1200 beetle piston conundrum

[stuey]

Ive been freshening up the girlfriends beetle engine and have hit on an issue- 3 of the pistons are seemingly different to the other 1 (stamped STD which I assume is standard not sexually transmitted disease) and when I have attempted to fit new piston rings they don't fit the "odd" 3 pistons.

The difference between the pistons seems to be the "3" have a dome to the top of the piston and the other is flat and of course the top 2 rings don't fit into the grooves (oil control rings fit all 4 however. Size wise the seem to be the same if my aldi calipers are to be believed- here is a pic of them can any one advise on why this/ what the hell I can do?



thanks in advance

[cairyhunt]

Have you cleaned ALL the crud from the grooves??

Do they measure the same as the other grooves?

Could always get some new B&P's

[stuey]

I did a bit more digging tonight and "think" I might have found the problem- the 3 domed pistons are for a 1300cc engine and although the bore is the same (77mm) I can only assume the 1300 uses a longer stroke crank to get the capacity.

anyway the 1300 piston rings are 2mm in thickness whereas the 1200 rings are 2.5mm I'll do some more measuring with the calipers tomorrow and if thats the case £20 worth of 1300cc rings will fix the issue (i hope!)

thanks for those suggestions- I think I will run the pistons through my wash tank again to be really sure!

[scaryoldcortina]

I'd have thought you also should look for another 1300 piston then! I can't see an odd piston doing the balance of the engine any favours (not that beetles are renowned for smooth running or anything )

Might also be doing odd things to the compression ratio too.

[stuey]

Apr 14, 2012 21:44:55 GMT scaryoldcortina said:

I'd have thought you also should look for another 1300 piston then! I can't see an odd piston doing the balance of the engine any favours (not that beetles are renowned for smooth running or anything )

Might also be doing odd things to the compression ratio too.

tbh it had been running superbly it did norfolk to somerset and back and didnt miss a beat but whilst its out and on the bench its a good thought- off to ebay with me!

[RobinJI]

Yeah, the lack of a domed top on the odd piston is a concern too. As Scary hints at, it'll be doing odd things to the compression, namely you'll likely have lower compression in that cylinder, so as well as the imbalance from the most likely miss-matched piston weights the engine will be getting less power from that cylinder. Finding a 4th matching piston would be a good idea.

[waveman1500]

Just out of interest, which cylinder did the flat-top come out of? If it's number 3, then I would guess that the person who built the motor was trying to be clever and reduce the compression on #3, because that is always the piston which runs hottest and hence wears out first on Beetle engines. It's probably not a bad idea. My old tired 1500cc Beetle had over 100psi compression in #1,2 and 4 and 70psi in #3, and that ran fine, so they don't mind having one cylinder down on compression!

[sparkyt]

I can sell you a full engine for 75 notes which you can strip the barrels pistons heads out of .. to rebuild your motor pm me if your interested