Will these heads work with my engine?

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RubNDub
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Joined: Wed Dec 13, 2017 1:41 pm

Will these heads work with my engine?

Post by RubNDub » Wed Dec 13, 2017 2:10 pm

Hi,

I have a stock 40hp 1200. Right now it has single port heads. I know I could swap out to a 1600 block, but I want to see what I can do with this engine so please forgive me.

I found a set of dual port heads, 113 101 375A, and I'm wondering if these could be made to work with my engine?

I'm particularly confused about what the "Barrel Opening" actually is and how it impacts fitment...I understand the piston barrel needs to fit to the heads on one end, and fit to the case on the other, but I'm unclear on what measurements are acceptable or needed for head to barrel fitment. I measured the OD of my barrels on the header end, and it reads very roughly at around 90.xx mm.

This makes me wonder how they could possibly fit...If I just bolt them on, wont they just slide around? Seems ungood. Could I can spacer machined to centre the barrel to the header maybe?

I asked the owner if he thought they would fit. Here is his reply:
yout barrel opening is 90.5mm, mine is 94mm

it will work you got to use 40 hp rockers and push rods and push rod tubes...the head will bolt up and work fine, just keep in mind the 40 hp is 20 mm shorter so you got to use all 40 hp top end rocker arms, push rods and push rod tubes

cncwhiz
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Re: Will these heads work with my engine?

Post by cncwhiz » Fri Dec 15, 2017 2:40 pm

I believe that 40 HP used a 83 mm piston. 1600 is a 85.5 mm piston. I think the owner is wrong as far as if dual port heads will fit 83 mm barrels. We used to build hopped up 40 hps back in the day. We used 1600 parts to build them but the case had to be bored out.

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Marc
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Re: Will these heads work with my engine?

Post by Marc » Sat Dec 16, 2017 12:26 am

Stock 1200 is 77mm bore (as is 1300). They use 32 and 33mm intake valves, respectively. 15/1600 intake valves are 35.5mm and WILL NOT fit into a 77mm bore if the heads are flycut too much and/or the valve lift is too much - but VW made a stock 1300 dual port for sale in countries which taxed by displacement which had the 35.5mm valves with a 77mm bore (113 101 375 NO SUFFIX LETTER), so with a stock C.R. and cam it can be done...you just need to watch out if you're doing other modifications. I've used those 1300DP heads on hi-perf 1300 engines and found it necessary to grind away some of the cylinderwall I.D. near the top for valve clearance. When you consider the valve-shrouding that's disgusting, but it's what we had to do in that class.
Some of the seller's comments regarding rockerarm ratios are nonsense (1965 and newer 1200s use the identical 1.1:1 rockers as 13/15/1600s) but you will need to change at least some of the head studs, and the ideal pushrod length may vary somewhat if ideal rockerarm geometry is to be achieved.
As for the 90 vs 94mm O.D. discrepancy, odds are that the chambers on the 113 101 375A heads are too large to realize a functional compression ratio on a 1200 and they'll have to be flycut to reduce the chamber volume - have that done at the 90mm diameter, leaving an unused step in the head, and the 77mm barrels will center just fine. If for some strange reason the heads DON'T need cutting, simply fabricate some spacers from strips of sheetmetal and insert them into the heads before bolting them down. That'll help to center the heads over the cylinders as you fight the preload of the pushrod tubes. Once torqued down the heads/cylinders aren't going to be going anywhere so the diameter discrepancy is really only an issue while assembling.

Bear in mind that when VW built the 1300DP they fitted it with a smaller-I.D. intake manifold that used the small 30PICT bolt pattern, along with end castings having smaller internal passages - they recognized that the 1600DP parts were just too big to use on a 1300 due to the lower flow velocity which would only exacerbate the "flat-spot" issues for which the 1600DPs are infamous. If you want to build a 1200DP with a single carb you'd better also source the 1300DP intake manifold & end castings.

You'll also want to acquire some 1600DP overcylinder tins. They'll need to be shortened at the case end due to the shorter stoke/narrower package of the 1200. Alternatively you could simply snip out the intake manifold openings, but that won't provide the additional air deflectors which are present on the DP sheetmetal.

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