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40hp Cylinder to Head Fit
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DeVertible
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PostPosted: Tue Jun 04, 2019 11:27 am    Post subject: 40hp Cylinder to Head Fit Reply with quote

I started to assemble the top end of a 40hp engine and found considerable 'slop' between the cylinder and the combustion chamber. I have no references that tell me tolerances ro know what is acceptable. They are new AA cylinders mating ro newly rebuilt 373 heads. I tried other heads and found a better fit but still sloppy. My first build so comments from experienced builders spreciated. There appears to be quite a variance in the diameter of combustion chambers between the 40hp heads a have on hand. . . Say . . . Half of a mm.
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DeVertible
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PostPosted: Tue Jun 04, 2019 5:05 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

Well. . . I guess I'm just in denial. . . My lovely 373 rebuilt heads must have been altered at some point in their long history. Combustion chambers are 90.5 mm. About .5 mm wider than the flange on the cylinders. Just wrong. Any thoughts or remedies welcome.
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mukluk
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PostPosted: Tue Jun 04, 2019 5:51 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

I just measured a spare stock 40hp head out in the garage, both chambers came in at just under 90.6mm bore. Yours sounds normal to me.

I would expect a bit of slop in the fit from the cylinders to the heads given the dissimilar metals they're constructed from and the amount of heat generated in the combustion chambers.
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Mr. Mike
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PostPosted: Tue Jun 04, 2019 6:05 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

I think you're fine. The sealing occurs between the top of the cylinder & the head and you will lap these two together to ensure their perspective surfaces are flat (search here or get the Tom Wilson book). I'm currently assembling a big bore 40hp and I have a little slop too. Likely for expansion of the steel cylinder in the aluminum head.
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DeVertible
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PostPosted: Tue Jun 04, 2019 8:25 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

Thanks for the votes of confidence. My build is also an 83 mm . I have two sets of 371 round boss heads and they mic at 89.9. This just didn't seem right but I just can think of a reason the heads would be so much larger than the others. Couldnt find any discussion anywhere on this issue Not a reason earth that I can think of they would be machined that way purposely. Will definitely be lapping the fit. Hope to get this resolved with a bit more guidance from the vw gurus. Thank you.
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Mr. Mike
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PostPosted: Tue Jun 04, 2019 10:02 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

DeVertible wrote:
Thanks for the votes of confidence. My build is also an 83 mm . I have two sets of 371 round boss heads and they mic at 89.9. This just didn't seem right but I just can think of a reason the heads would be so much larger than the others. Couldnt find any discussion anywhere on this issue Not a reason earth that I can think of they would be machined that way purposely. Will definitely be lapping the fit. Hope to get this resolved with a bit more guidance from the vw gurus. Thank you.


Sure thing! I'm closer to pedestrian than guru on the VW bell curve... I measured my NewOS and NotOS square bossed 40hp heads, and they're both about 90.5mm; the OD on my new AA 83mm cylinders measure 'bout 89.7mm.
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herbie1200
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PostPosted: Tue Jun 04, 2019 11:28 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

My opinion: there is a reason why the head studs have to be "torqued" in two steps:

The first step, with a lower torque, to let heads to find their centered position

The second step, with the final torque and a different sequence, to actually lock the heads on the cylinders.
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glutamodo Premium Member
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PostPosted: Wed Jun 05, 2019 2:37 pm    Post subject: Re: 40hp Cylinder to Head Fit Reply with quote

I think it needs to be a little more than 90mm, but not a whole lot. I just pulled up a very old photo of the Mexican square boss 40HP heads I'm now running on my 62, and also found this other photo where I took measure of the intake ports and head openings.

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


Yes, the initial torque pattern is to compress the pushrod tubes and get the head in place against the cylinders. Some people go really wild and torque the heads in small increasing amounts switching from side to side as they go. It is said that this causes more balanced, uh, what would you call it, "applied stress" to the overall block structure. (not sure if I ever really bought that myself, as the case is in two halves)

I'd more like to have head openings and cylinders that meet up square and clean. I've also many times lapped cylinders into their intended head openings.
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