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Wed Dec 20, 2017 10:36 am

Hello,
Is it possible to bore that much from stock late case (90 mm) 77mm liners to fit big bore 40hp 83mm pistons. There are no sets for late case I think and pistons are readily available. I'm mostly concern about overheating because of thin walls.
Thanks for any input.
Best Regards
Peter

Thu Dec 21, 2017 2:10 am

I think they would be to thin, you would be cutting about 1/8" of a inch from the wall of the cylinder, A&A makes a 80mm kit for the 36 hp that pretty cheap but you have to open the heads and case up to fit them, this should be a 1303cc motor, if you put a 69mm stroker crank in it it would be a 1387cc.
I think you would need 1/8" cylinder spacers to do it and it will make your motor wider which may cause a exhaust and intake problem.

https://aapistons.com/products/vw-80mm-type-1-cylinder-kit-36hp-big-bore

Casey

Fri Dec 22, 2017 3:59 am

You can find old stock 83mm (1500) piston/cylinder kits if you keep looking. I have a set of NOS 69mm-stroke 83mm COFAP pistons and cylinders that will go onto a rather old-school engine I've got the parts to put together (1493 single port, will have dual 34 ICTs on it, mostly intended as a beefy good-fuel-economy engine to replace the 1385 big-bore 40 engine in my 62)

Of course, 1200 and 1500 piston/cylinders also differ in that they have different lengths and connecting rod/piston pin diameters.

Fri Dec 22, 2017 8:12 am

Thanks for replies.
On pictures it looks like big bore kit has all external dimensions the same as 1200 barell. The wall is thinner than 1500 but similiar to 1600. Will talk with my machinist. I'm upgrading my cooling system with dog house so shouldn't have problems with overheating.

Fri Dec 22, 2017 9:42 am

I alluded to this above, but keep in mind that the 1200 uses a 20mm pin at the connecting rod and 13/15/1600 use a 22mm pin!! (not very interchangeable, in other words)

Fri Dec 22, 2017 9:51 am

I'm aware of all the differencies, trying to build fast 1200 looking daily driver motor. With 1200 rod and 69 crank you can get more low end grunt but still be in a very decent rod/stroke ratio of 1.884:1. Also I a have late mexican 1200 with 90 barell mounting size.

Fri Dec 22, 2017 10:33 am

OK, cool, just wanted you to be aware just in case you didn't know all about those differences.

Wait a sec though, when they shifted 1200 production to Mexico during model year 1978, the cylinders reverted to the 1969/older specification of 87mm openings at the bottom.

Sat Dec 23, 2017 6:56 am

Thanks for that tip and for photos, which are worth more than thousand words. Didn't expect for so many good info. I will have to check case opening dimension. Also will cut one junk head to see what can be done to enlarge ports for bigger valves and carbs.