My M20 needs a new exhaust valve insert. I lost compression today and found a bit of the insert had broken off. I got the part in the exhaust so no other damage.
Should I replace both inserts?
Is there anyone making/have inserts for the M20 or will I need to buy inserts and turn to suit.
Fitting the inserts Is it a matter of heating the barrel and putting the insert in the freezer to fit once at the right size.
Thank you
Robert
An M20 is not fitted with hardened valve seats as standard, the seats being cut directly into the iron of the barrel..The exhaust side will sometimes have been done to help with running on lead free fuel..I found that running around locally there wasn't a problem with the original exhaust seat but I did experience problems on longer runs that included motorway work. That meant running longer distances at cruising speeds when problems are more likely...The hardened seat cured that problem...
It isn't necessary to do the inlet side for that reason as that is only passing cold charge...However, if the seats have been excessively cut and the valves have become pocketed into the barrel both seats can be done to restore the valve seats (and valves) to the correct height..A pocketed inlet valve reduces gas flow, particularly at small valve openings, something that can't afford to be wrong on the already inefficient M20...
There are many different sizes of hardened seats available as stock items so getting the right ones for the M20 is no problem...You should also check the exhaust valve to ensure the 'lip' of the valve between its tapered face and the top of the valve head is still wide enough...That 'lip' allows the exhaust valve to cope with the high operating temperatures without failure of the edge of the valve...
Personally I wouldn't recommend attempting the job yourself unless you have all the correct equipment to hand..I take mine to my local engine shop where he supplies and fits the seats and has a Serdi head machine for accurately cutting the seats after fitment...To that end you should also check the valve stem to guide clearances to ensure they are within tolerance...Ian
