TheSamba.com :: View topic - which pistons for 2.0l type 4 CJ Did I buy the wrong ones?

Sun Feb 14, 2010 11:13 am

Hi, i'm buying new barrels and pistons and see there are flat and dished piston tops. The ones on the engine are currently dished but i've been offered flat top ones which are boxed and new and very cheap.

Will I need to make any cylinder head mods?

I live in the UK, the engine is for a 1974 type 2 camper. There are no emission laws for an engine of this age, and we don't have a 'hot' climate :D everything else on the engine is currently stock

any advice greatly received

Sun Feb 14, 2010 11:17 am

don't go cheap.

you'll have a hard time getting compression ratio set properly with flat-topped pistons. the other advice i always give people who are buying engine parts is have the engine torn down and measured before you start buying parts. it's easy to make mistakes on parts purchases if you start buying parts too early in the process or buy parts based on price.

Sun Feb 14, 2010 11:40 am

germansupplyscott wrote:

you'll have a hard time getting compression ratio set properly with flat-topped pistons.

Do you mean because it will raise it too quickly?

Sun Feb 14, 2010 11:43 am

thanks for the advice. I should have come on here sooner before stripping things down. This engine has been in a shed for 8 years. The stage i'm at;

Heads in being cleaned up and new valves fitted.
Case being cracked open, cleaned, measured for clearances and tolerances - any parts replaced as required.

I want to stay with a 'stock' engine at the moment due to cost

I should have been clearer, by cheap I meant inexpensive... these are good quality, Mahle for 914 engine kit brand new in original box, half the price!

I'm told these are a direct bolt on with no further mods required to the heads or deck height etc. If all this will do is increase compression but without any detriment to the longevity/reliability of the engine i'm happy to do this but i'm no expert mechanic.

Sun Feb 14, 2010 3:18 pm

reko wrote: thanks for the advice. I should have come on here sooner before stripping things down. This engine has been in a shed for 8 years. The stage i'm at;

Heads in being cleaned up and new valves fitted.
Case being cracked open, cleaned, measured for clearances and tolerances - any parts replaced as required.

I want to stay with a 'stock' engine at the moment due to cost

I should have been clearer, by cheap I meant inexpensive... these are good quality, Mahle for 914 engine kit brand new in original box, half the price!

I'm told these are a direct bolt on with no further mods required to the heads or deck height etc. If all this will do is increase compression but without any detriment to the longevity/reliability of the engine i'm happy to do this but i'm no expert mechanic.

It is not a big deal to go from dished to flat top with a 2 liter engine equipped with the dual carbs. So long as you maintain a decent deck height, and keep the timing at 25*-28* @ 3,400 rpm without hoses on the distributor, you'll be fine. If you have a functional vacuum retard distributor and original carbs, you do want to have the 10* ATDC timing specification. My very first rebuild from the dealer, I ran domed Porsche pistons. Very nice reliable 100,000 mile Mahles.
Colin

Sat Feb 27, 2010 1:04 pm

so anyway, I bought the barrels and pistons, they are not mahle, they are AA. Opened the box and found that the overall length of the piston is shorter than the original, and the distance between the top of the gudgeon pin, and the top of the piston is shorter (see photo of new and old piston on the same gudgeon pin) Is this because they are flat, or have I bought the wrong ones?

The new barrels are the same dimensions as the old ones

Sat Feb 27, 2010 1:45 pm

measure the diameter of your old pistons. they look like 1800 ones to me.

Sat Feb 27, 2010 1:45 pm

I believe that those are for a Porshe type 4. Bus type 4 has dished pistons.

Sat Feb 27, 2010 2:05 pm

AA 94mm flat top pistons had the wrong pin height and won't run all that well unless you shave something like 0.060 off the length of the jugs. I have heard that this has been corrected, but haven't viewed one to see for myself.

Sat Feb 27, 2010 2:08 pm

Methinks you picked up the wrong PC set for a bus application. For starters, I don't see ANY dish in the new one.

Sat Feb 27, 2010 2:15 pm

The diameter is 94mm, so diameter is correct and the barrel dimensions are correct. The pistons which came off the engine are dished as its a bus type 4. I was advised that the flat top pistons would be the same (aside from being flat obviously) and that they were a universal fit for Bus type 4/porsche 914 2000cc engine, with the comment being they would raise compression slightly.

sorry wildthing, you lost me at pin height and jugs. I'm a complete novice here, hence me buying the wrong damn things!

Sat Feb 27, 2010 2:20 pm

You're going to have to mock those up and check deck height. Universal fit? That statement makes me leery.

Sat Feb 27, 2010 2:21 pm

The pin height is the distance from the center of the wrist pin to the top of the flat area around the rim of the piston lid. As can be seen in your picture the pin heights are not the same between the two pistons. The jugs are the cylinders. Cylinder length can be changed on a VW engine to make up for varying pin heights. If you change the cylinder length you will also have to change the lengths of your push rods so that your valve geometry is correct.

Sat Feb 27, 2010 2:50 pm

ok, so i've just checked the accurate measurements.

New piston, 94mm diameter, flat topped. Top of gudgeon pin to top of crown = 23mm

Old piston, 94mm diameter, dished top. Top of gudgeon pin to top of crown (the flat bit round the edge) 30mm.

So just to confirm, I have bought the wrong set-up?

Sat Feb 27, 2010 2:51 pm

If the above comments can't help you then the short answer is: 10-4. Roger that.

Have you purchased Tom Wilson's book?

Sat Feb 27, 2010 3:07 pm

what do your rods look like? i think your original pistons have pin height for a 66mm stroke - i.e. for a 1700/1800.

Sat Feb 27, 2010 3:11 pm

ok i'm learning as i go. I'm reading tom wilson now. The engine I bought was a 2000cc. Or rather, thats what I was told. I've just found a stamp inside the old piston, 93 v 33+ which I assume means 93mm, therefore a 1800 engine.

Now i'm pissed

Sat Feb 27, 2010 4:14 pm

don't worry about the stamps or casting numbers - what is the actual measurement of the piston diameter? (it isn't that the marks are irrelevant, only that measurements are more relevant)

like i said the original pistons really look to me like 1800's because of the dish shape, and the pin height looks like 66mm stroke. but you say above you measured them to be 94mm which doesn't jive with that. post a photo of the connecting rod.

Sat Feb 27, 2010 8:35 pm

also can you post a pic of the bottom or 'inside' of those pistons? I am not convinced those are mahle.

Sun Feb 28, 2010 12:50 am

ok, i've remeasured and more accurately, with the crud off its 93mm The connecting rods have the small 'pad' on the top as descirbed by Tom Wilson. The inside of the old pistons are stamped Mahle, 93 v 33+.

i'm pissed at my ignorance more than having been duped