Forum

[John R]

I'm doing major work on my 350, including new main bearings and probably some performance tuning. I don't want to spoil it for "a ha'porth of tar", so If it would benefit from a new piston, new barrel or a rebore, it will get it. But I also don't want unneccessary cost and I don't want to be running in a new piston if I don't have to. How do you determine piston wear? The bore looks to be in good condition. 

[Peter]

Year and milage?

[simon]

You need to get a micrometre to measure the thrust faces of the piston and then measure the bore to see what the clearance is. Any more than 5 thou is getting tired and 8+ and it'll be clattering line a combine harvester.

[John R]

'89 and all the sixes. But that's not the no-brainer it sounds as I suspect it's not on the original bore/piston.

[Dennis C]

An easy way to measure wear is to remove the piston rings and place the piston in the bore at  the bottom where no wear has taken place and using a feeled guage set measure the gap at the bottom of the piston skirt, then move it up to the top where the wear takes place and measure it again.

[PeteF]

If you do decide the bore is worn I wonder if a rebore is cost effective as a new barrel is pretty cheap.

[Anders F. R.]

When I got my model G with poor compression etc., I dismantled the top end and took the cylinder and piston to a former speedway racer and engine tuner that is living nearby and is very helpful, and had him have a look at it.


It turns out it is not only about "the clearance between piston and cylinder"..... In my case the cylinder was both oval and conical, and the piston was not round either. So there were many different clearances...... A full rebore and a new piston was very much needed.


So what I am saying is that it may not be straightforward to measure cylinder and piston wear. A bit of experience may help.


Regards, Anders F. R.

[Alan R]

Hi there John R---You say you're doing "major work", with new mains and ---"probally some tuning work"?? At that level of engine dismantling it sounds like you're committed to a full overhaul--engine out , stripped, washed and laid -out for inspection ??  So naturally that will be including replacement seals, gaskets and the like. Irrespective of what year, milage etc what counts is what is in front of you on the bench right now. 1st}--Is the piston/barrel the correct one for the engine ie It's not some previous owners idea of the perfect special ?? Let's assume all is well on that score. 2nd}--What size piston is it ?? ie standard, +20, +40 etc. This should be a stamped No. on the top of the piston unless it is standard in which case there might be STD. or---nothing at all !!. In any event, the point I'm getting at is this-----If your piston is at the last overbore size then it really would be "cheapskate" if your measurements indicate that you might get another 2-3,000 miles, to put that assy. back in. Now on the other side of this coin----if you have a STD barrel/piston and you get similar readings , then a re-bore + piston to the next size is probally better. Dennis C has the right approach for the initial checks but as Anders F R  found out it's not just about the numbers. Also--pistons are not round when new (expantion etc)-- Now then a new barrel over a half-worn piston-?? Do you REALLY want that for your lovely, painstaking re-build ?? Have you an engineering friend / machine shop/ evening class  nearby--?? Ask them to pop a bore gauge down the barrel----3 readings in line with the gudgeon pin--3 at 90 degrees to it. Won't cost much and it will be accurate!!!!!! Finally------BE PATIENT. Whatever decision you take there will be a certain amount of running-in to do. Good luck and do keep us all informed of your progress. 

[John R]

First, thanks everyone.


 As fritzboy says, I am  embarking on a major strip down and rebuild, so as well as the new bearings that i know i need, I will be replacing all gaskets and seals. I will also give consideration to a roller big end, and looking at the oil pump spindle. (although I have no real worries about the current one).


 I am thinking of a gas flowed head; possiby having the work done by Tollgate because their tuning matches to a 26mm Amal, which I already have. They also rerccomend a high-comp piston. Lumpy cams might also be woth a look.


 Now as we all agree, no point in spoiling the ship for a haporth of tar. New barrels are not that expensive. But I'm not made of money!