question.

Will I run into issues with my engine if I put those domed piston without any spacer/shims nor machining?

Thanks again for help and answering my questions.

/Jerome
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Mr. Motorhead
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PostPosted: Fri Jan 27, 2017 2:06 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

And our survey says *bing!* No it will not cause any real problems. If you have the USA spec engine (most likely) of 6.6:1 stock compression the dome tops will bring you to around 7:1 or there about depending on the shape of the dome. Large, open chambers on the heads allow this. Make sure to check before assembling that your dome does not interfere (hit) with the head. If it does just simply clearance with a die grinder (.040" minimum).
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grailoc
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PostPosted: Fri Jan 27, 2017 4:43 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

thank you Matt,
this is the answer I was looking for.

Here is a picture of my head, do you think it will clear the space?
my car is a US spec bought new in Germany with the US program back in 56.

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

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Mr. Motorhead
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PostPosted: Fri Jan 27, 2017 7:00 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

grailoc wrote:
thank you Matt,
this is the answer I was looking for.

Here is a picture of my head, do you think it will clear the space?
my car is a US spec bought new in Germany with the US program back in 56.

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


As you are looking at that head picture, the two places that -may- need some trimming are the two sides on ether side of the spark plug hole. That wholly depends on the style domes you have, there are three I know of. Most clear just fine but it never hurts to make sure for your own piece of mind. I always do a .040" minimum clearance.
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grailoc
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PostPosted: Fri Jan 27, 2017 7:29 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

ok I see,

here is a picture of the piston I am looking to install:
Image may have been reduced in size. Click image to view fullscreen.

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

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PostPosted: Sat Jan 28, 2017 11:41 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

VW added doomed pistons on 1200 40hp in the seventies, so I will be very surprised if you got problems with the 36hp, with lower compression than the 40 hp.

But check the deck height, if it is 40 hp pistons you got, they are 1mm higher from the piston pin.
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grailoc
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PostPosted: Sat Jan 28, 2017 3:50 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

here is a picture of the reference of the kit:
seems like KS non domed piston kit reference is 91-347-90, but I have no clue that reference below is specific to 36hp domed piston.
Image may have been reduced in size. Click image to view fullscreen.

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gregson1
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PostPosted: Sun Jan 29, 2017 11:48 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

Are you doing the build? If you want to remove all doubt, measure it. You can do this by 'mocking up' the build. Get some solder or modeling clay and lay out some THIN strips of it across the top of the pistons, then slowly roll it though the cycles. Don't get crazy with the amount of clay. Take it apart and check that the impressions in the clay or thickness of the solder. If it's less than 040" you could have some clearancing to do. I've also done this to verify the clearance of the case and piston skirts.
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grailoc
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PostPosted: Sun Jan 29, 2017 12:21 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

gregson1 wrote:
Are you doing the build? If you want to remove all doubt, measure it. You can do this by 'mocking up' the build. Get some solder or modeling clay and lay out some THIN strips of it across the top of the pistons, then slowly roll it though the cycles. Don't get crazy with the amount of clay. Take it apart and check that the impressions in the clay or thickness of the solder. If it's less than 040" you could have some clearancing to do. I've also done this to verify the clearance of the case and piston skirts.


I will not do the build this is an hold VW mechanic who will proceed, however I wanted to make sure I buy the correct piston set before being able to measuring it.
thanks for the tip with modeling clay.
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PostPosted: Wed Feb 01, 2017 12:05 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

Kjell Roar wrote:
VW added doomed pistons on 1200 40hp in the seventies, so I will be very surprised if you got problems with the 36hp, with lower compression than the 40 hp.

But check the deck height, if it is 40 hp pistons you got, they are 1mm higher from the piston pin.


Ok thanks,
Here is a picture of my stock piston. I got 28mm between the pin and the deck.

So if the piston I am about the buy were for 40hp then the distance would be around 29mm?

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

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PostPosted: Wed Feb 08, 2017 10:48 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

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

now this is a high compression piston ! made by jahns back in the day. 8.5 to one. maybe use them on my next build ? Very Happy
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PostPosted: Wed Feb 08, 2017 12:11 pm    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

sgmalt46 wrote:
Image may have been reduced in size. Click image to view fullscreen.

now this is a high compression piston ! made by jahns back in the day. 8.5 to one. maybe use them on my next build ? Very Happy


indeed these have a lot bigger dome than the one I just bought,
I will post closer pic of mine when I receive them.
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PostPosted: Thu Feb 09, 2017 10:59 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

grailoc wrote:

I will post closer pic of mine when I receive them.

When you post your photo, I would be interested to know how tall the crown of the dome is compared to the top of the piston edge.
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PostPosted: Sat Feb 18, 2017 6:27 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

Here are some picture difference between the domed piston and OG piston:

the deck of the dome is about 2.2mm taller than the og piston.

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

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


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

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My 59 low light ghia: http://www.thesamba.com/vw/forum/viewtopic.php?p=7852268
My 56 beetle convertible: http://www.thesamba.com/vw/forum/viewtopic.php?p=7796259
My 60 SO23 westy: http://www.thesamba.com/vw/forum/viewtopic.php?t=341539
Check my blog: http://us-road-trip-vw-bus.skynetblogs.be/
and my garage build thread: http://www.garagejournal.com/forum/showthread.php?t=199438

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grailoc
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PostPosted: Sat Feb 18, 2017 6:35 am    Post subject: Re: Domed piston into stock 36hp engine Reply with quote

and the NOS setup:

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

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My 59 low light ghia: http://www.thesamba.com/vw/forum/viewtopic.php?p=7852268
My 56 beetle convertible: http://www.thesamba.com/vw/forum/viewtopic.php?p=7796259
My 60 SO23 westy: http://www.thesamba.com/vw/forum/viewtopic.php?t=341539
Check my blog: http://us-road-trip-vw-bus.skynetblogs.be/
and my garage build thread: http://www.garagejournal.com/forum/showthread.php?t=199438

My other vehicles
KG 64
vespa 1961 Acma & spanish GT 160
2 Lambretta LI 150
2 1971 Honda CT70
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